PART II.

CHIEFLY SCIENTIFIC.

Aber im stillen Gemach entwirft bedeutende Zirkel Sinnend der Weise, beschleicht forschend den schaffenden Geist, Prueft der Stoffe Gewalt, der Magnete Hassen und Lieben, Folgt durch die Luefte dem Klang, folgt durch den Aether dem Strahl, Sucht das vertraute Gesetz in des Zufalls grausenden Wundern, Sucht den ruhenden Pol in der Erscheinungen Flucht.

Schiller.

ON LIGHT AND HEAT.

(1.)

[Sidenote: THEORIES OF LIGHT.]

What is Light? The ancients supposed it to be something emitted by the eyes, and for ages no notion was entertained that it required time to pass through space. In the year 1676 Roemer first proved that the light from Jupiter's satellites required a certain time to cross the earth's orbit. Bradley afterwards found that, owing to the velocity with which the earth flies through space, the rays of the stars are slightly inclined, just as rain-drops which descend vertically appear to meet us when we move swiftly through the shower. In Kew Gardens there is a sun-dial commemorative of this discovery, which is called the _aberration of light_. Knowing the velocity of the earth, and the inclination of the stellar rays, Bradley was able to calculate the velocity of light; and his result agrees closely with that of Roemer. Celestial distances were here involved, but a few years ago M. Fizeau, by an extremely ingenious contrivance, determined the time required by light to pass over a distance of about 9000 yards; and his experiment is quite in accordance with the results of his predecessors.

But what is it which thus moves? Some, and among the number Newton, imagined light to consist of particles darted out from luminous bodies. This is the so-called Emission-Theory, which was held by some of the greatest men: Laplace, for example, accepted it; and M. Biot has developed it with a lucidity and power peculiar to himself. It was first opposed by the astronomer Huyghens, and afterwards by Euler, both of whom supposed light to be a kind of undulatory motion; but they were borne down by their great antagonists, and the emission-theory held its ground until the commencement of the present century, when Thomas Young, Professor of Natural Philosophy in the Royal Institution, reversed the scientific creed by placing the Theory of Undulation on firm foundations. He was followed by a young Frenchman of extraordinary genius, who, by the force of his logic and the conclusiveness of his experiments, left the Wave-Theory without a competitor. The name of this young Frenchman was Augustin Fresnel.

Since his time some of the ablest minds in Europe have been applied to the investigation of this subject; and thus a mastery, almost miraculous, has been attained over the grandest and most subtle of natural phenomena. True knowledge is always fruitful, and a clear conception regarding any one natural agent leads infallibly to better notions regarding others. Thus it is that our knowledge of light has corrected and expanded our knowledge of _heat_, while the latter, in its turn, will assuredly lead us to clearer conceptions regarding the other forces of Nature.

I think it will not be a useless labour if I here endeavour to state, in a simple manner, our present views of light and heat. Such knowledge is essential to the explanation of many of the phenomena referred to in the foregoing pages; and even to the full comprehension of the origin of the glaciers themselves. A few remarks on the nature of sound will form a fit introduction.

[Sidenote: NATURE OF SOUND.]

It is known that sound is conveyed to our organs of hearing by the air: a bell struck in a vacuum emits no sound, and even when the air is thin the sound is enfeebled. Hawksbee proved this by the air-pump; De Saussure fired a pistol at the top of Mont Blanc,--I have repeated the experiment myself, and found, with him, that the sound is feebler than at the sea level. Sound is not produced by anything projected through the air. The explosion of a gun, for example, is sent forward by a motion of a totally different kind from that which animates the bullet projected from the gun: the latter is a motion of _translation_; the former, one of _vibration_. To use a rough comparison, sound is projected through the air as a push is through a crowd; it is the propagation of a _wave_ or _pulse_, each particle taking up the motion of its neighbour, and delivering it on to the next. These aerial waves enter the external ear, meet a membrane, the so-called tympanic membrane, which is drawn across the passage at a certain place, and break upon it as sea-waves do upon the shore. The membrane is shaken, its tremors are communicated to the auditory nerve, and transmitted by it to the brain, where they produce the impression to which we give the name of sound.

[Sidenote: CAUSE OF MUSIC.]

In the tumult of a city, pulses of different kinds strike irregularly upon the tympanum, and we call the effect _noise_; but when a succession of impulses reach the ear _at regular intervals_ we feel the effect as _music_. Thus, a vibrating string imparts a series of shocks to the air around it, which are transmitted with perfect regularity to the ear, and produce a _musical note_. When we hear the song of a soaring lark we may be sure that the entire atmosphere between us and the bird is filled with pulses, or undulations, or waves, as they are often called, produced by the little songster's organ of voice. This organ is a vibrating instrument, resembling, in principle, the reed of a clarionet. Let us suppose that we hear the song of a lark, elevated to a height of 500 feet in the air. Before this is possible, the bird must have agitated a sphere of air 1000 feet in diameter; that is to say, it must have communicated to 17,888 tons of air a motion sufficiently intense to be appreciated by our organs of hearing.

[Sidenote: CAUSE OF PITCH.]

Musical sounds differ in _pitch_: some notes are high and shrill, others low and deep. Boys are chosen as choristers to produce the shrill notes; men are chosen to produce the bass notes. Now, the sole difference here is, that the boy's organ vibrates _more rapidly_ than the man's--it sends a greater number of impulses per second to the ear. In like manner, a short string emits a higher note than a long one, because it vibrates more quickly. The greater the number of vibrations which any instrument performs in a given time, the higher will be the pitch of the note produced. The reason why the hum of a gnat is shriller than that of a beetle is that the wings of the small insect vibrate more quickly than those of the larger one. We can, with suitable arrangements, make those sonorous vibrations visible to the eye;[A] and we also possess instruments which enable us to tell, with the utmost exactitude, the number of vibrations due to any particular note. By such instruments we learn that a gnat can execute many thousand flaps of its little wings in a second of time.

[Sidenote: NATURE OF LIGHT.]

In the study of nature the coarser phenomena, which come under the cognizance of the senses, often suggest to us the finer phenomena which come under the cognizance of the mind; and thus the vibrations which produce sound, and which, as has been stated, can be rendered visible to the eye by proper means, first suggested that _light_ might be due to a somewhat similar action. This is now the universal belief. A luminous body is supposed to have its atoms, or molecules, in a state of intense vibration. The motions of the atoms are supposed to be communicated to a medium suited to their transmission, as air is to the transmission of sound. This medium is called the _luminiferous ether_, and the little billows excited in it speed through it with amazing celerity, enter the pupil of the eye, pass through the humours, and break upon the retina or optic nerve, which is spread out at the back of the eye. Hence the tremors they produce are transmitted along the nerve to the brain, where they announce themselves as _light_. The swiftness with which the waves of light are propagated through the ether, is however enormously greater than that with which the waves of sound pass through the air. An aerial wave of sound travels at about the rate of 1100 feet in a second: a wave of light leaves 192,000 miles behind it in the same time.

[Sidenote: CAUSE OF COLOUR.]

Thus, then, in the case of sound, we have the sonorous body, the air, and the auditory nerve, concerned in the phenomenon; in the case of light, we have the luminous body, the ether, and the optic nerve. The fundamental analogy of sound and light is thus before us, and it is easily remembered. But we must push the analogy further. We know that the white light which comes to us from the sun is made up of an infinite number of coloured rays. By refraction with a prism we can separate those rays from each other, and arrange them in the series of colours which constitute the solar spectrum. The rainbow is an imperfect or _impure_ spectrum, produced by the drops of falling rain, but by prisms we can unravel the white light into pure red, orange, yellow, green, blue, indigo, and violet. Now, this spectrum is to the eye what the gamut is to the ear; each colour represents a note, and _the different colours represent notes of different pitch_. The vibrations which produce the impression of red are _slower_, and the waves which they produce are _longer_, than those to which we owe the sensation of violet; while the vibrations which excite the other colours are intermediate between these two extremes. This, then, is the second grand analogy between light and sound: _Colour answers to Pitch_. There is therefore truth in the figure when we say that the gentian of the Alps sings a shriller note than the wild rhododendron, and that the red glow of the mountains at sunset is of a lower pitch than the blue of the firmament at noon.

[Sidenote: LENGTH OF ETHEREAL WAVES.]

These are not fanciful analogies. To the mind of the philosopher these waves of ether are almost as palpable and certain as the waves of the sea, or the ripples on the surface of a lake. The length of the waves, both of sound and light, and the number of shocks which they respectively impart to the ear and eye, have been the subjects of the strictest measurement. Let us here go through a simple calculation. It has been found that 39,000 waves of red light placed end to end would make up an inch. How many inches are there in 192,000 miles? My youngest reader can make the calculation for himself, and find the answer to be 12,165,120,000 inches. It is evident that, if we multiply this number by 39,000, we shall obtain the number of waves of red light in 192,000 miles; this number is 474,439,680,000,000. _All these waves enter the eye in one second_; thus the expression "I see red colour," strictly means, "My eye is now in receipt of four hundred and seventy-four millions of millions of impulses per second." To produce the impression of violet light a still greater number of impulses is necessary; the wave-length of violet is the 1/57500th part of an inch, and the number of shocks imparted in a second by waves of this length is, in round numbers, six hundred and ninety-nine millions of millions. The other colours of the spectrum, as already stated, rise gradually in pitch from the red to the violet.

A very curious analogy between the eye and ear may here be noticed. The range of seeing is different in different persons--some see a longer spectrum than others; that is to say, rays which are obscure to some are luminous to others. Dr. Wollaston pointed out a similar fact as regards hearing; the range of which differs in different individuals. Savart has shown that a good ear can hear a musical note produced by 8 shocks in a second; it can also hear a note produced by 24,000 shocks in a second; but there are ears in which the range is much more limited. It is possible indeed to produce a sound which shall be painfully shrill to one person, while it is quite unheard by another. I once crossed a Swiss mountain in company with a friend; a donkey was in advance of us, and the dull tramp of the animal was plainly heard by my companion; but to me this sound was almost masked by the shrill chirruping of innumerable insects which thronged the adjacent grass; my friend heard nothing of this, it lay quite beyond his range of hearing.

A third and most important analogy between sound and light is now to be noted; and it will be best understood by reference to something more tangible than either. When a stone is thrown into calm water a series of rings spread themselves around the centre of disturbance. If a second stone be thrown in at some distance from the first, the rings emanating from both centres will cross each other, and at those points where the ridge of one wave coincides with the ridge of another the water will be lifted to a greater height. At those points, on the contrary, where the ridge of one wave crosses the furrow of another, we have both obliterated, and the water restored to its ordinary level. Where two ridges or two furrows unite, we have a case of _coincidence_; but where a ridge and a furrow unite we have what is called _interference_. It is quite possible to send two systems of waves into the same channel, and to hold back one system a little, so that its ridges shall coincide with the furrows of the other system. The "interference" would be here complete, and the waves thus circumstanced would mutually destroy each other, smooth water being the result. In this way, by the addition of motion to motion, _rest_ may be produced.

[Sidenote: LIGHT ADDED TO LIGHT MAKES DARKNESS.]

In a precisely similar manner two systems of sonorous waves can be caused to interfere and mutually to destroy each other: thus, by adding sound to sound, _silence_ may be produced. Two beams of light also may be caused to interfere and effect their mutual extinction: thus, by adding light to light, we can produce _darkness_. Here indeed we have a critical analogy between sound and light--_the_ one, in fact, which compels the most profound thinkers of the present day to assume that light, like sound, is a case of undulatory motion.

We see here the vision of the intellect prolonged beyond the boundaries of sense into the region of what might be considered mere imagination. But, unlike other imaginations, we can bring ours to the test of experiment; indeed, so great a mastery have we obtained over these waves, which eye has not seen, nor ear heard, that we can with mathematical certainty cause them to coincide or to interfere, to help each other or to destroy each other, at pleasure. It is perhaps possible to be a little more precise here. Let two stones--with a small distance between them--be dropped into water at the same moment; a system of circular waves will be formed round each stone. Let the distance from one little crest to the next following one be called _the length of the wave_, and now let us inquire what will take place at a point equally distant from the places where the two stones were dropped in. Fixing our attention upon the ridge of the first wave in each case, it is manifest that, as the water propagates both systems with the same velocity, the two foremost ridges will reach the point in question at the same moment; the ridge of one would therefore coincide with the ridge of the other, and the water at this point would be lifted to a height greater than that of either of the previous ridges.

[Sidenote: COINCIDENCE AND INTERFERENCE.]

Again, supposing that by any means we had it in our power to retard one system of waves so as to cause the first ridge of the one to be exactly one wave length behind the first ridge of the other, when they arrive at the point referred to. It is plain that the first ridge of the retarded system now falls in with the second ridge of the unretarded system, and we have another case of coincidence. A little reflection will show the same to be true when one system is retarded any number of _whole wave-lengths_; the first ridge of the retarded system will always, at the point referred to, coincide with a _ridge_ of the unretarded system.

But now suppose the one system to be retarded only _half a wave-length_; it is perfectly clear that, in this case the first ridge of the retarded system would fall in with the first _furrow_ of the unretarded system, and instead of coincidence we should have interference. One system, in fact, would tend to make a hollow at the point referred to, the other would tend to make a hill, and thus the two systems would oppose and neutralize each other, so that neither the hollow nor the hill would be produced; the water would maintain its ordinary level. What is here said of a single half-wave-length of retardation, is also true if the retardation amount to any _odd_ number of half-wave-lengths. In all such cases we should have the ridge of the one system falling in with the furrow of the other; a mutual destruction of the waves of both systems being the consequence. The same remarks apply when the point, instead of being equally distant from both stones, is an even or an odd number of semi-undulations farther from the one than from the other. In the former case we should have coincidence, and in the latter case interference, at the point in question.

[Sidenote: LIQUID WAVES.]

To the eye of a person who understands these things, nothing can be more interesting than the rippling of water under certain circumstances. By the action of interference its surface is sometimes shivered into the most beautiful mosaic, shifting and trembling as if with a kind of visible music. When the tide advances over a sea-beach on a calm and sunny day, and its tiny ripples enter, at various points, the clear shallow pools which the preceding tide had left behind, the little wavelets run and climb and cross each other, and thus form a lovely _chasing_, which has its counterpart in the lines of light converged by the ripples upon the sand underneath. When waves are skilfully generated in a vessel of mercury, and a strong light reflected from the surface of the metal is received upon a screen, the most beautiful effects may be observed. The shape of the vessel determines, in part, the character of the figures produced; in a circular dish of mercury, for example, a disturbance at the centre propagates itself in circular waves, which after reflection again encircle the centre. If the point of disturbance be a little removed from the centre, the intersections of the direct and reflected waves produce the magnificent chasing shown in the annexed figure (16), which I have borrowed from the excellent work on Waves by the Messrs. Weber. The luminous figure reflected from such a surface is exceedingly beautiful. When the mercury is lightly struck by a glass point, in a direction concentric with the circumference of the vessel, the lines of light run round the vessel in mazy coils, interlacing and unravelling themselves in the most wonderful manner. If the vessel be square, a splendid mosaic is produced by the crossing of the direct and reflected waves. Description, however, can give but a feeble idea of these exquisite effects;--

"Thou canst not wave thy staff in the air, Or dip thy paddle in the lake, But it carves the brow of beauty there, And the ripples in rhymes the oar forsake."

[Sidenote: CHASING PRODUCED BY WAVES.]

[Sidenote: EFFECT OF RETARDATION.]

Now, all that we have said regarding the retardation of the waves of water, by a whole undulation and a semi-undulation, is perfectly applicable to the case of light. Two luminous points may be placed near to each other so as to resemble the two stones dropped into the water; and when the light of these is properly received upon a screen, or directly upon the retina, we find that at some places the action of the rays upon each other produces darkness, and at others augmented light. The former places are those where the rays emitted from one point are an _odd_ number of semi-undulations in advance of the rays sent from the other; the latter places are those where the difference of path described by the rays is either nothing, or an _even_ number of semi-undulations. Supposing _a_ and _b_ (Fig. 17) to be two such sources of light, and S R a screen on which the light falls; at a point _l_, equally distant from _a_ and _b_, we have _light_; at a point _d_, where _a d_ is half an undulation longer than _b d_, we have darkness; at _l'_, where _a l'_ is a whole wave-length, or two semi-undulations, longer than _b l'_, we again have light; and at a point _d'_, where the difference is three semi-undulations, we have darkness; and thus we obtain a series of bright and dark spaces as we recede laterally from the central point _l_.

Let a bit of tin foil be closely pasted upon a piece of glass, and the edge of a penknife drawn across the foil so as to produce a slit. Looking through this slit at a small and distant light, we find the light spread out in a direction at right angles to the slit, and if the light looked at be _monochromatic_, that is, composed of a single colour, we shall have a series of bright and dark bars corresponding to the points at which the rays from the different points of the slit alternately coincide and interfere upon the retina. By properly drawing a knife across a sheet of letter-paper a suitable slit may also be obtained; and those practised in such things can obtain the effect by looking through their fingers or their eyelashes.

[Sidenote: CHROMATIC EFFECTS.]

But if the light looked at be white, the light of a candle for example, or of a jet of gas, instead of having a series of bright and dark bars, we have the bars _coloured_. And see how beautifully this harmonizes with what has been already said regarding the different lengths of the waves which produce different colours. Looking again at Fig. 17 we see that a certain obliquity is necessary to cause one ray to be a whole undulation in advance of the other at the point _l'_; but it is perfectly manifest that the obliquity must depend upon the length of the undulation; a long undulation would require a greater obliquity than a short one; red light, for example, requires a greater obliquity than blue light; so that if the point _l'_ represents the place where the first bar of red light would be at its maximum strength, the maximum for blue would lie a little to the left of _l'_; the different colours are in this way separated from each other, and exhibit themselves as distinct fringes when a distant source of white light is regarded through a narrow slit.

By varying the shape of the aperture we alter the form of the chromatic image. A circular aperture, for example, placed in front of a telescope through which a point of white light is regarded, is seen surrounded by a concentric system of coloured rings. If we multiply our slits or apertures the phenomena augment in complexity and splendour. To give some notion of this I have copied from the excellent work of M. Schwerd the annexed figure (Fig. 18) which represents the gorgeous effect observed when a distant point of light is looked at through two gratings with slits of different widths.[B] A bird's feather represents a peculiar system of slits, and the effect observed on properly looking through it is extremely interesting.

[Sidenote: COLOURS OF THIN FILMS.]

There are many ways by which the retardation necessary to the production of interference is effected. The splendid colours of a soap-bubble are entirely due to interference; the beam falling upon the transparent film is partially reflected at its outer surface, but a portion of it enters the film and is reflected at its _inner_ surface. The latter portion having crossed the film and returned, is retarded, in comparison with the former, and, if the film be of suitable thickness, these two beams will clash and extinguish each other, while another thickness will cause the beams to coincide and illuminate the film with a light of greater intensity. From what has been said it must be manifest that to make two red beams thus coincide a thicker film would be required than would be necessary for two blue or green beams; thus, when the thickness of the bubble is suitable for the development of red, it is not suitable for the development of green, blue, &c.; the consequence is that we have different colours at different parts of the bubble. Owing to its compactness and to its being shaded by a covering of debris from the direct heat of the sun, the ice underneath the moraines of glaciers appears sometimes of a pitchy blackness. While cutting such ice with my axe I have often been surprised and delighted by sudden flashes of coloured light which broke like fire from the mass. These flashes were due to internal rupture, by which fissures were produced as thin as the film of a soap-bubble; the colours being due to the interference of the light reflected from the opposite sides of the fissures.

If spirit of turpentine, or olive oil, be thrown upon water, it speedily spreads in a thin film over the surface, and the most gorgeous chromatic phenomena may be thus produced. Oil of lemons is also peculiarly suited to this experiment. If water be placed in a tea-tray, and light of sufficient intensity be suffered to fall upon it, this light will be reflected from the upper and under surfaces of the film of oil, and the colours thus produced may be received upon a screen, and seen at once by many hundred persons. If the oil of cinnamon be used, fine colours are also obtained, and the breaking up of this film exhibits a most interesting case of molecular action. By using a kind of varnish, instead of oil, Mr. Delarue has imparted such tenacity to these films that they may be removed from the water on which they rest and preserved for any length of time. By such films the colours of certain beetles, and of the wings of certain insects, may be accurately imitated; and a rook's feather may be made to shine with magnificent iridescences. The colours of tempered metals, and the beautiful metallochrome of Nobili are also due to a similar cause.

[Sidenote: DIFFRACTION.]

These colours are called the colours of _thin plates_, and are distinguished in treatises on optics from the coloured bars and fringes above referred to, which are produced by _diffraction_, or the bending of the waves round the edge of an object. One result of this bending, which is of interest to us, was obtained by the celebrated Thomas Young. Permitting a beam of sunlight to enter a dark room through an aperture made with a fine needle, and placing in the path of the beam a bit of card one-thirtieth of an inch wide, he found the shadow of this card, or rather the line on which its shadow might be supposed to fall, always _bright_; and he proved the effect to be due to the bending of the waves of ether round the two edges of the card, and their coincidence at the other side. It has, indeed, been shown by M. Poisson, that the centre of the shadow of a small circular opaque disk which stands in the way of a beam diverging from a point is exactly as much illuminated as if the disk were absent. The singular effects described by M. Necker in the letter quoted at page 178 at once suggest themselves here; and we see how possible it is for the solar rays, in grazing a distant tree, so to bend round it as to produce upon the retina, where shadow might be expected, the impression of a tree of light.[C] Another effect of diffraction is especially interesting to us at present. Let the seed of lycopodium be scattered over a glass plate, or even like a cloud in the air, and let a distant point of light be regarded through it; the luminous point will appear surrounded by a series of coloured rings, and when the light is intense, like the electric or the Drummond light, the effect is exceedingly fine.

[Sidenote: CLOUD IRIDESCENCE, ETC., EXPLAINED.]

And now for the application of these experiments. I have already mentioned a series of coloured rings observed around the sun by Mr. Huxley and myself from the Rhone glacier; I have also referred to the cloud iridescences on the Aletschhorn; and to the colours observed during my second ascent of Monte Rosa, the magnificence of which is neither to be rendered by pigments nor described in words. All these splendid phenomena are, I believe, produced by diffraction, the vesicles or spherules of water in the case of the cloud acting the part of the sporules in the case of the lycopodium. The coloured fringe which surrounds the _Spirit of the Brocken_, and the spectra which I have spoken of as surrounding the sun, are also produced by diffraction. By the interference of their rays in the earth's atmosphere the stars can momentarily quench themselves; and probably to an intermittent action of this kind their twinkling, and the swift chromatic changes already mentioned, are due. Does not all this sound more like a fairy tale than the sober conclusions of science? What effort of the imagination could transcend the realities here presented to us? The ancients had their spheral melodies, but have not we ours, which only want a sense sufficiently refined to hear them? Immensity is filled with this music; wherever a star sheds its light its notes are heard. Our sun, for example, thrills concentric waves through space, and every luminous point that gems our skies is surrounded by a similar system. I have spoken of the rising, climbing and crossing of the tiny ripples of a calm tide upon a smooth strand; but what are they to those intersecting ripples of the "uncontinented deep" by which Infinity is engine-turned! Crossing solar and stellar distances, they bring us the light of sun and stars; thrilled back from our atmosphere, they give us the blue radiance of the sky; rounding liquid spherules, they clash at the other side, and the survivors of the tumult bear to our vision the wondrous cloud-dyes of Monte Rosa.

FOOTNOTES:

[A] The vibrations of the air of a room in which a musical instrument is sounded may be made manifest by the way in which fine sand arranges itself upon a thin stretched membrane over which it is strewn; and indeed Savart has thus rendered visible the vibrations of the tympanum itself. Every trace of sand was swept from a paper drum held in the clock-tower of Westminster when the Great Bell was sounded. Another way of showing the propagation of aerial pulses is to insert a small gas jet into a vertical glass tube about a foot in length, in which the flame may be caused to burn tranquilly. On pitching the voice to the note of an open tube a foot long, the little flame quivers, stretches itself, and responds by producing a clear melodious note of the same pitch as that which excited it. The flame will continue its song for hours without intermission.

[B] I am not aware whether in his own country, or in any other, a recognition at all commensurate with the value of the performance has followed Schwerd's admirable essay entitled 'The Phenomena of Diffraction deduced from the Theory of Undulation.'

[C] I think, however, that the strong irradiation from the glistening sides of the twigs and branches must also contribute to the result.

[Sidenote: RADIANT HEAT.]

(2.)

Thus, then, we have been led from Sound to Light, and light now in its turn will lead us to _Radiant Heat_; for in the order in which they are here mentioned the conviction arose that they are all three different kinds of motion. It has been said that the beams of the sun consist of rays of different colours, but this is not a complete statement of the case. The sun emits a multitude of rays which are perfectly non-luminous; and the same is true, in a still greater degree, of our artificial sources of illumination. Measured by the quantity of heat which they produce, 90 per cent. of the rays emanating from a flame of oil are obscure; while 99 out of every 100 of those which emanate from an alcohol flame are of the same description.[A]

[Sidenote: OBSCURE RAYS.]

In fact, the visible solar spectrum simply embraces an interval of rays of which the eye is formed to take cognizance, but it by no means marks the limits of solar action. Beyond the violet end of the spectrum we have obscure rays capable of producing chemical changes, and beyond the red we have rays possessing a high heating power, but incapable of exciting the impression of light. This latter fact was first established by Sir William Herschel, and it has been amply corroborated since.

The belief now universally prevalent is, that the rays of heat differ from the rays of light simply as one colour differs from another. As the waves which produce red are longer than those which produce yellow, so the waves which produce this obscure heat are longer than those which produce red. In fact, it may be shown that the longest waves never reach the retina at all; they are completely absorbed by the humours of the eye.

What is true of the sun's obscure rays is also true of calorific rays emanating from any obscure source,--from our own bodies, for example, or from the surface of a vessel containing boiling water. We must, in fact, figure a warm body also as having its particles in a state of vibration. When these motions are communicated from particle to particle of the body the heat is said to be _conducted_; when, on the contrary, the particles transmit their vibrations through the surrounding ether, the heat is said to be _radiant_. This radiant heat, though obscure, exhibits a deportment exactly similar to light. It may be refracted and reflected, and collected in the focus of a mirror or of a suitable lens. The principle of interference also applies to it, so that by adding heat to heat we can produce _cold_. The identity indeed is complete throughout, and, recurring to the analogy of sound, we might define this radiant heat to be light of too low a pitch to be visible.

I have thus far spoken of _obscure_ heat only; but the selfsame ray may excite both light and heat. The red rays of the spectrum possess a very high heating power. It was once supposed that the heat of the spectrum was an essence totally distinct from its light; but a profounder knowledge dispels this supposition, and leads us to infer that the selfsame ray, falling upon the nerves of feeling, excites heat, and falling upon the nerves of seeing, excites light. As the same electric current, if sent round a magnetic needle, along a wire, and across a conducting liquid, produces different physical effects, so also the same agent acting upon different organs of the body affects our consciousness differently.

FOOTNOTES:

[A] Melloni.

(3.)

[Sidenote: HEAT A KIND OF MOTION.]

Heat has been defined in the foregoing section as a motion of the molecules or atoms of a body; but though the evidence in favour of this view is at present overwhelming, I do not ask the reader to accept it as a certainty, if he feels sceptically disposed. In this case, I would only ask him to accept it as a symbol. Regarded as a mere physical image, a kind of paper-currency of the mind, convertible, in due time, into the gold of truth, the hypothesis will be found exceedingly useful.

All known bodies possess more or less of this molecular motion, and all bodies are communicating it to the ether in which they are immersed. Ice possesses it. Ice before it melts attains a temperature of 32 deg. Fahr., but the substance in winter often possesses a temperature far below 32 deg., so that in rising to 32 deg. it is _warmed_. In experimenting with ice I have often had occasion to cool it to 100 deg. and more below the freezing point, and to warm it afterwards up to 32 deg.

If then we stand before a wall of ice, the wall radiates heat to us, and we also radiate heat to it; but the quantity which we radiate being greater than that which the ice radiates, we lose more than we gain, and are consequently chilled. If, on the contrary, we stand before a warm stove, a system of exchanges also takes place; but here the quantity we receive is in excess of the quantity lost, and we are warmed by the difference.

In like manner the earth radiates heat by day and by night into space, and against the sun, moon, and stars. By day, however, the quantity received is greater than the quantity lost, and the earth is warmed; by night the conditions are reversed; the earth radiates more heat than is sent to her by the moon and stars, and she is consequently cooled.

But here an important point is to be noted:--the earth receives the heat of the sun, moon, and stars, in great part as _luminous_ heat, but she gives it out as _obscure_ heat. I do not now speak of the heat reflected by the earth into space, as the light of the moon is to us; but of the heat which, after it has been absorbed by the earth, and has contributed to warm it, is radiated into space, as if the earth itself were its independent source. Thus we may properly say that the heat radiated from the earth is _different in quality_ from that which the earth has received from the sun.

[Sidenote: QUALITIES OF HEAT.]

In one particular especially does this difference of quality show itself; besides being non-luminous, the heat radiated from the earth is more easily intercepted and absorbed by almost all transparent substances. A vast portion of the sun's rays, for example, can pass instantaneously through a thick sheet of water; gunpowder could easily be fired by the heat of the sun's rays converged by passing through a thick water lens; the drops upon leaves in greenhouses often act as lenses, and cause the sun to burn the leaves upon which they rest. But with regard to the rays of heat emanating from an obscure source, they are all absorbed by a layer of water less than the 20th of an inch in thickness: water is opaque to such rays, and cuts them off almost as effectually as a metallic screen. The same is true of other liquids, and also of many transparent solids.

[Sidenote: THE ATMOSPHERE LIKE A RATCHET.]

Assuming the same to be true of gaseous bodies, that they also intercept the obscure rays much more readily than the luminous ones, it would follow that while the sun's rays penetrate our atmosphere with freedom, the change which they undergo in warming the earth deprives them in a measure of this penetrating power. They can reach the earth, but _they cannot get back_; thus the atmosphere acts the part of a ratchet-wheel in mechanics; it allows of motion in one direction, but prevents it in the other.

De Saussure, Fourier, M. Pouillet, and Mr. Hopkins have developed this speculation, and drawn from it consequences of the utmost importance; but it nevertheless rested upon a basis of conjecture. Indeed some of the eminent men above-named deemed its truth beyond the possibility of experimental verification. Melloni showed that for a distance of 18 or 20 feet the absorption of obscure rays by the atmosphere was absolutely inappreciable. Hence, the _total_ absorption being so small as to elude even Melloni's delicate tests, it was reasonable to infer that _differences_ of absorption, if such existed at all, must be far beyond the reach of the finest means which we could apply to detect them.

[Sidenote: DIFFERENCES OF ABSORPTION BY GASES.]

This exclusion of one of the three states of material aggregation from the region of experiment was, however, by no means satisfactory; for our right to infer, from the deportment of a solid or a liquid towards radiant heat, the deportment of a gas, is by no means evident. In both liquids and solids we have the molecules closely packed, and more or less chained by the force of cohesion; in gases, on the contrary, they are perfectly free, and widely separated. How do we know that the interception of radiant heat by liquids and solids may not be due to an arrangement and comparative rigidity of their parts, which gases do not at all share? The assumption which took no note of such a possibility seemed very insecure, and called for verification.

My interest in this question was augmented by the fact, that the assumption referred to lies, as will be seen, at the root of the glacier question. I therefore endeavoured to fill the gap, and to do for gases and vapours what had been already so ably done for liquids and solids by Melloni. I tried the methods heretofore pursued, and found them unavailing; oxygen, hydrogen, nitrogen, and atmospheric air, examined by such methods, showed no action upon radiant heat. Nature was dumb, but the question occurred, "Had she been addressed in the proper language?" If the experimentalist is convinced of this, he will rest content even with a negative; but the absence of this conviction is always a source of discomfort, and a stimulus to try again.

The principle of the method finally applied is all that can here be referred to; and it, I hope, will be quite intelligible. Two beams of heat, from two distinct sources, were allowed to fall upon the same instrument,[A] and to contend there for mastery. When both beams were perfectly equal, they completely neutralized each other's action; but when one of them was in any sensible degree stronger than the other, the predominance of the former was shown by the instrument. It was so arranged that one of the conflicting beams passed through a tube which could be exhausted of air, or filled with any gas; thus varying at pleasure the medium through which it passed. The question then was, supposing the two beams to be equal when the tube was filled with air, will the exhausting of the tube disturb the equality? The answer was affirmative; the instrument at once showed that a greater quantity of heat passed through the vacuum than through the air.

The experiment was so arranged that the effect thus produced was very large as measured by the indications of the instrument. But the action of the simple gases, oxygen, hydrogen, and nitrogen, was incomparably less than that produced by some of the compound gases, while these latter again differed widely from each other. Vapours exhibited differences of equal magnitude. The experiments indeed proved that gaseous bodies varied among themselves, as to their power of transmitting radiant heat, just as much as liquids and solids. It was in the highest degree interesting to observe how a gas or vapour of perfect transparency, as regards light, acted like an opaque screen upon the heat. To the eye, the gas within the tube might be as invisible as the air itself, while to the radiant heat it behaved like a cloud which it was almost impossible to penetrate.

[Sidenote: SELECTED HEAT.]

Applying the same method, I have found that from the sun, from the electric light, or from the lime-light, a large amount of heat can be selected, which is unaffected not only by air, but by the most energetic gases that experiment has revealed to me; while this same heat, when it has its _quality_ changed by being rendered obscure, is powerfully intercepted. Thus the bold and beautiful speculation above referred to has been made an experimental fact; the radiant heat of the sun does certainly pass through the atmosphere to the earth with greater facility than the radiant heat of the earth can escape into space.

[Sidenote: POSSIBLE HEAT OF NEPTUNE.]

It is probable that, were the earth unfurnished with this atmospheric swathing, its conditions of temperature would be such as to render it uninhabitable by man; and it is also probable that a suitable atmosphere enveloping the most distant planet might render it, as regards temperature, perfectly habitable. If the planet Neptune, for example, be surrounded by an atmosphere which permits the solar and stellar rays to pass towards the planet, but cuts off the escape of the warmth which they excite, it is easy to see that such an accumulation of heat may at length take place as to render the planet a comfortable habitation for beings constituted like ourselves.[B]

But let us not wander too far from our own concerns. Where radiant heat is allowed to fall upon an absorbing substance, a certain thickness of the latter is always necessary for the absorption. Supposing we place a thin film of glass before a source of heat, a certain percentage of the heat will pass through the glass, and the remainder will be absorbed. Let the transmitted portion fall upon a second film similar to the first, a smaller percentage than before will be absorbed. A third plate would absorb still less, a fourth still less; and, after having passed through a sufficient number of layers, the heat would be so _sifted_ that all the rays capable of being absorbed by glass would be abstracted from it. Suppose all these films to be placed together so as to form a single thick plate of glass, it is evident that the plate must act upon the heat which falls upon it, in such a manner that the major portion is absorbed _near the surface at which the heat enters_. This has been completely verified by experiment.

[Sidenote: COLD OF UPPER ATMOSPHERE.]

Applying this to the heat radiated from the earth, it is manifest that the greatest quantity of this heat will be absorbed by the lowest atmospheric strata. And here we find ourselves brought, by considerations apparently remote, face to face with the fact upon which the existence of all glaciers depends, namely, the comparative coldness of the upper regions of the atmosphere. The sun's rays can pass in a great measure through these regions without heating them; and the earth's rays, which they might absorb, hardly reach them at all, but are intercepted by the lower portions of the atmosphere.[C]

Another cause of the greater coldness of the higher atmosphere is the expansion of the denser air of the lower strata when it ascends. The dense air makes room for itself by pushing back the lighter and less elastic air which surrounds it: _it does work_, and, to perform this work, a certain amount of heat must be consumed. It is the consumption of this heat--its absolute annihilation as heat--that chills the expanded air, and to this action a share of the coldness of the higher atmosphere must undoubtedly be ascribed. A third cause of the difference of temperature is the large amount of heat communicated, _by way of contact_, to the air of the earth's surface; and a fourth and final cause is the loss endured by the highest strata through radiation into space.

FOOTNOTES:

[A] The opposite faces of a thermo-electric pile.

[B] See a most interesting paper on this subject by Mr. Hopkins in the Cambridge 'Transactions,' May, 1856.

[C] See M. Pouillet's important Memoir on Solar Radiation. Taylor's Scientific Memoirs, vol. iv. p. 44.

ORIGIN OF GLACIERS.

(4.)

[Sidenote: THE SNOW-LINE.]

Having thus accounted for the greater cold of the higher atmospheric regions, its consequences are next to be considered. One of these is, that clouds formed in the lower portions of the atmosphere, in warm and temperate latitudes, usually discharge themselves upon the earth as rain; while those formed in the higher regions discharge themselves upon the mountains as snow. The snow of the higher atmosphere is often melted to rain in passing through the warmer lower strata: nothing indeed is more common than to pass, in descending a mountain, from snow to rain; and I have already referred to a case of this kind. The appearance of the grassy and pine-clad alps, as seen from the valleys after a wet night, is often strikingly beautiful; the level at which the snow turned to rain being distinctly marked upon the slopes. Above this level the mountains are white, while below it they are green. The eye follows this _snow-line_ with ease along the mountains, and when a sufficient extent of country is commanded its regularity is surprising.

The term "snow-line," however, which has been here applied to a local and temporary phenomenon, is commonly understood to mean something else. In the case just referred to it marked the place where the supply of solid matter from the upper atmospheric regions, during a single fall, was exactly equal to its consumption; but the term is usually understood to mean the line along which the quantity of snow which falls _annually_ is melted, and no more. Below this line each year's snow is completely cleared away by the summer heat; above it a residual layer abides, which gradually augments in thickness from the snow-line upwards.

[Sidenote: MOUNTAINS UNLOADED BY GLACIERS.]

Here then we have a fresh layer laid on every year; and it is evident that, if this process continued without interruption, every mountain which rises above the snow-line must augment annually in height; the waters of the sea thus piled, in a solid form, upon the summits of the hills, would raise the latter to an indefinite elevation. But, as might be expected, the snow upon steep mountain-sides frequently slips and rolls down in avalanches into warmer regions, where it is reduced to water. A comparatively small quantity of the snow is, however, thus got rid of, and the great agent which Nature employs to relieve her overladen mountains is the glaciers.

Let us here avoid an error which may readily arise out of the foregoing reflections. The principal region of clouds and rain and snow extends only to a limited distance upwards in the atmosphere; the highest regions contain very little moisture, and were our mountains sufficiently lofty to penetrate those regions, the quantity of snow falling upon their summits would be too trifling to resist the direct action of the solar rays. These would annually clear the summits to a certain level, and hence, were our mountains high enough, we should have a superior, as well as an inferior, snow-line; the region of perpetual snow would form a belt, below which, in summer, snowless valleys and plains would extend, and above which snowless summits would rise.

(5.)

[Sidenote: WHITE AND BLUE ICE.]

At its origin then a glacier is snow--at its lower extremity it is ice. The blue blocks that arch the source of the Arveiron were once powdery snow upon the slopes of the Col du Geant. Could our vision penetrate into the body of the glacier, we should find that the change from white to blue essentially consists in the gradual expulsion of the air which was originally entangled in the meshes of the fallen snow. Whiteness always results from the intimate and irregular mixture of air and a transparent solid; a crushed diamond would resemble snow; if we pound the most transparent rock-salt into powder we have a substance as white as the whitest culinary salt; and the colourless glass vessel which holds the salt would also, if pounded, give a powder as white as the salt itself. It is a law of light that in passing from one substance to another possessing a different power of refraction, a portion of it is always reflected. Hence when light falls upon a transparent solid mixed with air, at each passage of the light from the air to the solid and from the solid to the air a portion of it is reflected; and, in the case of a powder, this reflection occurs so frequently that the passage of the light is practically cut off. Thus, from the mixture of two perfectly transparent substances, we obtain an opaque one; from the intimate mixture of air and water we obtain foam; clouds owe their opacity to the same principle; and the condensed steam of a locomotive casts a shadow upon the fields adjacent to the line, because the sunlight is wasted in echoes at the innumerable limiting surfaces of water and air.

[Sidenote: AIR-BUBBLES IN ICE.]

The snow which falls upon high mountain-eminences has often a temperature far below the freezing point of water. Such snow is _dry_, and if it always continued so the formation of a glacier from it would be impossible. The first action of the summer's sun is to raise the temperature of the superficial snow to 32 deg., and afterwards to melt it. The water thus formed percolates through the colder mass underneath, and this I take to be the first active agency in expelling the air entangled in the snow. But as the liquid trickles over the surfaces of granules colder than itself it is partially deposited in a solid form on these surfaces, thus augmenting the size of the granules, and cementing them together. When the mass thus formed is examined, the air within it is found as _round bubbles_. Now it is manifest that the air caught in the irregular interstices of the snow can have no tendency to assume this form so long as the snow remains solid; but the process to which I have referred--the saturation of the lower portions of the snow by the water produced by the melting of the superficial portions--enables the air to form itself into globules, and to give the ice of the _neve_ its peculiar character. Thus we see that, though the sun cannot get directly at the deeper portions of the snow, by liquefying the upper layer he charges it with heat, and makes it his messenger to the cold subjacent mass.

The frost of the succeeding winter may, I think, or may not, according to circumstances, penetrate through this layer, and solidify the water which it still retains in its interstices. If the winter set in with clear frosty weather, the penetration will probably take place; but if heavy snow occur at the commencement of winter, thus throwing a protective covering over the _neve_, freezing to any great depth may be prevented. Mr. Huxley's idea seems to be quite within the range of possibility, that water-cells may be transmitted from the origin of the glacier to its end, retaining their contents always liquid.

[Sidenote: SNOW PRESSED TO ICE.]

It was formerly supposed, and is perhaps still supposed by many, that the snow of the mountains is converted into the ice of the glacier by the process of saturation and freezing just indicated. But the frozen layer would not yet resemble glacier ice; it is only at the deeper portions of the _neve_ that we find an approximation to the true ice of the glacier. This brings us to the second great agent in the process of glacification, namely, pressure. The ice of the _neve_ at 32 deg. may be squeezed or crushed with extreme facility; and if the force be applied slowly and with caution, the yielding of the mass may be made to resemble the yielding of a plastic body. In the depths of the _neve_, where each portion of the ice is surrounded by a resistant mass, rude crushing is of course out of the question. The layers underneath yield with extreme slowness to the pressure of the mass above them; they are squeezed, but not rudely fractured; and even should rude fracture occur, the ice, as shall subsequently be shown, possesses the power of restoring its own continuity. Thus, then, the lower portions of the _neve_ are removed by pressure more and more from the condition of snow, the air-bubbles which give to the _neve_-ice its whiteness are more and more expelled, and this process, continued throughout the entire glacier, finally brings the ice to that state of magnificent transparency which we find at the termination of the glacier of Rosenlaui and elsewhere. This is all capable of experimental proof. The Messrs. Schlagintweit compressed the snow of the _neve_ to compact ice; and I have myself frequently obtained slabs of ice from snow in London.

COLOUR OF WATER AND ICE.

(6.)

The sun is continually sending forth waves of different lengths, all of which travel with the same velocity through the ether. When these waves enter a prism of glass they are retarded, but in different degrees. The shorter waves suffer the greatest retardation, and in consequence of this are most deflected from their straight course. It is this property which enables us to separate one from the other in the solar spectrum, and this separation proves that the waves are by no means inextricably entangled with each other, but that they travel independently through space.

In consequence of this independence, the same body may intercept one system of waves while it allows another to pass: on this quality, indeed, depend all the phenomena of colour. A red glass, for example, is red because it is so constituted that it destroys the shorter waves which produce the other colours, and transmits only the waves which produce red. I may remark, however, that scarcely any glass is of a pure colour; along with the predominant waves, some of the other waves are permitted to pass. The colours of flowers are also very impure; in fact, to get pure colours we must resort to a delicate prismatic analysis of white light.

[Sidenote: LONG WAVES MOST ABSORBED.]

It has already been stated that a layer of water less than the twentieth of an inch in thickness suffices to stop and destroy all waves of radiant heat emanating from an obscure source. The longer waves of the obscure heat cannot get through water, and I find that all transparent compounds which contain _hydrogen_ are peculiarly hostile to the longer undulations. It is, I think, the presence of this element in the humours of the eye which prevents the extra red rays of the solar spectrum from reaching the retina. It is interesting to observe that while bisulphide of carbon, chloride of phosphorus, and other liquids which contain no hydrogen, permit a large portion of the rays emanating from an iron or copper ball, at a heat below redness, to pass through them with facility, the same thickness of substances equally transparent, but which contain hydrogen, such as ether, alcohol, water, or the vitreous humour of the eye of an ox, completely intercepts these obscure rays. The same is true of solid bodies; a very slight thickness of those which contain hydrogen offers an impassable barrier to all rays emanating from a non-luminous source.[A] But the heat thus intercepted is by no means lost; its _radiant form_ merely is destroyed. Its waves are shivered upon the particles of the body, but they impart warmth to it, while the heat which retains its radiant form contributes in no way to the warmth of the body through which it passes.

[Sidenote: FINAL COLOUR OF ICE AND WATER BLUE.]

Water then absorbs all the extra red rays of the sun, and if the layer be thick enough it invades the red rays themselves. Thus the greater the distance the solar beams travel through pure water the more are they deprived of those components which lie at the red end of the spectrum. The consequence is, that the light finally transmitted by the water, and which gives to it its colour, is _blue_.

[Sidenote: EXPERIMENT.]

I find the following mode of examining the colour of water both satisfactory and convenient:--A tin tube, fifteen feet long and three inches in diameter, has its two ends stopped securely by pieces of colourless plate glass. It is placed in a horizontal position, and pure water is poured into it through a small lateral pipe, until the liquid reaches half way up the glasses at the ends; the tube then holds a semi-cylinder of water and a semi-cylinder of air. A white plate, or a sheet of white paper, well illuminated, is then placed at a little distance from one end of the tube, and is looked at through the tube. Two semicircular spaces are then seen, one by the light which has passed through the air, the other by the light which has passed through the water; and their proximity furnishes a means of comparison, which is absolutely necessary in experiments of this kind. It is always found that, while the former semicircle remains white, the latter one is vividly coloured.[B]

When the beam from an electric lamp is sent through this tube, and a convex lens is placed at a suitable distance from its most distant end, a magnified image of the coloured and uncoloured semicircles may be projected upon a screen. Tested thus, I have sometimes found, after rain, the ordinary pipe-water of the Royal Institution quite opaque; while, under other circumstances, I have found the water of a clear green. The pump-water of the Institution thus examined exhibits a rich sherry colour, while distilled water is blue-green.

The blueness of the Grotto of Capri is due to the fact that the light which enters it has previously traversed a great depth of clear water. According to Bunsen's account, the _laugs_, or cisterns of hot water, in Iceland must be extremely beautiful. The water contains silica in solution, which, as the walls of the cistern arose, was deposited upon them in fantastic incrustations. These, though white, when looked at through the water appear of a lovely blue, which deepens in tint as the vision plunges deeper into the liquid.

[Sidenote: ICE OPAQUE TO RADIANT HEAT.]

Ice is a crystal formed from this blue liquid, the colour of which it retains. Ice is the most opaque of transparent solids to radiant heat, as water is the most opaque of liquids. According to Melloni, a plate of ice one twenty-fifth of an inch thick, which permits the rays of light to pass without sensible absorption, cuts off 94 per cent. of the rays of heat issuing from a powerful oil lamp, 99-1/2 per cent. of the rays issuing from incandescent platinum, and the whole of the rays issuing from an obscure source. The above numbers indicate how large a portion of the rays emitted by our artificial sources of light is obscure.

When the rays of light pass through a sufficient thickness of ice the longer waves are, as in the case of water, more and more absorbed, and the final colour of the substance is therefore blue. But when the ice is filled with minute air-bubbles, though we should loosely call it _white_, it may exhibit, even in small pieces, a delicate blue tint. This, I think, is due to the frequent interior reflection which takes place at the surfaces of the air-cells; so that the light which reaches the eye from the interior may, in consequence of its having been reflected hither and thither, really have passed through a considerable thickness of ice. The same remark, as we have already seen, applies to the delicate colour of newly fallen snow.

FOOTNOTES:

[A] What is here stated regarding hydrogen is true of all the liquids and solids which have hitherto been examined,--but whether any exceptions occur, future experience must determine. It is only when in combination that it exhibits this impermeability to the obscure rays.

[B] In my own experiments I have never yet been able to obtain a pure blue, the nearest approach to it being a blue-green.

COLOURS OF THE SKY.

(7.)

[Sidenote: NEWTON'S HYPOTHESIS.]

In treating of the Colours of Thin Plates we found that a certain thickness was necessary to produce blue, while a greater thickness was necessary for red. With that wonderful power of generalization which belonged to him, Newton thus applies this apparently remote fact to the blue of the sky:--"The blue of the first order, though very faint and little, may possibly be the colour of some substances, and particularly the azure colour of the skies seems to be of this order. For all vapours, when they begin to condense and coalesce into small parcels, become first of that bigness whereby such an azure is reflected, before they can constitute clouds of other colours. And so, this being the first colour which vapours begin to reflect, it ought to be the colour of the finest and most transparent skies, in which vapours are not arrived at that grossness requisite to reflect other colours, as we find it is by experience."

M. Clausius has written a most interesting paper, which he endeavours to show that the minute particles of water which are supposed by Newton to reflect the light, cannot be little globes entirely composed of water, but bladders or hollow spheres; the vapour must be in what is generally termed the _vesicular_ state. He was followed by M. Bruecke, whose experiments prove that the suspended particles may be so small that the reasoning of M. Clausius may not apply to them.

But why need we assume the existence of such particles at all?--why not assume that the colour of the air is blue, and renders the light of the sun blue, after the fashion of a blue glass or a solution of the sulphate of copper? I have already referred to the great variation which the colour of the firmament undergoes in the Alps, and have remarked that this seems to indicate that the blue depends upon some variable constituent of the atmosphere. Further, we find that the blue light of the sky is _reflected_ light; and there must be something in the atmosphere capable of producing this reflection; but this thing, whatever it is, produces another effect which the blue glass or liquid is unable to produce. These _transmit_ blue light, whereas, when the solar beams have traversed a great length of air, as in the morning or the evening, they are yellow, or orange, or even blood-red, according to the state of the atmosphere:--the transmitted light and the reflected light of the atmosphere are then totally different in colour.

[Sidenote: GOETHE'S HYPOTHESIS.]

Goethe, in his celebrated 'Farbenlehre,' gives a theory of the colour of the sky, and has illustrated it by a series of striking facts. He assumed two principles in the universe--Light and Darkness--and an intermediate stage of Turbidity. When the darkness is seen through a turbid medium on which the light falls, the medium appears blue; when the light itself is viewed through such a medium, it is yellow, or orange, or ruby-red. This he applies to the atmosphere, which sends us blue light, or red, according as the darkness of infinite space, or the bright surface of the sun, is regarded through it.

As a theory of colours Goethe's work is of no value, but the facts which he has brought forward in illustration of the action of turbid media are in the highest degree interesting. He refers to the blueness of distant mountains, of smoke, of the lower part of the flame of a candle (which if looked at with a white surface behind it completely disappears), of soapy water, and of the precipitates of various resins in water. One of his anecdotes in connexion with this subject is extremely curious and instructive. The portrait of a very dignified theologian having suffered from dirt, it was given to a painter to be cleaned. The clergyman was drawn in a dress of black velvet, over which the painter, in the first place, passed his sponge. To his astonishment the black velvet changed to the colour of blue plush, and completely altered the aspect of its wearer. Goethe was informed of the fact; the experiment was repeated in his presence, and he at once solved it by reference to his theory. The varnish of the picture when mixed with the water formed a turbid medium, and the black coat seen through it appeared blue; when the water evaporated the coat resumed its original aspect.

[Sidenote: SUSPENDED PARTICLES.]

With regard to the real explanation of these effects, it may be shown, that, if a beam of white light be sent through a liquid which contains extremely minute particles in a state of suspension, the short waves are more copiously reflected by such particles than the long ones; blue, for example, is more copiously reflected than red. This may be shown by various fine precipitates, but the best is that of Bruecke. We know that mastic and various resins are soluble in alcohol, and are precipitated when the solution is poured into water: _Eau de Cologne_, for example, produces a white precipitate when poured into water. If however this precipitate be sufficiently diluted, it gives the liquid a bluish colour by reflected light. Even when the precipitate is very thick and gross, and floats upon the liquid like a kind of curd, its under portions often exhibit a fine blue. To obtain particles of a proper size, Bruecke recommends 1 gramme of colourless mastic to be dissolved in 87 grammes of alcohol, and dropped into a beaker of water, which is kept in a state of agitation. In this way a blue resembling that of the firmament may be produced. It is best seen when a black cloth is placed behind the glass; but in certain positions this blue liquid appears yellow; and these are the positions when the _transmitted_ light reaches the eye. It is evident that this change of colour must necessarily exist; for the blue being partially withdrawn by more copious reflection, the transmitted light must partake more or less of the character of the complementary colour; though it does not follow that they should be exactly complementary to each other.

[Sidenote: THE SUN THROUGH LONDON SMOKE.]

When a long tube is filled with clear water, the colour of the liquid, as before stated, shows itself by transmitted light. The effect is very interesting when a solution of mastic is permitted to drop into such a tube, and the fine precipitate to diffuse itself in the water. The blue-green of the liquid is first neutralized, and a yellow colour shows itself; on adding more of the solution the colour passes from yellow to orange, and from orange to blood-red. With a cell an inch and a half in width, containing water, into which the solution of mastic is suffered to drop, the same effect may be obtained. If the light of an electric lamp be caused to form a clear sunlike disk upon a white screen, the gradual change of this light by augmented precipitation into deep glowing red, resembling the colour of the sun when seen through fine London smoke, is exceedingly striking. Indeed the smoke acts, in some measure, the part of our finely-suspended matter.

[Sidenote: MORNING AND EVENING RED.]

By such means it is possible to imitate the phenomena of the firmament; we can produce its pure blue, and cause it to vary as in nature. The milkiness which steals over the heavens, and enables us to distinguish one cloudless day from another, can be produced with the greatest ease. The yellow, orange, and red light of the morning and evening can also be obtained: indeed the effects are so strikingly alike as to suggest a common origin--that the colours of the sky are due to minute particles diffused through the atmosphere. These particles are doubtless the condensed vapour of water, and its variation in quality and amount enables us to understand the variability of the firmamental blue, and of the morning and the evening red. Professor Forbes, moreover, has made the interesting observation that the steam of a locomotive, at a certain stage of its condensation, is blue or red according as it is viewed by reflected or transmitted light.

These considerations enable us to account for a number of facts of common occurrence. Thin milk, when poured upon a black surface, appears bluish. The milk is colourless; that is, its blueness is not due to _absorption_, but to a _separation_ of the light by the particles suspended in the liquid. The juices of various plants owe their blueness to the same cause; but perhaps the most curious illustration is that presented by a blue eye. Here we have no true colouring matter, no proper absorption; but we look through a muddy medium at the black choroid coat within the eye, and the medium appears blue.[A]

[Sidenote: COLOUR OF SWISS LAKES.]

Is it not probable that this action of finely-divided matter may have some influence on the colour of some of the Swiss lakes--as that of Geneva for example? This lake is simply an expansion of the river Rhone, which rushes from the end of the Rhone glacier, as the Arveiron does from the end of the Mer de Glace. Numerous other streams join the Rhone right and left during its downward course; and these feeders, being almost wholly derived from glaciers, join the Rhone charged with the finer matter which these in their motion have ground from the rocks over which they have passed. But the glaciers must grind the mass beneath them to particles of all sizes, and I cannot help thinking that the finest of them must remain suspended in the lake throughout its entire length. Faraday has shown that a precipitate of gold may require months to sink to the bottom of a bottle not more than five inches high, and in all probability it would require _ages_ of calm subsidence to bring _all_ the particles which the Lake of Geneva contains to its bottom. It seems certainly worthy of examination whether such particles suspended in the water contribute to the production of that magnificent blue which has excited the admiration of all who have seen it under favourable circumstances.

FOOTNOTES:

[A] Helmholtz, 'Das Sehen des Menschen.'

THE MORAINES.

(8.)

The surface of the glacier does not long retain the shining whiteness of the snow from which it is derived. It is flanked by mountains which are washed by rain, dislocated by frost, riven by lightning, traversed by avalanches, and swept by storms. The lighter debris is scattered by the winds far and wide over the glacier, sullying the purity of its surface. Loose shingle rattles at intervals down the sides of the mountains, and falls upon the ice where it touches the rocks. Large rocks are continually let loose, which come jumping from ledge to ledge, the cohesion of some being proof against the shocks which they experience; while others, when they hit the rocks, burst like bomb-shells, and shower their fragments upon the ice.

[Sidenote: LATERAL MORAINES.]

Thus the glacier is incessantly loaded along its borders with the ruins of the mountains which limit it; and it is evident that the quantity of rock and rubbish thus cast upon the glacier depends upon the character of the adjacent mountains. Where the summits are bare and friable, we may expect copious showers; where they are resistant, and particularly where they are protected by a covering of ice and snow, the quantity will be small. As the glacier moves downward, it carries with it the load deposited upon it. Long ridges of debris thus flank the glacier, and these ridges are called _lateral moraines_. Where two tributary glaciers join to form a trunk-glacier, their adjacent lateral moraines are laid side by side at the place of confluence, thus constituting a ridge which runs along the middle of the trunk-glacier, and which is called a _medial moraine_. The rocks and debris carried down by the glacier are finally deposited at its lower extremity, forming there a _terminal moraine_.

[Sidenote: MEDIAL AND TERMINAL MORAINES.]

It need hardly be stated that the number of medial moraines is only limited by the number of branch glaciers. If a glacier have but two branches, it will have only one medial moraine; if it have three branches, it will have two medial moraines; if _n_ branches, it will have _n_-1 medial moraines. The number of medial moraines, in short, is always _one less_ than the number of branches. A glance at the annexed figure will reveal the manner in which the lateral moraines of the Mer de Glace unite to form medial ones. (See Fig. 19.)

When a glacier diminishes in size it leaves its lateral moraines stranded on the flanks of the valleys. Successive shrinkings may thus occur, and _have_ occurred at intervals of centuries; and a succession of old lateral moraines, such as many glacier-valleys exhibit, is the consequence. The Mer de Glace, for example, has its old lateral moraines, which run parallel with its present ones. The glacier may also diminish _in length_ at distant intervals; the result being a succession of more or less concentric terminal moraines. In front of the Rhone-glacier we have six or seven such moraines, and the Mer de Glace also possesses a series of them.

Let us now consider the effect produced by a block of stone upon the surface of a glacier. The ice around it receives the direct rays of the sun, and is acted on by the warm air; it is therefore constantly melting. The stone also receives the solar beams, is warmed, and transmits its heat, by conduction, to the ice beneath it. If the heat thus transmitted to the ice through the stone be less than an equal space of the surrounding ice receives, it is manifest that the ice around the stone will waste more quickly than that beneath it, and the consequence is, that, as the surface sinks, it leaves behind it a pillar of ice, on which the block is elevated. If the stone be wide and flat, it may rise to a considerable height, and in this position it constitutes what is called a glacier-_table_. (See Fig. 6.)

[Sidenote: GLACIER TABLES ACCOUNTED FOR.]

Almost all glaciers present examples of such tables; but no glacier with which I am acquainted exhibits them in greater number and perfection than the Unteraar glacier, near the Grimsel. Vast masses of granite are thus poised aloft on icy pedestals; but a limit is placed to their exaltation by the following circumstance. The sun plays obliquely upon the table all day; its southern extremity receives more heat than its northern, and the consequence is, that it _dips_ towards the south. Strictly speaking, the plane of the dip rotates a little during the day, being a little inclined towards the east in the morning, north and south a little after noon, and inclined towards the west in the evening; so that, theoretically speaking, the block is a sun-dial, showing by its position the hour of the day. This rotation is, however, too small to be sensible, and hence _the dip of the stones upon a glacier sufficiently exposed to the sunlight, enables us at any time to draw the meridian line along its surface_. The inclination finally becomes so great that the block slips off its pedestal, and begins to form another, while the one which it originally occupied speedily disappears, under the influence of sun and air. Fig. 20 represents a typical section of a glacier-table, the sun's rays being supposed to fall in the direction of the shading lines.

[Sidenote: TYPE "TABLE."]

Stones of a certain size are always lifted in the way described. A considerable portion of the heat which a large block receives is wasted by radiation, and by communication to the air, so that the quantity which reaches the ice beneath is trifling. Such a mass is, of course, a protector of the ice beneath it. But if the stone be small, and dark in colour, it absorbs the heat with avidity, communicates it quickly to the ice with which it is in contact, and consequently sinks in the ice. This is also the case with bits of dirt and the finer fragments of debris; they sink in the glacier. Sometimes, however, a pretty thick layer of sand is washed over the ice from the moraines, or from the mountain-sides; and such sand-layers give birth to ice-cones, which grow to peculiarly grand dimensions on the Lower Aar glacier. I say "grow," but the truth, of course, is, that the surrounding ice wastes, while the portion underneath the sand is so protected that it remains as an eminence behind. At first sight, these sand-covered cones appear huge heaps of dirt, but on examination they are found to be cones of ice, and that the dirt constitutes merely a superficial covering.

Turn we now to the moraines. Protecting, as they do, the ice from waste, they rise, as might be expected, in vast ridges above the general surface of the glacier. In some cases the surrounding mass has been so wasted as to leave the spines of ice which support the moraines forty or fifty feet above the general level of the glacier. I should think the moraines of the Mer de Glace about the Tacul rise to this height. But lower down, in the neighbourhood of the Echelets, these high ridges disappear, and nought remains to mark the huge moraine but a strip of dirt, and perhaps a slight longitudinal protuberance on the surface of the glacier. How have the blocks vanished that once loaded the moraines near the Tacul? They have been swallowed in the crevasses which intersect the moraines lower down; and if we could examine the ice at the Echelets we should find the engulfed rocks in the body of the glacier.

[Sidenote: MORAINES ENGULFED AND DISGORGED.]

Cases occur, wherein moraines, after having been engulfed, and hidden for a time, are again entirely disgorged by the glacier. Two moraines run along the basin of the Talefre, one from the Jardin, the other from an adjacent promontory, proceeding parallel to each other towards the summit of the great ice-fall. Here the ice is riven, and profound chasms are formed, in which the blocks and shingle of the moraines disappear. Throughout the entire ice-fall the only trace of the moraines is a broad dirt-streak, which the eye may follow along the centre of the fall, with perhaps here and there a stone which has managed to rise from its frozen sepulchre. But the ice wastes, and at the base of the fall large masses of stone begin to reappear; these become more numerous as we descend; the smaller debris also appears, and finally, at some distance below the fall, the moraine is completely restored, and begins to exercise its protecting influence; it rises upon its ridge of ice, and dominates as before over the surface of the glacier.

[Sidenote: TRANSPARENCY OF ICE UNDER THE MORAINES.]

The ice under the moraines and sand-cones is of a different appearance from that of the surrounding glacier, and the principles we have laid down enable us to explain the difference. The sun's rays, striking upon the unprotected surface of the glacier, enter the ice to a considerable depth; and the consequence is, that the ice near the surface of the glacier is always disintegrated, being cut up with minute fissures and cavities, filled with water and air, which, for reasons already assigned, cause the glacier, when it is clean, to appear white and opaque. The ice under the moraines, on the contrary, is usually dark and transparent; I have sometimes seen it as black as pitch, the blackness being a proof of its great transparency, which prevents the reflection of light from its interior.

The ice under the moraines cannot be assailed in its depths by the solar heat, because this heat becomes _obscure_ before it reaches the ice, and as such it lacks the power of penetrating the substance. It is also communicated in great part by way of contact instead of by radiation. A thin film at the surface of the moraine-ice engages all the heat that acts upon it, its deeper portions remaining intact and transparent.

GLACIER MOTION.

PRELIMINARY.

(9.)

[Sidenote: NEVE AND GLACIER.]

Though a glacier is really composed of two portions, one above and the other below the snow-line, the term glacier is usually restricted to the latter, while the French term _neve_ is applied to the former. It is manifest that the snow which falls upon the glacier proper can contribute nothing to its growth or permanence; for every summer is not only competent to abolish the accumulations of the foregoing winter, but to do a great deal more. During each summer indeed a considerable quantity of the ice below the snow-line is reduced to water; so that, if the waste were not in some way supplied, it is manifest that in a few years the lower portion of the glacier must entirely disappear. The end of the Mer de Glace, for example, could never year after year thrust itself into the valley of Chamouni, were there not some agency by which its manifest waste is made good. This agency is the motion of the glacier.

To those unacquainted with the fact of their motion, but who have stood upon these vast accumulations of ice, and noticed their apparent fixity and rigidity, the assertion that a glacier moves must appear in the highest degree startling and incredible. They would naturally share the doubts of a certain professor of Tuebingen, who, after a visit to the glaciers of Switzerland, went home and wrote a book flatly denying the possibility of their motion. But reflection comes to the aid of sense, and qualifies first impressions. We ask ourselves how is the permanence of the glacier secured? How are the moraines to be accounted for? Whence come the blocks which we often find at the terminus of a glacier, and which we know belong to distant mountains? The necessity of motion to produce these results becomes more and more apparent, until at length we resort to actual experiment. We take two fixed points at opposite sides of the glacier, so that a block of stone which rests upon the ice may be in the straight line which unites the points; and we soon find that the block quits the line, and is borne downwards by the glacier. We may well realize the interest of the man who first engaged in this experiment, and the pleasure which he felt on finding that the block moved; for even now, after hundreds of observations on the motion of glaciers have been made, the actual observance of this motion for the first time is always accompanied by a thrill of delight. Such pleasure the direct perception of natural truth always imparts. Like Antaeus we touch our mother, and are refreshed by the contact.

[Sidenote: HUGI'S MEASUREMENTS.]

The fact of glacier-motion has been known for an indefinite time to the inhabitants of the mountains; but the first who made quantitative observations of the motion was Hugi. He found that from 1827 to 1830 his cabin upon the glacier of the Aar had moved 100 metres, or about 110 yards, downwards; in 1836 it had moved 714 metres; and in 1841 M. Agassiz found it at a distance of 1,428 metres from its first position. This is equivalent in round numbers to an average velocity of 100 metres a year. In 1840 M. Agassiz fixed the position of the rock known as the Hotel des Neufchatelois; and on the 5th of September, 1841, he found that it had moved 213 feet downward. Between this date and September, 1842, the rock moved 273 feet, thus accomplishing a distance of 486 feet in two years.

But much uncertainty prevailed regarding the motion of the boulders, for they sometimes rolled upon the glacier, and hence it was resolved to use stakes of wood driven into the ice. In the month of July, 1841, M. Escher de la Linth fixed a system of stakes, every two of which were separated from each other by a distance of 100 metres, across the great Aletsch glacier. A considerable number of other stakes were fixed _along_ the glacier, the longitudinal separation being also 100 metres. On the 8th of July the stakes stood at a depth of about three feet in the ice. On the 16th of August he returned to the glacier. Almost all the stakes had fallen, and no trace, even of the holes in which they had been sunk, remained. M. Agassiz was equally unsuccessful on the glacier of the Aar. It must therefore be borne in mind, that, previous to the introduction of the facile modes of measurement which we now employ, severe labour and frequent disappointment had taught observers the true conditions of success.

After his defeat upon the Aletsch, M. Escher joined MM. Agassiz and Desor on the Aar glacier, where, between the 31st of August and the 5th of September, they fixed in concert the positions of a series of blocks upon the ice, with the view of measuring their displacements the following year.

[Sidenote: AGASSIZ'S MEASUREMENTS.]

Another observation of great importance was also commenced in 1841. Warned by previous failures, M. Agassiz had iron boring-rods carried up the glacier, with which he pierced the ice at six places to a depth of ten feet, and at each place drove a wooden pile into the ice. These six stations were in the same straight line across the glacier; three of them standing upon the Finsteraar and three on the Lauteraar tributary. About this time also M. Agassiz conceived the idea of having the displacements measured the year following with precise instruments, and also of having constructed, by a professional engineer, a map of the entire glacier, on which all its visible "accidents" should be drawn according to scale. This excellent work was afterwards executed by M. Wild, now Professor of Geodesy and Topography in the Polytechnic School of Zuerich, and it is published as a separate atlas in connexion with M. Agassiz's 'Systeme Glaciaire.'

[Sidenote: PROF. J. D. FORBES INVITED.]

M. Agassiz is a naturalist, and he appears to have devoted but little attention to the study of physics. At all events, the physical portions of his writings appear to me to be very often defective. It was probably his own consciousness of this deficiency that led him to invoke the advice of Arago and others previous to setting out upon his excursions. It was also his desire "to see a philosopher so justly celebrated occupy himself with the subject," which induced him to invite Prof. J. D. Forbes of Edinburgh to be his guest upon the Aar glacier in 1841. On the 8th of August they met at the Grimsel Hospice, and for three weeks afterwards they were engaged together daily upon the ice, sharing at night the shelter of the same rude roof. It is in reference to this visit that Prof. Forbes writes thus at page 38 of the 'Travels in the Alps':--"Far from being ready to admit, as my sanguine companions wished me to do in 1841, that the theory of glaciers was complete, and the cause of their motion certain, after patiently hearing all they had to say and reserving my opinion, I drew the conclusion that no theory which I had then heard of could account for the few facts admitted on all hands." In 1842 Prof. Forbes repaired, as early as the state of the snow permitted, to the Mer de Glace; he worked there, in the first instance, for a week, and afterwards crossed over to Courmayeur to witness a solar eclipse. The result of his week's observations was immediately communicated to Prof. Jameson, then editor of the 'Edinburgh New Philosophical Journal.'

[Sidenote: CENTRE MOVES QUICKEST.]

In that letter he announces the fact, but gives no details of the measurement, that "the central part of the glacier moves faster than the edges in a very considerable proportion; quite contrary to the opinion generally entertained." He also announced at the same time the continuous hourly advance of the glacier. This letter bears the date, "Courmayeur, Piedmont, 4th July," but it was not published until the month of October following.

Meanwhile M. Agassiz, in company with M. Wild, returned to complete his experiment upon the glacier of the Aar. On the 20th of July, 1842, the displacements of the six piles which he had planted the year before were determined by means of a theodolite. Of the three upon the Finsteraar affluent, that nearest the side had moved 160 feet, the next 225 feet, while that nearest to the centre had moved 269 feet. Of those on the Lauteraar, that nearest the side had moved 125 feet, the next 210 feet, and that nearest the centre 246 feet. These observations were perfectly conclusive as to the quicker motion of the centre: they embrace a year's motion; and the magnitude of the displacements, causing errors of inches, which might seriously affect small displacements, to vanish, justifies us in ranking this experiment with the most satisfactory of the kind that have ever been made. The results were communicated to Arago in a letter dated from the glacier of the Aar, on the 1st of August, 1842; they were laid before the Academy of Sciences on the 29th of August, 1842, and are published in the 'Comptes Rendus' of the same date.

The facts, then, so far as I have been able to collect them, are as follows:--M. Agassiz commenced his experiment about ten months before Professor Forbes, and the results of his measurements, with quantities stated, were communicated to the French Academy about two months prior to the publication of the letter of Professor Forbes in the 'Edinburgh Philosophical Journal.' But the latter communication, announcing in general terms the fact of the speedier central motion, was dated from Courmayeur twenty-seven days before the date of M. Agassiz's letter from the glacier of the Aar.

[Sidenote: STATE OF THE QUESTION.]

The speedier motion of the central portion of a glacier has been justly regarded as one of cardinal importance, and no other observation has been the subject of such frequent reference; but the general impression in England is that M. Agassiz had neither part nor lot in the establishment of the above fact; and in no English work with which I am acquainted can I find any reference to the above measurements. Relying indeed upon such sources for my information, I remained ignorant of the existence of the paper in the 'Comptes Rendus' until my attention was directed to it by Professor Wheatstone. In the next following chapters I shall have to state the results of some of my own measurements, and shall afterwards devote a little time to the consideration of the cause of glacier-motion. In treating a question on which so much has been written, it is of course impossible, as it would be undesirable, to avoid subjecting both my own views and those of others to a critical examination. But in so doing I hope that no expression shall escape me inconsistent with the courtesy which ought to be habitual among philosophers or with the frank recognition of the just claims of my predecessors.

MOTION OF THE MER DE GLACE.

(10.)

[Sidenote: MY FIRST OBSERVATION.]

On Tuesday, the 14th of July, 1857, I made my first observation on the motion of the Mer de Glace. Accompanied by Mr. Hirst I selected on the steep slope of the Glacier des Bois a straight pinnacle of ice, the front edge of which was perfectly vertical. In coincidence with this edge I fixed the vertical fibre of the theodolite, and permitted the instrument to stand for three hours. On looking through it at the end of this interval, the cross hairs were found projected against the white side of the pyramid; the whole mass having moved several inches downwards.

The instrument here mentioned, which had long been in use among engineers and surveyors, was first applied to measure glacier-motion in 1842; by Prof. Forbes on the Mer de Glace, and by M. Agassiz on the glacier of the Aar. The portion of the theodolite made use of is easily understood. The instrument is furnished with a telescope capable of turning up and down upon a pivot, without the slightest deviation right or left; and also capable of turning right or left without the slightest deviation up or down. Within the telescope two pieces of spider's thread, so fine as to be scarcely visible to the naked eye, are drawn across the tube and across each other. When we look through the telescope we see these fibres, their point of intersection being exactly in the centre of the tube; and the instrument is furnished with screws by means of which this point can be fixed upon any desired object with the utmost precision.

[Sidenote: MODE OF MEASUREMENT.]

In setting a straight row of stakes across the glacier, our mode of proceeding was in all cases this:--The theodolite was placed on the mountain-side flanking the glacier, quite clear of the ice; and having determined the direction of a line perpendicular to the axis of the glacier, a well-defined object was sought at the opposite side of the valley as close as possible to this direction; the object being, in some cases, the sharp edge of a cliff; in others, a projecting corner of rock; and, in others, a well-defined mark on the face of the rock. This object and those around it were carefully sketched, so that on returning to the place it could be instantly recognized. On commencing a line the point of intersection of the two spiders' threads within the telescope was first fixed accurately upon the point thus chosen, and an assistant carrying a straight baton was sent upon the ice. By rough signalling he first stood near the place where the first stake was to be driven in; and the object end of the telescope was then lowered until he came within the field of view. He held his staff upright upon the ice, and, in obedience to signals, moved upwards or downwards until the point of intersection of the spiders-threads exactly hit the bottom of the baton; a concerted signal was then made, the ice was pierced with an auger to a depth of about sixteen inches, and a stake about two feet long was firmly driven into it. The assistant then advanced for some distance across the glacier; the end of the telescope was now gently raised until he and his upright staff again appeared in the field of view. He then moved as before until the bottom of his staff was struck by the point of intersection, and here a second stake was fixed in the ice. In this way the process was continued until the line of stakes was completed.

Before quitting the station, a plummet was suspended from a hook directly underneath the centre of the theodolite, and the place where the point touched the ground was distinctly marked. To measure the motion of the line of stakes, we returned to the place a day or two afterwards, and by means of the plummet were able to make the theodolite occupy the exact position which it occupied when the line was set out. The telescope being directed upon the point at the opposite side of the valley, and gradually lowered, it was found that no single stake along the line preserved its first position: they had all shifted downwards. The assistant was sent to the first stake; the point which it had first occupied was again determined, and its present distance from that point accurately measured. The same thing was done in the case of each stake, and thus the displacement of the whole row of stakes was ascertained.[A] The time at which the stake was fixed, and at which its displacement was measured, being carefully noted, a simple calculation determined _the daily motion_ of the stake.

[Sidenote: THE FIRST LINE.]

Thus, on the 17th of July, 1857, we set out our first line across the Mer de Glace, at some distance below the Montanvert; on the day following we measured the progress of the stakes. The observed displacements are set down in the following table:--

First Line.--Daily Motion.

No. of stake. Inches. West 1 moved 12-1/4 2 " 16-3/4 3 " 22-1/2 4 " ... 5 " 24-1/2 6 moved ... 7 " 26-1/4 8 " ... 9 " 28-3/4 10 " 35-1/2 East.

[Sidenote: THE CENTRE-POINT NOT THE QUICKEST.]

The theodolite in this case stood on the Montanvert side of the valley, and the stakes are numbered from this side. We see that the motion gradually augments from the 1st stake onward--the 1st stake being held back by the friction of the ice against the flanking mountain-side. The stakes 4, 6, and 8 have no motion attached to them, as an accident rendered the measurement of their displacements uncertain. But one remarkable fact is exhibited by this line; the 7th stake stood upon the _middle_ of the glacier, and we see that its motion is by no means the quickest; it is exceeded in this respect by the stakes 9 and 10.

The portion of the glacier on which the 10th stake stood was very much cut up by crevasses, and, while the assistant was boring it with his auger, the ice beneath him was observed, through the telescope, to slide suddenly forward for about 4 inches. The other stakes retained their positions, so that the movement was purely local. Deducting the 4 inches thus irregularly obtained, we should have a daily motion of 31-1/2 inches for stake No. 10. The place was watched for some time, but the slipping was not repeated; and a second measurement on the succeeding day made the motion of the 10th stake 32 inches, whilst that of the centre of the glacier was only 27.

Here, then, was a fact which needed explanation; but, before attempting this, I resolved, by repeated measurements in the same locality, to place the existence of the fact beyond doubt. We therefore ascended to a point upon the old and now motionless moraine, a little above the Montanvert Hotel; and choosing, as before, a well-defined object at the opposite side of the valley, we set between it and the theodolite a row of twenty stakes across the glacier. Their motions, measured on a subsequent day, and reduced to their daily rate, gave the results set down in the following table:--

Second Line.--Daily Motion.

No. of stake. Inches. West 1 moved 7-1/2 2 " 10-3/4 3 " 12-1/4 4 " 14-1/2 5 " 16 6 " 16-3/4 7 " 17-1/2 8 " 19 9 " 19-1/2 10 " 21 11 moved 21 12 " 22-1/2 13 " 21 14 " 22-1/2 15 " 20-1/2 16 " 21-3/4 17 " 22-1/4 18 " 25-1/4 19 " ... 20 " 25-3/4 East.

[Sidenote: CORROBORATIVE MEASUREMENTS.]

As regards the retardation of the side, we observe here the same fact as that revealed by our first line--the motion gradually augments from the first stake to the last. The stake No. 20 stood upon the dirty portion of the ice, which was derived from the Talefre tributary of the Mer de Glace, and far beyond the middle of the glacier. These measurements, therefore, corroborate that made lower down, as regards the non-coincidence of the point of swiftest motion with the centre of the glacier.

But it will be observed that the measurements do not show any retardation of the ice at the eastern extremity of the line of stakes--the motion goes on augmenting from the first stake to the last. The reason of this is, that in neither of the cases recorded were we able to get the line quite across the glacier; the crevasses and broken ice-ridges, which intercepted the vision, compelled us to halt before we came sufficiently close to the eastern side to make its retardation sensible. But on the 20th of July my friend Hirst sought out an elevated station on the Chapeau, or eastern side of the valley, whence he could command a view from side to side over all the humps and inequalities of the ice, the fixed point at the opposite side, upon which the telescope was directed, being the corner of a window of the Montanvert Hotel. Along this line were placed twelve stakes, the daily motions of which were found to be as follows:--

Third Line.--Daily Motion.

No. of stake. Inches. East 1 moved 19-1/2 2 " 22-3/4 3 " 28-3/4 4 " 30-1/4 5 " 33-3/4 6 " 28-1/4 7 moved 24-1/2 8 " 25 9 " 25 10 " 18 11 " ... 12 " 8-1/2 West.

The numbering of the stakes along this line commenced from the Chapeau-side of the glacier, and the retardation of that side is now manifest enough; the motion gradually augmenting from 19-1/2 to 33-1/2 inches. But, comparing the velocity of the two extreme stakes, we find that the retardation of stake 12 is much greater than that of stake 1. Stake 5, moreover, which moved with the _maximum_ velocity, was not upon the centre of the glacier, but much nearer to the eastern than to the western side.

[Sidenote: A NEW PECULIARITY OF GLACIER MOTION.]

It was thus placed beyond doubt that the point of maximum motion of the Mer de Glace, at the place referred to, is not the centre of the glacier. But, to make assurance doubly sure, I examined the comparative motion along three other lines, and found in all the same undeviating result.

This result is not only unexpected, but is quite at variance with the opinions hitherto held regarding the motion of the Mer de Glace. The reader knows that the trunk-stream is composed of three great tributaries from the Geant, the Lechaud, and the Talefre. The Glacier du Geant fills more than half of the trunk-valley, and the junction between it and its neighbours is plainly marked by the dirt upon the surface of the latter. In fact four medial moraines are crowded together on the eastern side of the glacier, and before reaching the Montanvert they have strewn their debris quite over the adjacent ice. A distinct limit is thus formed between the clean Glacier du Geant and the other dirty tributaries of the trunk-stream.

Now the eastern side of the Mer de Glace is observed on the whole to be much more fiercely torn than the western side, and this excessive crevassing has been referred to _the swifter motion of the Glacier du Geant_. It has been thought that, like a powerful river, this glacier drags its more sluggish neighbours after it, and thus tears them in the manner observed. But the measurement of the foregoing three lines shows that this cannot be the true cause of the crevassing. In each case the stakes which moved quickest _lay upon the dirty portion of the trunk-stream_, far to the east of the line of junction of the Glacier du Geant, which in fact moved slowest of all.

[Sidenote: LAW OF MOTION SOUGHT.]

The general view of the glacier, and of the shape of the valley which it filled, suggested to me that the analogy with a river might perhaps make itself good beyond the limits hitherto contemplated. The valley was not straight, but sinuous. At the Montanvert the convex side of the glacier was turned eastward; at some distance higher up, near the passages called _Les Ponts_, it was turned westward; and higher up again it was turned once more, for a long stretch, eastward. Thus between Trelaporte and the Ponts we had what is called a point of contrary flexure, and between the Ponts and the Montanvert a second point of the same kind.

[Sidenote: CONJECTURE REGARDING CHANGE OF FLEXURE.]

Supposing a river, instead of the glacier, to sweep through this valley; _its_ point of maximum motion would not always remain central, but would deviate towards that side of the valley to which the river turned its convex boundary. Indeed the positions of towns along the banks of a navigable river are mainly determined by this circumstance. They are, in most cases, situate on the convex sides of the bends, where the rush of the water prevents silting up. Can it be then that the ice exhibits a similar deportment? that the same principle which regulates the distribution of people along the banks of the Thames is also acting with silent energy amid the glaciers of the Alps? If this be the case, the position of the point of maximum motion ought, of course, to shift with the bending of the glacier. Opposite the Ponts, for example, the point ought to be on the Glacier du Geant, and westward of the centre of the trunk-stream; while, higher up, we ought to have another change to the eastern side, in accordance with the change of flexure.

On the 25th of July a line was set out across the glacier, one of its fixed termini being a mark upon the first of the three Ponts. The motion of this line, measured on a subsequent day, and reduced to its daily rate, was found to be as follows:--

Fourth Line.--Daily Motion.

No. of stake. Inches. East 1 moved 6-1/2 2 " 8 3 " 12-1/2 4 " 15-1/4 5 " 15-1/2 6 " 18-3/4 7 " 18-1/4 8 " 18-3/4 9 " 19-1/2 10 moved 21 11 " 20-1/2 12 " 23-1/4 13 " 23-1/4 14 " 21 15 " 22-1/4 16 " 17-1/4 17 " 15 West.

This line, like the third, was set out and numbered from the eastern side of the glacier, the theodolite occupying a position on the heights of the Echelets. A moment's inspection of the table reveals a fact different from that observed on the third line; _there_ the most easterly stake moved with more than twice the velocity of the most westerly one; _here_, on the contrary, the most westerly stake moves with more than twice the velocity of the most easterly one.

To enable me to compare the motion of the eastern and western halves of the glacier with greater strictness, my able and laborious companion undertook the task of measuring with a surveyor's chain the line just referred to; noting the pickets which had been fixed along the line, and the other remarkable objects which it intersected. The difficulty of thus directing a chain over crevasses and ridges can hardly be appreciated except by those who have tried it. Nevertheless, the task was accomplished, and the width of the Mer de Glace, at this portion of its course, was found to be 863 yards, or almost exactly half a mile.

Referring to the last table, it will be seen that the two stakes numbered 12 and 13 moved with a common velocity of 23-1/4 inches per day, and that their motion is swifter than that of any of the others. The point of swiftest motion may be taken midway between them, and this point was found by measurement to lie 233 yards _west_ of the dirt which marked the junction of the Glacier du Geant with its fellow tributaries: whereas, in the former cases, it lay a considerable distance _east_ of this limit. Its distance from the eastern side of the glacier was 601 yards, and from the western side 262 yards, being 170 yards west of the centre of the glacier.

[Sidenote: CONJECTURE TESTED.]

But the measurements enabled me to take the stakes in pairs, and to compare the velocity of a number of them which stood at certain distances from the eastern side of the valley, with an equal number which stood at the same distances from the western side. By thus arranging the points two by two, I was able to compare the motion of the entire body of the ice at the one side of the central line with that of the ice at the other side. Stake 17 stood about as far from the western side of the glacier as stake 3 did from its eastern side; 16 occupied the same relation to 4; 15, to 5; 13, to 7; and 12, to 9.

Calling each pair of points which thus stand at equal distances from the opposite sides _corresponding points_, the following little table exhibits their comparative motions:--

Numbers and Velocities of Corresponding Points on the Fourth Line.

No. Vel. No. Vel. No. Vel. No. Vel. No. Vel. West 17 15 16 17-1/4 15 22-1/4 13 23-1/4 12 23-1/4 East 3 12-1/2 4 15-1/4 5 15-1/2 7 18-1/4 9 19-1/2

[Sidenote: WESTERN HALF MOVES QUICKEST.]

The table explains itself. We see that while stake 17, which stands _west_ of the centre, moves 15 inches, stake 3, which stands an equal distance _east_ of the centre, moves only 12-1/2 inches. Comparing every pair of the other points, we find the same to hold good; the western stake moves in each case faster than the corresponding eastern one. Hence, _the entire western half of the Mer de Glace, at the place crossed by our fourth line, moves more quickly than the eastern half of the glacier_.

We next proceeded farther up, and tested the contrary curvature of the glacier, opposite to Trelaporte. The station chosen for this purpose was on a grassy platform of the promontory, whence, on the 28th of July, a row of stakes was fixed at right angles to the axis of the glacier. Their motions, measured on the 31st, gave the following results:--

Fifth Line.[B]--Daily Motion.

No. of stake. Inches. West 1 moved 11-1/4 2 " 13-1/2 3 " 12-3/4 4 " 15 5 " 15-1/4 6 " 16 7 " 17-1/4 8 " 19-1/4 9 moved 19-3/4 10 " 19 11 " 19-1/2 12 " 17-1/2 13 " 16 14 " 14-3/4 15 " 10 East.

This line was set out and numbered from the Trelaporte side of the valley, and was also measured by Mr. Hirst, over boulders, ice-ridges, chasms, and moraines. The entire width of the glacier here was found to be 893 yards, or somewhat wider than it is at the Ponts. It will also be observed that its motion is somewhat slower.

An inspection of the notes of this line showed me that stakes 3 and 14, 4 and 12, 7 and 10, were "corresponding points;" the first of each pair standing as far from the western side, as the second stood from the eastern. In the following table these points and their velocities are arranged exactly as in the case of the fourth line.

Numbers and Velocities of the Corresponding Points on the Fifth Line.

No. Vel. No. Vel. No. Vel. West 3 12-3/4 4 15 7 17-1/4 East 14 14-3/4 12 17-1/2 10 19

[Sidenote: EASTERN HALF MOVES QUICKEST.]

In each case we find that the stake on the eastern side moves more quickly than the corresponding one upon the western side: so that where the fifth line crosses the glacier _the eastern half of the Mer de Glace moves more quickly than the western half_. This is the reverse of the result obtained at our fourth line, but it agrees with that obtained on our first three lines, where the curvature of the valley is similar. The analogy between a river and a glacier moving through a sinuous valley is therefore complete.

Supposing the points of maximum motion to be determined for a great number of lines across the glacier, the line uniting all these points is what mathematicians would call the _locus_ of the point of maximum motion. At Trelaporte this line would lie east of the centre; at the Ponts it would lie west of the centre; hence, in passing from Trelaporte to the Ponts, it must cross the axis of the glacier. Again, at the Montanvert, it would lie east of the centre, and between the Ponts and the Montanvert the axis of the glacier would be crossed a second time. Supposing the dotted line in Fig. 21 to represent the middle line of the glacier, then the defined line would represent the locus of the point of maximum motion. _It is a curve more deeply sinuous than the valley itself, and it crosses the axis of the glacier at each point of contrary flexure._

[Sidenote: LOCUS OF POINT OF SWIFTEST MOTION.]

To complete our knowledge of the motion of the Mer de Glace, we afterwards determined the velocity of its two accessible tributaries--the Glacier du Geant, and the Glacier de Lechaud. On the 29th of July, a line of stakes was set out across the former, a little above the Tacul, and their motion was subsequently found to be as follows:

Sixth Line.--Daily Motion.

No. of stake. Inches. 1 moved 11 2 " 10 3 " 12 4 " 13 5 " 12 6 moved 12-3/4 7 " 10-1/2 8 " 10 9 " 9 10 " 5

The width of the glacier at this place we found to be 1134 yards, and its maximum velocity, as shown by the foregoing table, 13 inches a day.

On the 1st of August a line was set out across the Glacier de Lechaud, above its junction with the Talefre: it commenced beneath the block of stone known as the Pierre de Beranger. The displacements of the stakes, measured on the 3rd of August, gave the following results:--

Seventh Line.--Daily Motion.

No. of stake. Inches. 1 moved 4-1/2 2 " 8-1/4 3 " 9-1/2 4 " 9 5 " 8-1/2 6 moved 7-1/2 7 " 6-1/4 8 " 8-1/2 9 " 7 10 " 5-1/2

The width of the Glacier de Lechaud at this place was found to be 825 yards; its maximum motion, as shown by the table, being 9-1/2 inches a day. This is the slowest rate which we observed upon either the Mer de Glace or its tributaries. The width of the Talefre-branch, as it descends the cascade, or, in other words, before it is influenced by the pressure of the Lechaud, was found approximately to be 638 yards.

[Sidenote: SQUEEZING AT TRELAPORTE.]

The widths of the tributaries were determined for the purpose of ascertaining the amount of lateral compression endured by the ice in its passage through the neck of the valley at Trelaporte. Adding all together we have--

Geant 1134 yards. Lechaud 825 " Talefre 638 " Total 2597 yards.

These three branches, as shown by the actual measurement of our 5th line, are forced at Trelaporte through a channel 893 yards wide; the width of the trunk stream is a little better than one-third of that of its tributaries, and it passes through this gorge at a velocity of nearly 20 inches a day.

[Sidenote: THE LECHAUD A DRIBLET.]

Limiting our view to one of the tributaries only, the result is still more impressive. Previous to its junction with the Talefre, the Glacier de Lechaud stretches before the observer as a broad river of ice, measuring 825 yards across: at Trelaporte it is squeezed, in a frozen vice, between the Talefre on one side and the Geant on the other, to a driblet, measuring 85 yards in width, or about one-tenth of its former transverse dimension. It will of course be understood that it is the _form_ and not the _volume_ of the glacier that is affected to this enormous extent by the pressure.

Supposing no waste took place, the Glacier de Lechaud would force precisely the same amount of ice through the "narrows" at Trelaporte, in one day, as it sends past the Pierre de Beranger. At the latter place its velocity is about half of what it is at the former, but its width is more than nine times as great. Hence, if no waste took place, its _depth_, at Trelaporte, would be at _least_ 4-1/2 times its depth opposite the Pierre de Beranger. Superficial and subglacial melting greatly modify this result. Still I think it extremely probable that observations directed to this end would prove the comparative shallowness of the upper portions of the Glacier de Lechaud.

FOOTNOTES:

[A] Great care is necessary on the part of the man who measures the displacements. The staff ought to be placed along the original line, and the assistant ought to walk along it until the foot of a _perpendicular_ from the stake is attained. When several days' motion is to be measured, this precaution is absolutely necessary; the eye being liable to be grossly deceived in _guessing_ the direction of a perpendicular.

[B] The details of the measurement of the fourth and fifth lines are published in the 'Philosophical Transactions,' vol. cxlix., p. 261.

ICE-WALL AT THE TACUL.

VELOCITIES OF TOP AND BOTTOM.

(11.)

As regards the motion of the _surface_ of a glacier, two laws are to be borne in mind: 1st, that regarding the quicker movement of the centre; 2nd, that regarding the locus of the point of maximum motion. Our next care must be to compare the motion of the surface of a glacier with the motion of those parts which lie near its bed. Rendu first surmised that the bottom of the glacier was retarded by friction, and both Professor Forbes[A] and M. Martins[B] have confirmed the conjecture. Theirs are the only observations which we possess upon the subject; and I was particularly desirous to instruct myself upon this important head by measurements of my own.

[Sidenote: FIRST ATTEMPT AT MEASUREMENT.]

During the summer of 1857 the eastern side of the Glacier du Geant, near the Tacul, exposed a nearly vertical precipice of ice, measuring 140 feet from top to bottom. I requested Mr. Hirst to fix two stakes in the same vertical plane, one at the top of the precipice and one near the bottom. This he did upon the 3rd of August, and on the 5th I accompanied him to measure the progress of the stakes. On the summit of the precipice, and running along it, was the lateral moraine of the glacier. The day was warm and the ice liquefying rapidly, so that the boulders and debris, deprived incessantly of their support, came in frequent leaps and rushes down the precipice. Into this peril my guide was about to enter, to measure the displacement of the lower stake, while I was to watch, and call out the direction in which he was to run when a stone gave way. But I soon found that the initial motion was no sure index of the final motion. By striking the precipice, the stones were often deflected, and carried wide of their original direction. I therefore stopped the man, and sent him to the summit of the precipice to remove all the more dangerous blocks. This accomplished, he descended, and while I stood beside him, executed the required measurement. From the 3rd to the 5th of August the upper stake had moved twelve inches, and the lower one six.

Unfortunately some uncertainty attached itself to this result, due to the difficulty of fixing the lower stake. The guide's attention had been divided between his work and his safety, and he had to retreat more than a dozen times from the falling boulders and debris. I, on the other hand, was unwilling to accept an observation of such importance with a shade of doubt attached to it. Hence arose the desire to measure the motion myself. On the 11th of August I therefore reascended to the Tacul, and fixed a stake at the top of the precipice, and another at the bottom. While sitting on the old moraine looking at the two pickets, the importance of determining the motion of a point midway between the top and bottom forcibly occurred to me, but, on mentioning it to my guide, he promptly pronounced any attempt of the kind absurd.

[Sidenote: STAKES FIXED AT TOP, BOTTOM, AND CENTRE.]

On scanning the place carefully, however, the value of the observation appeared to me to outweigh the amount of danger. I therefore took my axe, placed a stake and an auger against my breast, buttoned my coat upon them, and cut an oblique staircase up the wall of ice, until I reached a height of forty feet from the bottom. Here the position of the stake being determined by Mr. Hirst, who was at the theodolite, I pierced the ice with the auger, drove in the stake, and descended without injury. During the whole operation however my guide growled audibly.

On the following morning we commenced the ascent of Mont Blanc, a narrative of which is given in Part I. We calculated on an absence of three days, and estimated that the stakes which had just been fixed would be ready for measurement on our return; but we did not reach Chamouni until the afternoon of Friday, the 14th. Heavy clouds settled, during our descent, upon the summits behind us, and a thunder-peal from the Aiguilles soon heralded a fall of rain, which continued without intermission till the afternoon of the 16th, when the atmosphere cleared, and showed the mountains clothed to their girdles with snow. The Montanvert was thickly covered, and on our way to it we met the servants in charge of the cattle, which had been driven below the snow-line to obtain food.

[Sidenote: THROUGH GLOOM TO THE TACUL.]

On Monday morning, the 17th, a dense fog filled the valley of the Mer de Glace. I watched it anxiously. The stakes which we had set at the Tacul had been often in my thoughts, and I wished to make some effort to save the labour and peril incurred in setting them from being lost. I therefore set out, in one of the clear intervals, accompanied by my friend and Simond, determined to measure the motion of the stakes, if possible, or to fix them more firmly, if they still stood. As we passed, however, from l'Angle to the glacier, the fog became so dense and blinding that we halted. At my request Mr. Hirst returned to the Montanvert; and Simond, leaving the theodolite in the shelter of a rock, accompanied me through the obscurity to the Tacul. We found the topmost stake still stuck by its point in the ice; but the two others had disappeared, and we afterwards discovered their fragments in a snow-buttress, which reared itself against the base of the precipice. They had been hit by the falling stones, and crushed to pieces. Having thus learned the worst, we descended to the Montanvert amid drenching rain.

[Sidenote: DESCENT OF BOULDERS.]

On the morning of the 18th there was no cloud to be seen anywhere, and the sunlight glistened brightly on the surface of the ice. We ascended to the Tacul. The spontaneous falling of the stones appeared more frequent this morning than I had ever seen it. The sun shone with unmitigated power upon the ice, producing copious liquefaction. The rustle of falling debris was incessant, and at frequent intervals the boulders leaped down the precipice, and rattled with startling energy amid the rocks at its base. I sent Simond to the top to remove the looser stones; he soon appeared, and urged the moraine-shingle in showers down the precipice, upon a bevelled slope of which some blocks long continued to rest. They were out of the reach of the guide's baton, and he sought to dislodge them by sending other stones down upon them. Some of them soon gave way, drawing a train of smaller shingle after them; others required to be hit many times before they yielded, and others refused to be dislodged at all. I then cut my way up the precipice in the manner already described, fixed the stake, and descended as speedily as possible. We afterwards fixed the bottom stake, and on the 20th the displacements of all three were measured.[C] The spaces passed over by the respective stakes in 24 hours were found to be as follows:--

Inches. Top stake 6.00 Middle stake 4.50 Bottom stake 2.56

[Sidenote: MOTION OF STAKES.]

The height of the precipice was 140.8 feet, but it sloped off at its upper portion. The height of the middle stake above the ground was 35 feet, and of the bottom one 4 feet. It is therefore proved by these measurements that the bottom of the ice-wall at the Tacul moves with less than half the velocity of the top; while the displacement of the intermediate stake shows how the velocity gradually increases from the bottom upwards.

FOOTNOTES:

[A] 'Edinb. Phil. Journ.,' Oct. 1846, p. 417.

[B] Agassiz, 'Systeme Glaciaire,' p. 522.

[C] On this latter occasion my guide volunteered to cut the steps for me up to the pickets; and I permitted him to do so. In fact, he was at least as anxious as myself to see the measurement carried out.

WINTER MOTION OF THE MER DE GLACE.

(12.)

The winter measurements were executed in the manner already described, on the 28th and 29th of December, 1859. The theodolite was placed on the mountain's side flanking the glacier, and a well-defined object was chosen at the opposite side of the valley, so that a straight line between this object and the theodolite was approximately perpendicular to the axis of the glacier. Fixing the telescope in the first instance with its cross hairs upon the object, its end was lowered until it struck the point upon the glacier at which a stake was to be fixed. Thanks to the intelligence of my assistants, after the fixing of the first stake they speedily took up the line at all other points, requiring very little correction to make their positions perfectly accurate. On the day following that on which the stakes were driven in, the theodolite was placed in the same position, and the distances to which the stakes had moved from their original positions were accurately determined. As already stated, the first line crossed the glacier about 80 yards above the Montanvert Hotel.

[Sidenote: HALF OF SUMMER MOTION.]

Line No. I.--Winter Motion in Twenty-four Hours.

No. of stake. Inches. West 1 7-1/4 2 11 3 13-1/2 4 13 5 13-3/4 6 14-1/4 7 15-3/4 8 15-3/4 9 12-1/4 10 12 11 6-1/2 East.

[Sidenote: THE SAME LAW IN SUMMER AND WINTER.]

The maximum here is fifteen and three-quarters inches; the maximum summer motion of the same portion of the glacier is about thirty inches. These measurements also show that in winter, as well as in summer, the side of the glacier opposite to the Montanvert moves quicker than that adjacent to it. The stake which moved with the maximum velocity was beyond the moraine of La Noire. The second line crossed the glacier about 130 yards below the Montanvert.

Line No. II.--Winter Motion in Twenty-four Hours.

No. of stake. Inches. 1 7-3/4 2 9-1/2 3 13-3/4 4 16 5 16 6 15-3/4 7 17-1/2 8 16-1/2 9 14-1/2 10 14

The maximum here is an inch and three-quarters greater than that of line No. 1. The summer maximum at this portion of the glacier also exceeds that of the part intersected by line No. 1. The surface of the glacier between the two lines is in a state of tension which relieves itself by a system of transverse fissures, and thus permits of the quicker advance of the forward portion.

My desire, in making these measurements, was, in the first place, to raise the winter observations of the motion to the same degree of accuracy as that already possessed by the summer ones. Auguste Balmat had already made a series of winter observations on the Mer de Glace; but they were made in the way employed before the introduction of the theodolite by Agassiz and Forbes, and shared the unavoidable roughness of such a mode of measurement. They moreover gave us no information as to the motion of the different parts of the glacier along the same transverse line, and this, for reasons which will appear subsequently, was the point of chief interest to me.

CAUSE OF GLACIER-MOTION.

DE SAUSSURE'S THEORY.

(13.)

Perhaps the first attempt at forming a glacier-theory is that of Scheuchzer in 1705. He supposed the motion to be caused by the conversion of water into ice within the glacier; the known and almost irresistible expansion which takes place on freezing, furnishing the force which pushed the glacier downward. This idea was illustrated and developed with so much skill by M. de Charpentier, that his name has been associated with it; and it is commonly known as the Theory of Charpentier, or the Dilatation-Theory. M. Agassiz supported this theory for a time, but his own thermometric experiments show us that the body of the glacier is at a temperature of 32 deg. Fahr.; that consequently there is no interior magazine of cold to freeze the water with which the glacier is supposed to be incessantly saturated. So that these experiments alone, if no other grounds existed, would prove the insufficiency of the theory of dilatation. I may however add, that the arguments most frequently urged against this theory deal with an assumption, which I do not think its author ever intended to make.

[Sidenote: THE GLACIER SLIDES.]

Another early surmise was that of Altmann and Gruener (1760), both of whom conjectured that the glacier slid along its bed. This theory received distinct expression from De Saussure in 1799; and has since been associated with the name of that great alpine traveller, being usually called the 'Theory of Saussure,' and sometimes the 'Sliding Theory.' It is briefly stated in these words:--

"Almost every glacier reposes upon an inclined bed, and those of any considerable size have beneath them, even in winter, currents of water which flow between the ice and the bed which supports it. It may therefore be understood that these frozen masses, drawn down the slope on which they repose, disengaged by the water from all adhesion to the bottom, sometimes even raised by this water, must glide by little and little, and descend, following the inclinations of the valleys, or of the slopes which they cover. It is this slow but continual sliding of the ice on its inclined base which carries it into the lower valleys."[A]

[Sidenote: STRAINED INTERPRETATION.]

De Saussure devoted but little time to the subject of glacier-motion; and the absence of completeness in the statement of his views, arising no doubt from this cause, has given subsequent writers occasion to affix what I cannot help thinking a strained interpretation to the sliding theory. It is alleged that he regarded a glacier as a perfectly rigid body; that he considered it to be "a mass of ice of small depth, and considerable but uniform breadth, sliding down a uniform valley, or pouring from a narrow valley into a wider one."[B] The introduction "of the smallest flexibility or plasticity" is moreover emphatically denied to him.[C]

It is by no means probable that the great author of the 'Voyages' would have subscribed to this "rigid" annotation. His theory, be it remembered, is to some extent _true_: the glacier moves over its bed in the manner supposed, and the rocks of Britain bear to this day the traces of these mighty sliders. De Saussure probably contented himself with a general statement of what he believed to be the substantial cause of the motion. He visited the Jardin, and saw the tributaries of the Mer de Glace turning round corners, welding themselves together, and afterwards moving through a sinuous trunk-valley; and it is scarcely credible that in the presence of such facts he would have denied all flexibility to the glacier.

The statement that he regarded a glacier to be a mass of ice of uniform width, is moreover plainly inconsistent with the following description of the glacier of Mont Dolent: "Its most elevated plateau is a great circus, surrounded by high cliffs of granite, of pyramidal forms; thence the glacier descends through a gorge, in which _it is narrowed_; but after having passed the gorge, it _enlarges again_, spreading out like a fan. Thus it has on the whole the form of a sheaf tied in the middle and dilated at its two extremities."[D]

[Sidenote: GLACIER OF MONT DOLENT.]

Curiously enough this very glacier, and these very words, are selected by M. Rendu as illustrative of the plasticity of glaciers. "Nothing," he says, "shows better the extent to which a glacier moulds itself to its locality than the form of the glacier of Mont Dolent in the Valley of Ferret;" and he adds, in connexion with the same passage, these remarkable words:--"There is a multitude of facts which would seem to necessitate the belief that the substance of glaciers enjoys a kind of ductility which permits it to mould itself to the locality which it occupies, to grow thin, to swell, and to narrow itself like a soft paste."[E]

FOOTNOTES:

[A] 'Voyages,' Sec. 535.

[B] James D. Forbes, 'Occasional Papers on the Theory of Glaciers,' 1859, p. 100.

[C] "I adhere to the definition as excluding the introduction of the smallest flexibility or plasticity." 'Occ. Pap.,' p. 96.

[D] 'Voyages,' tome ii. p. 290.

[E] In connexion with this brief sketch of the 'Sliding Theory,' it ought to be stated, that Mr. Hopkins has proved experimentally, that ice may descend an incline at a sensibly uniform rate, and that the velocity is augmented by increasing the weight. In this remarkable experiment the motion was due to the slow disintegration of the lower surface of the ice. See 'Phil. Mag.,' 1845, vol. 26.

RENDU'S THEORY.

(14.)

[Sidenote: RENDU'S CHARACTER.]

M. Rendu, Bishop of Annecy, to whose writings I have just referred, died last autumn.[A] He was a man of great repute in his diocese, and we owe to him one of the most remarkable essays upon glaciers that have ever appeared. His knowledge was extensive, his reasoning close and accurate, and his faculty of observation extraordinary. With these were associated that intuitive power, that presentiment concerning things as yet untouched by experiment, which belong only to the higher class of minds. Throughout his essay a constant effort after quantitative accuracy reveals itself. He collects observations, makes experiments, and tries to obtain numerical results; always taking care, however, so to state his premises and qualify his conclusions that nobody shall be led to ascribe to his numbers a greater accuracy than they merit. It is impossible to read his work, and not feel that he was a man of essentially truthful mind, and that science missed an ornament when he was appropriated by the Church.

The essay above referred to is printed in the tenth volume of the Memoirs of the Royal Academy of Sciences of Savoy, published in 1841, and is entitled, '_Theorie des Glaciers de la Savoie, par M. le Chanoine Rendu, Chevalier du Merite Civil et Secretaire perpetuel_.' The paper had been written for nearly two years, and might have remained unprinted, had not another publication on the same subject called it forth.

I will place a few of the leading points of this remarkable production before the reader; commencing with a generalization which is highly suggestive of the character of the author's mind.

[Sidenote: "THEORIE DES GLACIERS DE LA SAVOIE."]

He reflects on the accumulation of the mountain-snows, each year adding fifty-eight inches of ice to a glacier. This would make Mont Blanc four hundred feet higher in a century, and four thousand feet higher in a thousand years. "It is evident," he says, "that nothing like this occurs in nature." The escape of the ice then leads him to make some general remarks on what he calls the "law of circulation." "The conserving will of the Creator has employed for the permanence of His work the great law of _circulation_, which, strictly examined, is found to reproduce itself in all parts of nature. The waters circulate from the ocean to the air, from the air to the earth, and from the earth to the ocean.... The elements of organic substances circulate, passing from the solid to the liquid or aeriform condition, and thence again to the state of solidity or of organisation. That universal agent which we designate by the names of fire, light, electricity, and magnetism, has probably also a _circulation_ as wide as the universe." The italics here are Rendu's own. This was published in 1841, but written, we are informed, nearly two years before. In 1842 Mr. Grove wrote thus:--"Light, heat, magnetism, motion, and chemical affinity, are all convertible material affections." More recently Helmholtz, speaking of the "circuit" formed by "heat, light, electricity, magnetism, and chemical affinity," writes thus:--"Starting from each of these different manifestations of natural forces, we can set every other in action." I quote these passages because they refer to the same agents as those named by M. Rendu, and to which he ascribes "_circulation_." Can it be doubted that this Savoyard priest had a premonition of the Conservation of Force? I do not want to lay more stress than it deserves upon a conjecture of this kind; but its harmony with an essay remarkable for its originality gives it a significance which, if isolated, it might not possess.

[Sidenote: GLACIERS RIGHTLY DIVIDED.]

With regard to the glaciers, Rendu commences by dividing them into two kinds, or rather the selfsame glacier into two parts, one of which he calls the "_glacier reservoir_," the other the "_glacier d'ecoulement_,"--two terms highly suggestive of the physical relationship of the _neve_ and the glacier proper. He feeds the reservoirs from three sources, the principal one of which is the snow, to which he adds the rain, and the vapours which are condensed upon the heights without passing into the state of either rain or snow. The conversion of the snow into ice he supposes to be effected by four different causes, the most efficacious of which is _pressure_.[B] It is needless to remark that this quite agrees with the views now generally entertained.

In page 60 of the volume referred to there is a passage which shows that the "veined structure" of the glacier had not escaped him, though it would seem that he ascribed it to stratification. "When," he writes, "we perceive the profile of a glacier on the walls of a crevasse, we see different layers distinct in colour, but more particularly in density; some seem to have the hardness, as they have the greenish colour, of glass; others preserve the whiteness and porosity of the snow." There is also a very close resemblance between his views of the influence of "time and cohesion" and those of Prof. Forbes. "We may conclude," he writes, "that _time_, favouring the action of _affinity_, and the pressure of the layers one upon the other, causes the little crystals of which snow is composed to approach each other, bring them into contact, and convert them into ice."[C] Regelation also appears to have attracted his notice.[D] "When we fill an ice-house," he writes, "we break the ice into very small fragments; afterwards we wet it with water 8 or 10 degrees above zero (Cent.) in temperature; but, notwithstanding this, the whole is converted into a compact mass of ice." He moreover maintains, in almost the same language as Prof. Forbes,[E] the opinion, that ice has always an inner temperature lower than zero (Cent.). He believed this to be a property "inherent to ice." "Never," he says, "can a calorific ray pass the first surface of ice to raise the temperature of the interior."[F]

[Sidenote: OBSERVATIONS AND HYPOTHESES.]

He notices the direction of the glacier as influencing the wasting of its ridges by the sun's heat; ascribing to it the effect to which I have referred in explaining the wave-like forms upon the surface of the Mer de Glace. His explanation of the Moulins, too, though insufficient, assigns a true cause, and is an excellent specimen of physical reasoning.

With regard to the diminution of the _glaciers reservoirs_, or, in other words, to the manner in which the ice disappears, notwithstanding the continual additions made to it, we have the following remarkable passage:--"In seeking the cause of the diminution of glaciers, it has occurred to my mind that the ice, notwithstanding its hardness and its rigidity, can only support a given pressure without breaking or being squeezed out. According to this supposition, whenever the pressure exceeds that force, there will be rupture of the ice, and a flow in consequence. Let us take, at the summit of Mont Blanc, a column of ice reposing on a horizontal base. The ice which forms the first layer of that column is compressed by the weight of all the layers above it; but if the solidity of the said first layer can only support a weight equal to 100, when the weight exceeds this amount there will be rupture and spreading out of the ice of the base. Now, something very similar occurs in the immense crust of ice which covers the summits of Mont Blanc. This crust appears to augment at the upper surface and to diminish by the sides. To assure oneself that the movement is due to the force of pressure, it would be necessary to make a series of experiments upon the solidity of ice, such as have not yet been attempted."[G] I may remark that such experiments substantially verify M. Rendu's notion.

But it is his observations and reasoning upon the _glaciers d'ecoulement_ that chiefly interest us. The passages in his writings where he insists upon the power of the glaciers to mould themselves to their localities, and compares them to a soft paste, to lava at once ductile and liquid, are well known from the frequent and flattering references of Professor Forbes; but there are others of much greater importance, which have hitherto remained unknown in this country. Regarding the motion of the Mer de Glace, Rendu writes as follows:--

[Sidenote: MEASUREMENT OF MOTION.]

[Sidenote: THE SIDES OF THE GLACIER RETARDED.]

"I sought to appreciate the quantity of its motion; but I could only collect rather vague data. I questioned my guides regarding the position of an enormous rock at the edge of the glacier, but still upon the ice, and consequently partaking of its motion. The guides showed me the place where it stood the preceding year, and where it had stood two, three, four, and five years previously; they showed me the place where it would be found in a year, in two years, &c.; _so certain are they of the regularity of the motion_. Their reports, however, did not always agree precisely with each other, and their indications of time and distance lack the precision without which we proceed obscurely in the physical sciences. In reducing these different indications to a mean, I found the total advance of the glacier to be about 40 feet a year. During my last journey I obtained more certain data, which I have stated in the preceding chapter. _The enormous difference between the two results arises from the fact that the latter observations were made at the centre of the glacier_, WHICH MOVES MORE RAPIDLY, _while the former were made at the side, where the ice_ IS RETAINED BY THE FRICTION AGAINST ITS ROCKY WALLS."[H]

An opinion, founded on a grave misapprehension which Rendu enables us to correct, is now prevalent in this country, not only among the general public, but also among those of the first rank in science. The nature of the mistake will be immediately apparent. At page 128 of the 'Travels in the Alps' its distinguished author gives a sketch of the state of our knowledge of glacier-motion previous to the commencement of his inquiries. He cites Ebel, Hugi, Agassiz, Bakewell, De la Beche, Shirwell, Rendu, and places them in open contradiction to each other. Rendu, he says, gives the motion of the Mer de Glace to be "242 feet per annum; 442 feet per annum; a foot a day; 400 feet per annum, and 40 feet per annum, or _one-tenth_ of the last!" ... and he adds, "I was not therefore wrong in supposing that the actual progress of a glacier was yet a new problem when I commenced my observations on the Mer de Glace in 1842."[I]

In the 'North British Review' for August, 1859, a writer equally celebrated for the brilliancy of his discoveries and the vigour of his pen, collected the data furnished by the above paragraph into a table, which he introduced to his readers in the following words:--"It is to Professor Forbes alone that we owe the first and most correct researches respecting the motion of glaciers; and in proof of this, we have only to give the following list of observations which had been previously made.

Observers. Name of glacier. Annual rate of motion.

Ebel Chamouni 14 feet Ebel Grindelwald 25 " Hugi Aar 240 " Agassiz Aar 200 " Bakewell Mer de Glace 540 " De la Beche Mer de Glace 600 " Shirwell Mer de Glace 300 " M. Rendu Mer de Glace 365 " Saussure's Ladder Mer de Glace 375 "

... Such was the state of our knowledge when Professor Forbes undertook the investigation of the subject."

I am persuaded that the writer of this article will be the first to applaud any attempt to remove an error which, advanced on his great authority, must necessarily be widely disseminated. The numbers in the above table certainly differ widely, and it is perhaps natural to conclude that such discordant results can be of no value; but the fact really is that _every one of them may be perfectly correct_. This fact, though overlooked by Professor Forbes, was clearly seen by Rendu, who pointed out with perfect distinctness the sources from which the discrepancies were derived.

[Sidenote: DISCREPANCIES EXPLAINED.]

"It is easy," he says, "to comprehend that it is impossible to obtain a general measure,--that there ought to be one for each particular glacier. The nature of the slope, the number of changes to which it is subjected, the depth of the ice, the width of the couloir, the form of its sides, and a thousand other circumstances, must produce variations in the velocity of the glacier, and these circumstances cannot be everywhere absolutely the same. Much more, it is not easy to obtain this velocity for a single glacier, and for this reason. In those portions where the inclination is steep, the layer of ice is thin, and its velocity is great; in those where the slope is almost nothing, the glacier swells and accumulates; the mass in motion being double, triple, &c., the motion is only the half, the third, &c.

[Sidenote: LIQUID MOTION ASCRIBED TO GLACIER.]

"But this is not all," adds M. Rendu: "_Between the Mer de Glace and a river, there is a resemblance so complete that it is impossible to find in the latter a circumstance which does not exist in the former._ In currents of water the motion is not uniform, neither throughout their width nor throughout their depth; _the friction of the bottom, that of the sides_, the action of obstacles, cause the motion to vary, _and only towards the middle of the surface is this entire...._"[J]

In 1845 Professor Forbes appears to have come to the same conclusion as M. Rendu; for after it had been proved that the centre of the Aar glacier moved quicker than the side in the ratio of fourteen to one, he accepted the result in these words:--"The movement of the centre of the glacier is to that of a point five metres from the edge as FOURTEEN to ONE: such is the effect of plasticity!"[K] Indeed, if the differences exhibited in the table were a proof of error, the observations of Professor Forbes himself would fare very ill. The measurements of glacier-motion made with his own hands vary from less than 42 feet a year to 848 feet a year, the minimum being less than _one-twentieth_ of the maximum; and if we include the observations made by Balmat, the fidelity of which has been certified by Professor Forbes, the minimum is only _one-thirty-seventh_ of the maximum.

[Sidenote: NORTH BRITISH REVIEW.]

There is another point connected with Rendu's theory which needs clearing up:--"The idea," writes the eminent reviewer, "that a glacier is a semifluid body is no doubt startling, especially to those who have seen the apparently rigid ice of which it is composed. M. Rendu himself shrank from the idea, and did not scruple to say that 'the rigidity of a mass of ice was in direct opposition to it;' and we think that Professor Forbes himself must have stood aghast when his fancy first associated the notion of imperfect fluidity with the solid or even the fissured ice of the glacier, and when he saw in his mind's eye the glaciers of the Alps flowing like a river along their rugged bed. A truth like this was above the comprehension and beyond the sympathy of the age; and it required a moral power of no common intensity to submit it to the ordeal of a shallow philosophy, and the sneers of a presumptuous criticism."

These are strong words; but the fact is that, so far from "shrinking" from the idea, Rendu affirmed, with a clearness and an emphasis which have not been exceeded since, that all the phenomena of a river were reproduced upon the Mer de Glace; its deeps, its shallows, its widenings, its narrowings, its rapids, its places of slow motion, and the quicker flow of its centre than of its sides. He did not shrink from accepting a difference between the central and lateral motion amounting to a ratio of ten to one--a ratio so large that Professor Forbes at one time regarded the acceptance of it as a simple absurdity. In this he was perhaps justified; for his own first observations, which, however valuable, were hasty and incomplete, gave him a maximum ratio of about one and a half to one, while the ratio in some cases was nearly one of _equality_. The observations of Agassiz however show that the ratio, instead of being ten to one, may be _infinity_ to one; for the lateral ice may be so held back by a local obstacle that in the course of a year it shall make no sensible advance at all.

[Sidenote: THE ICE AND THE GLACIER.]

From one thing only did M. Rendu shrink; and it is _the_ thing regarding which we are still disunited. He shrank from stating the physical quality of the ice in virtue of which a glacier moved like a river. He demands experiments upon snow and ice to elucidate this subject. The very observations which Professor Forbes regards as proofs are those of which we require the physical explanation. It is not the viscous flow, if you please to call it such, of the glacier as a whole that here concerns us; but it is the quality of the _ice_ in virtue of which this kind of motion is accomplished. Professor Forbes sees this difference clearly enough: he speaks of "fissured ice" being "flexible" in hand specimens; he compares the glacier to a mixture of ice and sand; and finally, in a more matured paper, falls back for an explanation upon the observations of Agassiz regarding the capillaries of the glacier.[L]

FOOTNOTES:

[A] Expressions such as "last summer," "last autumn," "recently," will be taken throughout in the sense which they had in the early half of 1860, when this book was first published.--L. C. T.

[B] 'Memoir,' p. 77.

[C] P. 75.

[D] P. 71.

[E] 'Philosophical Magazine,' 1859.

[F] 'Memoir,' p. 69.

[G] Page 80.

[H] Page 95.

[I] At page 38 of the 'Travels' the following passage also occurs:--"I believe that I may safely affirm that not one observation of the rate of motion of a glacier, either on the average or at any particular season of the year, existed when I commenced my experiments in 1842."

[J] 'Theorie,' p. 96.

[K] 'Occ. Pap.,' p. 74.

[L] In all that has been written upon glaciers in this country the above passages from the writings of Rendu are unquoted; and many who mingled very warmly in the discussions of the subject were, until quite recently, ignorant of their existence. I was long in this condition myself, for I never supposed that passages which bear so directly upon a point so much discussed, and of such cardinal import, could have been overlooked; or that the task of calling attention to them should devolve upon myself nearly twenty years after their publication. Now that they are discovered, I conceive no difference of opinion can exist as to the propriety of placing them in their true position.

(15.)

The measurements of Agassiz and Forbes completely verify the anticipations of Rendu; but no writer with whom I am acquainted has added anything essential to the Bishop's statements as to the identity of glacier and liquid motion. He laid down the conditions of the problem with perfect clearness, and, as regards the distribution of merit, the point to be decided is the relative importance of his idea, and of the measurements which were subsequently made.

[Sidenote: OBSERVATIONS OF FORBES.]

The observations on which Professor Forbes based the analogy between a glacier and a river are the following:--In 1842 he fixed four marks upon the Mer de Glace a little below the Montanvert, the first of which was 100 yards distant from the side of the glacier, while the last was at the centre "or a little beyond it." The relative velocity of these four points was found to be

1.000 1.332 1.356 1.367.

The first observations were made upon two of these points, two others being subsequently added. Professor Forbes also determined the velocity of two points on the Glacier du Geant, and found the ratio of motion, in the first instance, to be as 14 to 32. Subsequent measurements, however, showed the ratio to be as 14 to 18, the larger motion belonging to the station nearest to the centre of the glacier. These are the only measurements which I can find in his large work that establish the swifter motion of the centre of the glacier; and in these cases the velocity of the centre is compared with that of _one side_ only. In no instance that I am aware of, either in 1842 or subsequent years, did Professor Forbes extend his measurements quite across a glacier; and as regards completeness in this respect, no observations hitherto made can at all compare with those executed at the instance of Agassiz upon the glacier of the Aar.

In 1844 Professor Forbes made a series of interesting experiments on a portion of the Mer de Glace near l'Angle. He divided a length of 90 feet into 45 equal spaces, and fixed pins at the end of each. His theodolite was placed upon the ice, and in seventeen days he found that the ice 90 feet nearer the centre than the theodolite had moved 26 inches past the latter. These measurements were undertaken for a special object, and completely answered the end for which they were intended.

In 1846 Professor Forbes made another important observation. Fixing three stakes at the heights of 8, 54, and 143 feet above the bed of the glacier, he found that in five days they moved respectively 2.87, 4.18, and 4.66 feet. The stake nearest the bed moved most slowly, thus showing that the ice is retarded by friction. This result was subsequently verified by the measurements of M. Martins, and by my own.

If we add to the above an observation made during a short visit to the Aletsch glacier in 1844, which showed its lateral retardation, I believe we have before us the whole of the measurements executed by Professor Forbes, which show the analogy between the motion of a glacier and that of a viscous body.

[Sidenote: MEASUREMENTS OF AGASSIZ.]

Illustrative of the same point, we have the elaborate and extensive series of measurements executed by M. Wild under the direction of M. Agassiz upon the glacier of the Aar in 1842, 1843, 1844, and 1845, which exhibit on a grand scale, and in the most conclusive manner, the character of the motion of this glacier; and also show, on close examination, an analogy with fluid motion which neither M. Agassiz nor Professor Forbes suspected. The former philosopher publishes a section in his 'Systeme Glaciaire,' entitled 'Migrations of the Centre;' in which he shows that the middle of the glacier is not always the point of swiftest motion. The detection of this fact demonstrates the attention devoted by M. Agassiz to the discussion of his observations, but he gives no clue to the cause of the variation. On inspecting the shape of the valley through which the Aar glacier moves, I find that these "migrations" follow the law established in 1857 upon the Mer de Glace, and enunciated at page 286.

To sum up this part of the question:--The _idea_ of semi-fluid motion belongs entirely to Rendu; the _proof_ of the quicker central flow belongs in part to Rendu, but almost wholly to Agassiz and Forbes; the proof of the retardation of the bed belongs to Forbes alone; while the discovery of the locus of the point of maximum motion belongs, I suppose, to me.

FORBES'S THEORY.

(16.)

The formal statement of this theory is given in the following words:--"A glacier is an imperfect fluid, or viscous body, which is urged down slopes of a certain inclination by the mutual pressure of its parts." The consistency of the glacier is illustrated by reference to treacle, honey, and tar, and the theory thus enunciated and exemplified is called the 'Viscous Theory.'

It has been the subject of much discussion, and great differences of opinion are still entertained regarding it. Able and sincere men take opposite sides; and the extraordinary number of Reviews which have appeared upon the subject during the last two years show the interest which the intellectual public of England take in the question. The chief differences of opinion turn upon the inquiry as to what Professor Forbes really meant when he propounded the viscous theory; some affirm one thing, some another, and, singularly enough, these differences continue, though the author of the theory has at various times published expositions of his views.

[Sidenote: "FACTS AND PRINCIPLES."]

The differences referred to arise from the circumstances that a sufficient distinction has not been observed between _facts_ and _principles_, and that the viscous theory has assumed various forms since its first promulgation. It has been stated to me that the theory of Professor Forbes is "the congeries of facts" which he has discovered. But it is quite evident that no recognition, however ample, of these facts would be altogether satisfactory to Professor Forbes himself. He claims recognition of his _theory_,[A] and no writer with whom I am acquainted makes such frequent use of the term. What then can the viscous theory mean apart from the facts? I interpret it as furnishing the principle from which the facts follow as physical consequences--that the glacier moves as a river because the ice is viscous. In this sense only can Professor Forbes's views be called a theory; in any other, his experiments are mere illustrations of the facts of glacier motion, which do not carry us a hair's breadth towards their physical cause.

[Sidenote: VISCOUS THEORY;--WHAT IS IT?]

What then is the meaning of viscosity or viscidity? I have heard it defined by men of high culture as "gluey tenacity;" and such tenacity they once supposed a glacier to possess. If we dip a spoon into treacle, honey, or tar, we can draw the substance out into filaments, and the same may be done with melted caoutchouc or lava. All these substances are viscous, and all of them have been chosen to illustrate the physical property in virtue of which a glacier moves. Viscosity then consists in the power of being drawn out when subjected to a force of tension, the substance, after stretching, being in a state of molecular equilibrium, or, in other words, devoid of that elasticity which would restore it to its original form. This certainly was the idea attached to Professor Forbes's words by some of his most strenuous supporters, and also by eminent men who have never taken part in any controversy on the subject. Mr. Darwin, for example, speaks of felspathic rocks being "stretched" while flowing slowly onwards in a pasty condition, in precisely the same manner as Professor Forbes believes that the ice of moving glaciers is stretched and fissured; and Professor Forbes himself quotes these words of Mr. Darwin as illustrative of his theory.[B]

The question now before us is,--Does a glacier exhibit that power of yielding to a force of tension which would entitle its ice to be regarded as a viscous substance?

[Sidenote: THEORY TESTED.]

With a view to the solution of this question Mr. Hirst took for me the inclinations of the Mer de Glace and all its tributaries in 1857; the effect of a change of inclination being always noted. I will select from those measurements a few which bear more specially upon the subject now under consideration, commencing with the Glacier des Bois, down which the ice moves in that state of wild dislocation already described. The inclination of the glacier above this cascade is 5 deg. 10', and that of the cascade itself is 22 deg. 20', the change of inclination being therefore 17 deg. 10'.

In Fig. 22 I have protracted the inclination of the cascade and of the glacier above it; the line A B representing the former and B C the latter. Now a stream of molten lava, of treacle, or tar, would, in virtue of its viscosity, be able to flow over the brow at B without breaking across; but this is not the case with the glacier; it is so smashed and riven in crossing this brow, that, to use the words of Professor Forbes himself, "it pours into the valley beneath in a cascade of icy fragments."

[Sidenote: INCLINATIONS OF THE MER DE GLACE.]

But this reasoning will appear much stronger when we revert to other slopes upon the Mer de Glace. For example, its inclination above l'Angle is 4 deg., and it afterwards descends a slope of 9 deg. 25', the change of inclination being 5 deg. 25'. If we protract these inclinations to scale, we have the line A B, Fig. 23, representing the steeper slope, and B C that of the glacier above it. One would surely think that a viscous body could cross the brow B without transverse fracture, but this the glacier cannot do, and Professor Forbes himself pronounces this portion of the Mer de Glace impassable. Indeed it was the profound crevasses here formed which placed me in a difficulty already referred to. Higher up again, the glacier is broken on passing from a slope of 3 deg. 10' to one of 5 deg. Such observations show how differently constituted a glacier is from a stream of lava in a "pasty condition," or of treacle, honey, tar, or melted caoutchouc, to all which it has been compared. In the next section I shall endeavour to explain the origin of the crevasses, and shall afterwards make a few additional remarks on the alleged viscosity of ice.

FOOTNOTES:

[A] "Mr. Hopkins," writes Professor Forbes, "has done me the honour, in the memoirs before alluded to, to mention with approbation my observations and experiments on the subject of glaciers. He has been more sparing either in praise or criticism of the theory which I have founded upon them. Had Mr. Hopkins," &c.--_Eighth Letter_; 'Occ. Papers,' p. 66.

[B] 'Occ. Papers,' p. 92.

THE CREVASSES.

(17.)

[Sidenote: CREVASSES CAUSED BY THE MOTION.]

Having made ourselves acquainted with the motion of the glacier, we are prepared to examine those rents, fissures, chasms, or, as they are most usually called, _Crevasses_, by which all glaciers are more or less intersected. They result from the motion of the glacier, and the laws of their formation are deduced immediately from those of the motion. The crevasses are sometimes very deep and numerous, and apparently without law or order in their distribution. They cut the ice into long ridges, and break these ridges transversely into prisms; these prisms gradually waste away, assuming, according to the accidents of their melting, the most fantastic forms. I have seen them like the mutilated statuary of an ancient temple, like the crescent moon, like huge birds with outstretched wings, like the claws of lobsters, and like antlered deer. Such fantastic sculpture is often to be found on the ice cascades, where the riven glacier has piled vast blocks on vaster pedestals, and presented them to the wasting action of sun and air. In Fig. 24 I have given a sketch of a mass of ice of this character, which stood in 1859 on the dislocated slope of the Glacier des Bois.

[Sidenote: FANTASTIC ICE-MASSES.]

It is usual for visitors to the Montanvert to descend to the glacier, and to be led by their guides to the edges of the crevasses, where, being firmly held, they look down into them; but those who have only made their acquaintance in this way know but little of their magnitude and beauty in the more disturbed portions of glaciers. As might be expected, they have been the graves of many a mountaineer; and the skeletons found upon the glacier prove that even the chamois itself, with its elastic muscles and admirable sureness of foot, is not always safe among the crevasses. They are grandest in the higher ice-regions, where the snow hangs like a coping over their edges, and the water trickling from these into the gloom forms splendid icicles. The Goerner Glacier, as we ascend it towards the old Weissthor, presents many fine examples of such crevasses; the ice being often torn in a most curious and irregular manner. You enter a porch, pillared by icicles, and look into a cavern in the very body of the glacier, encumbered with vast frozen bosses which are fringed all round by dependent icicles. At the peril of your life from slipping, or from the yielding of the stalactites, you may enter these caverns, and find yourself steeped in the blue illumination of the place. Their beauty is beyond description; but you cannot deliver yourself up, heart and soul, to its enjoyment. There is a strangeness about the place which repels you, and not without anxiety do you look from your ledge into the darkness below, through which the sound of subglacial water sometimes rises like the tolling of distant bells. You feel that, however the cold splendours of the place might suit a purely spiritual essence, they are not congenial to flesh and blood, and you gladly escape from its magnificence to the sunshine of the world above.

[Sidenote: BIRTH OF A CREVASSE.]

From their numbers it might be inferred that the formation of crevasses is a thing of frequent occurrence and easy to observe; but in reality it is very rarely observed. Simond was a man of considerable experience upon the ice, but the first crevasse he ever saw formed was during the setting out of one of our lines, when a narrow rent opened beneath his feet, and propagated itself through the ice with loud cracking for a distance of 50 or 60 yards. Crevasses always commence in this way as mere narrow cracks, which open very slowly afterwards. I will here describe the only case of crevasse-forming which has come under my direct observation.

On the 31st of July, 1857, Mr. Hirst and myself, having completed our day's work, were standing together upon the Glacier du Geant, when a loud dull sound, like that produced by a heavy blow, seemed to issue from the body of the ice underneath the spot on which we stood. This was succeeded by a series of sharp reports, which were heard sometimes above us, sometimes below us, sometimes apparently close under our feet, the intervals between the louder reports being filled by a low singing noise. We turned hither and thither as the direction of the sounds varied; for the glacier was evidently breaking beneath our feet, though we could discern no trace of rupture. For an hour the sounds continued without our being able to discover their source; this at length revealed itself by a rush of air-bubbles from one of the little pools upon the surface of the glacier, which was intersected by the newly-formed crevasse. We then traced it for some distance up and down, but hardly at any place was it sufficiently wide to permit the blade of my penknife to enter it. M. Agassiz has given an animated description of the terror of his guides upon a similar occasion, and there was an element of awe in our own feelings as we heard the evening stillness of the glacier thus disturbed.

[Sidenote: MECHANICAL ORIGIN.]

With regard to the mechanical origin of the crevasses the most vague and untenable notions had been entertained until Mr. Hopkins published his extremely valuable papers. To him, indeed, we are almost wholly indebted for our present knowledge of the subject, my own experiments upon this portion of the glacier-question being for the most part illustrations of the truth of his reasoning. To understand the fissures in their more complex aspects it is necessary that we should commence with their elements. I shall deal with the question in my own way, adhering, however, to the mechanical principles upon which Mr. Hopkins has based his exposition.

Let A B, C D, be the bounding sides of a glacier moving in the direction of the arrow; let _m_, _n_ be two points upon the ice, one, _m_, close to the retarding side of the valley, and the other, _n_, at some distance from it. After a certain time, the point _m_ will have moved downwards to _m'_, but in consequence of the swifter movement of the parts at a distance from the sides, _n_ will have moved in the same time to _n'_. Thus the line _m n_, instead of being at right angles to the glacier, takes up the oblique position _m' n'_; but to reach from _m'_ to _n'_ the line _m n_ would have to stretch itself considerably; every other line that we can draw upon the ice parallel to _m' n'_ is in a similar state of tension; or, in other words, the sides of the glacier are acted upon by an oblique pull towards the centre. Now, Mr. Hopkins has shown that the direction in which this oblique pull is strongest encloses an angle of 45 deg. with the side of the glacier.

[Sidenote: LINE OF GREATEST STRAIN.]

What is the consequence of this? Let A B, C D, Fig. 26, represent, as before, the sides of the glacier, moving in the direction of the arrow; let the shading lines enclose an angle of 45 deg. with the sides. _Along_ these lines the marginal ice suffers the greatest strain, and, consequently _across_ these lines and at right angles to them, the ice tends to break and to form _marginal crevasses_. The lines, _o p_, _o p_, mark the direction of these crevasses; they are at right angles to the line of greatest strain, and hence also enclose an angle of 45 deg. with the side of the valley, _being obliquely pointed upwards_.

[Sidenote: MARGINAL AND TRANSVERSE CREVASSES.]

This latter result is noteworthy; it follows from the mechanical data that the swifter motion of the centre tends to produce marginal crevasses which are inclined from the side of the glacier towards its source, and not towards its lower extremity. But when we look down upon a glacier thus crevassed, the first impression is that the sides have been dragged down, and have left the central portions behind them; indeed, it was this very appearance that led M. de Charpentier and M. Agassiz into the error of supposing that the sides of a glacier moved more quickly than its middle portions; and it was also the delusive aspect of the crevasses which led Professor Forbes to infer the slower motion of the eastern side of the Mer de Glace.

The retardation of the ice is most evident near the sides; in most cases, the ice for a considerable distance right and left of the central line moves with a sensibly uniform velocity; there is no dragging of the particles asunder by a difference of motion, and, consequently, a compact centre is perfectly compatible with fissured sides. Nothing is more common than to see a glacier with its sides deeply cut, and its central portions compact; this, indeed, is always the case where the glacier moves down a bed of uniform inclination.

But supposing that the bed is not uniform--that the valley through which the glacier moves changes its inclination abruptly, so as to compel the ice to pass over a brow; the glacier is then circumstanced like a stick which we try to break by holding its two ends and pressing it against the knee. The brow, where the bed changes its inclination, represents the knee in the case of the stick, while the weight of the glacier itself is the force that tends to break it. It breaks; and fissures are formed across the glacier, which are hence called _transverse crevasses_.

[Sidenote: GRINDELWALD GLACIER.]

No glacier with which I am acquainted illustrates the mechanical laws just developed more clearly and fully than the Lower glacier of Grindelwald. Proceeding along the ordinary track beside the glacier, at about an hour's distance from the village the traveller reaches a point whence a view of the glacier is obtained from the heights above it. The marginal fissures are very cleanly cut, and point nearly in the direction already indicated; the glacier also changes its inclination several times along the distance within the observer's view. On crossing each brow the glacier is broken across, and a series of transverse crevasses is formed, which follow each other down the slope. At the bottom of the slope tension gives place to pressure, the walls of the crevasses are squeezed together, and the chasms closed up. They remain closed along the comparatively level space which stretches between the base of one slope and the brow of the next; but here the glacier is again transversely broken, and continues so until the base of the second slope is reached, where longitudinal pressure instead of longitudinal strain begins to act, and the fissures are closed as before. In Fig. 27A I have given a sketchy section of a portion of the glacier, illustrating the formation of the crevasses at the top of a slope, and their subsequent obliteration at its base.

[Sidenote: COMPRESSION AND TENSION.]

Another effect is here beautifully shown, namely, the union of the transverse and marginal crevasses to form continuous fissures which stretch quite across the glacier. Fig. 27B will illustrate my meaning, though very imperfectly; it represents a plan of a portion of the Lower Grindelwald glacier, with both marginal and transverse fissures drawn upon it. I have placed it under the section so that each part of it may show in plan the portion of the glacier which is shown in section immediately above it. It shows how the marginal crevasses remain after the compression of the centre has obliterated the transverse ones; and how the latter join on to the former, so as to form continuous fissures, which sweep across the glacier in vast curves, with their convexities turned upwards. The illusion before referred to is here strengthened; the crevasses turn, so to say, _against_ the direction of motion, instead of forming loops, with their convexities pointing downwards, and thus would impress a person unacquainted with the mechanical data with the idea that the glacier margins moved more quickly than the centre. The figures are intended to convey the idea merely; on the actual slopes of the glacier between twenty and thirty chasms may be counted: also the word "compression" ought to have been limited to the level portions of the sketch.

[Sidenote: LONGITUDINAL CREVASSES.]

Besides the two classes of fissures mentioned we often find others, which are neither marginal nor transverse. The terminal portions of many glaciers, for example, are in a state of compression; the snout of the glacier abuts against the ground, and having to bear the thrust of the mass behind it, if it have room to expand laterally, the ice will yield, and _longitudinal crevasses_ will be formed. They are of very common occurrence, but the finest example of the kind is perhaps exhibited by the glacier of the Rhone. After escaping from the steep gorge which holds the cascade, this glacier encounters the bottom of a comparatively wide and level valley; the resistance to its forward motion is augmented, while its ability to expand laterally is increased; it has to bear a longitudinal thrust, and it splits at right angles to the pressure [strain?]. A series of fissures is thus formed, the central ones of which are truly longitudinal; but on each side of the central line the crevasses diverge, and exhibit a fan-like arrangement. This disposition of the fissures is beautifully seen from the summit of the Mayenwand on the Grimsel Pass.

Here then we have the elements, so to speak, of glacier-crevassing, and through their separate or combined action the most fantastic cutting up of a glacier may be effected. And see how beautifully these simple principles enable us to account for the remarkable crevassing of the eastern side of the Mer de Glace. Let A B, C D, be the opposite sides of a portion of the glacier, near the Montanvert; C D being east, and A B west, the glacier moving in the direction of the arrow; let the points _m n_ represent the extremities of our line of stakes, and let us suppose an elastic string stretched across the glacier from one to the other. We have proved that the point of maximum motion here lies much nearer to the side C D than to A B. Let _o_ be this point, and, seizing the string at _o_, let it be drawn in the direction of motion until it assumes the position, _m_, _o'_, _n_. It is quite evident that _o' n_ is in a state greater tension than _o' m_, and the ice at the eastern side of the Mer de Glace is in a precisely similar mechanical condition. It suffers a greater strain than the ice at the opposite side of the valley, and hence is more fissured and broken. Thus we see that the crevassing of the eastern side of the glacier is a simple consequence of the quicker motion of that side, and does not, as hitherto supposed, demonstrate its slower motion. The reason why the eastern side of the glacier, as a whole, is much more fissured than the western side is, that there are two long segments which turn their convex curvature eastward, and only one segment of the glacier which turns its convexity westward.

[Sidenote: CREVASSING OF CONVEX SIDE.]

The lower portion of the Rhone glacier sweeps round the side of the valley next the Furca, and turns throughout a convex curve to this side: the crevasses here are wide and frequent, while they are almost totally absent at the opposite side of the glacier. The lower Grindelwald glacier turns at one place a convex curve towards the Eiger, and is much more fissured at that side than at the opposite one; indeed, the fantastic ice-splinters, columns, and minarets, which are so finely exhibited upon this glacier, are mainly due to the deep crevassing of the convex side. Numerous other illustrations of the law might, I doubt not, be discovered, and it would be a pleasant and useful occupation to one who takes an interest in the subject, to determine, by strict measurements upon other glaciers, the locus of the point of maximum motion, and to observe the associated mechanical effects.

[Sidenote: BERGSCHRUNDS.]

The appearance of crevasses is often determined by circumstances more local and limited than those above indicated; a boss of rock, a protuberance on the side of the flanking mountain, anything, in short, which checks the motion of one part of the ice and permits an adjacent portion to be pushed away from it, produces crevasses. Some valleys are terminated by a kind of mountain-circus with steep sides, against which the snow rises to a considerable height. As the mass is urged downwards, the lower portion of the snow-slope is often torn away from its higher portion, and a chasm is formed, which usually extends round the head of the valley. To such a crevasse the specific name _Bergschrund_ is applied in the Bernese Alps; I have referred to one of them in the account of the "Passage of the Strahleck."

(18.)

The phenomena described and accounted for in the last chapter have a direct bearing upon the question of viscosity. In virtue of the quicker central flow the lateral ice is subject to an oblique strain; but, instead of stretching, it breaks, and marginal crevasses are formed. We also see that a slight curvature in the valley, by throwing an additional strain upon one half of the glacier, produces an augmented crevassing of that side.

But it is known that a substance confessedly viscous may be broken by a sudden shock or strain. Professor Forbes justly observes that sealing-wax at moderate temperatures will mould itself (with time) to the most delicate inequalities of the surface on which it rests, but may at the same time be shivered to atoms by the blow of a hammer. Hence, in order to estimate the weight of the objection that a glacier breaks when subjected to strain, we must know the conditions under which the force is applied.

The Mer de Glace has been shown (p. 287) to move through the neck of the valley at Trelaporte at the rate of twenty inches a day. Let the sides of this page represent the boundaries of the glacier at Trelaporte, and any one of its lines of print a transverse slice of ice. Supposing the line to move down the page as the slice of ice moves down the valley, then the bending of the ice in twenty-four hours, shown on such a scale, would only be sufficient to push forward the centre in advance of the sides by a very small fraction of the width of the line of print. To such an extremely gradual strain the ice is unable to accommodate itself without fracture.

[Sidenote: NUMERICAL TEST OF VISCOSITY.]

Or, referring to actual numbers:--the stake No. 15 on our 5th line, page 284, stood on the lateral moraine of the Mer de Glace; and between it and No. 14 a distance of 190 feet intervened. Let A B, Fig. 29, be the side of the glacier, moving in the direction of the arrow, and let _a b c d_ be a square upon the glacier with a side of 190 feet. The whole square moves with the ice, but the side _b d_ moves quickest; the point _a_ moving 10 inches, while _b_ moves 14.75 inches in 24 hours; the differential motion therefore amounts to an inch in five hours. Let _a b' d' c_ be the shape of the figure after five hours' motion; then the line _a b_ would be extended to _a b'_ and _c d_ to _c d'_.

The extension of _these_ lines does not however express the _maximum_ strain to which the ice is subjected. Mr. Hopkins has shown that this takes place along the line _a d_; in five hours then this line, if capable of stretching, would be stretched to _a d'_. From the data given every boy who has mastered the 47th Proposition of the First Book of Euclid can find the length both of _a d_ and _a d'_; the former is 3224.4 inches, and the latter is 3225.1, the difference between them being seven-tenths of an inch.

This is the amount of yielding required from the ice in five hours, but it cannot grant this; the glacier breaks, and numerous marginal crevasses are formed. It must not be forgotten that the evidence here adduced merely shows what ice cannot do; what it _can_ do in the way of viscous yielding we do not know: there exists as yet no single experiment on great masses or small to show that ice possesses in any sensible degree that power of being drawn out which seems to be the very essence of viscosity.

I have already stated that the crevasses, on their first formation, are exceedingly narrow rents, which widen very slowly. The new crevasse observed by our guide required several days to attain a width of three inches; while that observed by Mr. Hirst and myself did not widen a single inch in three days. This, I believe, is the general character of the crevasses; they form suddenly and open slowly. Both facts are at variance with the idea that ice is viscous; for were this substance capable of stretching at the slow rate at which the fissures widen, there would be no necessity for their formation.

[Sidenote: STRETCHING OF ICE NOT PROVED.]

It cannot be too clearly and emphatically stated that the _proved_ fact of a glacier conforming to the law of semi-fluid motion is a thing totally different from the _alleged_ fact of its being viscous. Nobody since its first enunciation disputed the former. I had no doubt of it when I repaired to the glaciers in 1856; and none of the eminent men who have discussed this question with Professor Forbes have thrown any doubt upon his measurements. It is the assertion that small pieces of ice are proved to be viscous[A] by the experiments made upon glaciers, and the consequent impression left upon the public mind--that ice possesses the "gluey tenacity" which the term viscous suggests--to which these observations are meant to apply.

FOOTNOTES:

[A] "The viscosity, though it cannot be traced in the parts _if very minute_ nevertheless _exists_ there, as unequivocally proved by experiments on the large scale."--Forbes in 'Phil. Mag.,' vol. x., p. 301.

HEAT AND WORK.

(19.)

[Sidenote: CONNEXION OF NATURAL FORCES.]

Great scientific principles, though usually announced by individuals, are often merely the distinct expression of thoughts and convictions which had long been entertained by all advanced investigators. Thus the more profound philosophic thinkers had long suspected a certain equivalence and connexion between the various forces of nature; experiment had shown the direct connexion and mutual convertibility of many of them, and the spiritual insight, which, in the case of the true experimenter, always surrounds and often precedes the work of his hands, revealed more or less plainly that natural forces either had a common root, or that they formed a circle, whose links were so connected that by starting from any one of them we could go through the circuit, and arrive at the point from which we set out. For the last eighteen years this subject has occupied the attention of some of the ablest natural philosophers, both in this country and on the Continent. The connexion, however, which has most occupied their minds is that between _heat_ and _work_; the absolute numerical equivalence of the two having, I believe, been first announced by a German physician named Mayer, and experimentally proved in this country by Mr. Joule.

[Sidenote: MECHANICAL EQUIVALENT OF HEAT.]

A lead bullet may be made hot enough to burn the hand, by striking it with a hammer, or by rubbing it against a board; a clever blacksmith can make a nail red-hot by hammering it; Count Rumford boiled water by the heat developed in the boring of cannon, and inferred from the experiment that heat was not what it was generally supposed to be, an imponderable fluid, but a kind of motion generated by the friction. Now Mr. Joule's experiments enable us to state the exact amount of heat which a definite expenditure of mechanical force can originate. I say _originate_, not drag from any hiding-place in which it had concealed itself, but actually bring into existence, so that the total amount of heat in the universe is thereby augmented. If a mass of iron fall from a tower 770 feet in height, we can state the precise amount of heat developed by its collision with the earth. Supposing all the heat thus generated to be concentrated in the iron itself, its temperature would thereby be raised nearly 10 deg. Fahr. Gravity in this case has expended a certain amount of force in pulling the iron to the earth, and this force is the _mechanical equivalent_ of the heat generated. Furthermore, if we had a machine so perfect as to enable us to apply all the heat thus produced to the raising of a weight, we should be able, by it, to lift the mass of iron to the precise point from which it fell.

But the heat cannot lift the weight and still continue heat; this is the peculiarity of the modern view of the matter. The heat is consumed, used up, it is no longer heat; but instead of it we have a certain amount of gravitating force stored up, which is ready to act again, and to regenerate the heat when the weight is let loose. In fact, when the falling weight is stopped by the earth, the motion of its mass is converted into a motion of its molecules; when the weight is lifted by heat, molecular motion is converted into ordinary mechanical motion, but for every portion of either of them brought into existence an equivalent portion of the other must be consumed.

What is true for masses is also true for atoms. As the earth and the piece of iron mutually attract each other, and produce heat by their collision, so the carbon of a burning candle and the oxygen of the surrounding air mutually attract each other; they rush together, and on collision the arrested motion becomes heat. In the former case we have the conversion of gravity into heat, in the latter the conversion of chemical affinity into heat; but in each case the process consists in the generation of motion by attraction, and the subsequent change of that motion into motion of another kind. Mechanically considered, the attraction of the atoms and its results is precisely the same as the attraction of the earth and weight and _its_ results.

[Sidenote: HEAT PRODUCED IF THE EARTH STRUCK THE SUN.]

But what is true for an atom is also true for a planet or a sun. Supposing our earth to be brought to rest in her orbit by a sudden shock, we are able to state the exact amount of heat which would be thereby generated. The consequence of the earth's being thus brought to rest would be that it would fall into the sun, and the amount of heat which would be generated by this second collision is also calculable. Helmholtz has calculated that in the former case the heat generated would be equal to that produced by the combustion of fourteen earths of solid coal, and in the latter case the amount would be 400 times greater.

[Sidenote: SHIFTING OF ATOMS.]

Whenever a weight is lifted by a steam-engine in opposition to the force of gravity an amount of heat is consumed equivalent to the work done; and whenever the molecules of a body are shifted in opposition to their mutual attractions work is also performed, and an equivalent amount of heat is consumed. Indeed the amount of work done in the shifting of the molecules of a body by heat, when expressed in ordinary mechanical work, is perfectly enormous. The lifting of a heavy weight to the height of 1000 feet may be as nothing compared with the shifting of the atoms of a body by an amount so small that our finest means of measurement hardly enable us to determine it. Different bodies give heat different degrees of trouble, if I may use the term, in shifting their atoms and putting them in new places. Iron gives more trouble than lead; and water gives far more trouble than either. The heat expended in this molecular work is lost as heat; it does not show itself as temperature. Suppose the heat produced by the combustion of an ounce of candle to be concentrated in a pound of iron, a certain portion of that heat would go to perform the molecular work to which I have referred, and the remainder would be expended in raising the temperature of the body; and if the same amount of heat were communicated to a pound of iron and to a pound of lead, the balance in favour of temperature would be greater in the latter case than in the former, because the heat would have less molecular work to do; the lead would become more heated than the iron. To raise a pound of iron a certain number of degrees in temperature would, in fact, require more than three times the absolute quantity of heat which would be required to raise a pound of lead the same number of degrees. Conversely, if we place the pound of iron and the pound of lead, heated to the same temperature, into ice, we shall find that the quantity of ice melted by the iron will be more than three times that melted by the lead. In fact, the greater amount of molecular work invested in the iron now comes into play, the atoms again obey their own powerful forces, and an amount of heat corresponding to the energy of these forces is generated.

This molecular work is that which has usually been called _specific heat_, or _capacity for heat_. According to the _materialistic_ view of heat, bodies are figured as sponges, and heat as a kind of fluid absorbed by them, different bodies possessing different powers of absorption. According to the _dynamic_ view, as already explained, heat is regarded as a motion, and capacity for heat indicates the quantity of that motion consumed in internal changes.

The greatest of these changes occurs when a body passes from one state of aggregation to another, from the solid to the liquid, or from the liquid to the aeriform state; and the quantity of heat required for such changes is often enormous. To convert a pound of ice at 32 deg. Fahr. into water _at the same temperature_ would require an amount of heat competent, if applied as mechanical force, to lift the same pound of ice to a height of 110,000 feet; it would raise a ton of ice nearly 50 feet, or it would lift between 49 and 50 tons to a height of one foot above the earth's surface. To convert a pound of water at 212 deg. into a pound of steam at the same temperature would require an amount of heat which would perform nearly seven times the amount of mechanical work just mentioned.

[Sidenote: HEAT CONSUMED IN MOLECULAR WORK.]

This heat is entirely expended in _interior work_,[A] and does nothing towards augmenting the temperature; the water is at the temperature of the ice which produced it, both are 32 deg.; and the steam is at the temperature of the water which produced it, both are 212 deg. The whole of the heat is consumed in producing the change of aggregation; I say "_consumed_," not hidden or "latent" in either the water or the steam, but absolutely non-existent as heat. The molecular forces, however, which the heat has sacrificed itself to overcome are able to reproduce it; the water in freezing and the steam in condensing give out the exact amount of heat which they consumed when the change of aggregation was in the opposite direction.

At a temperature of several degrees below its freezing point ice is much harder than at 32 deg. I have more than once cooled a sphere of the substance in a bath of solid carbonic acid and ether to a temperature of 100 deg. below the freezing point. During the time of cooling the ice crackled audibly from its contraction, and afterwards it quite resisted the edge of a knife; while at 32 deg. it may be cut or crushed with extreme facility. The cold sphere was subjected to pressure; it broke with the detonation of a vitreous body, and was taken from the press a white opaque powder; which, on being subsequently raised to 32 deg. and again compressed, was converted into a pellucid slab of ice.

[Sidenote: ICE NEAR THE MELTING POINT.]

But before the temperature of 32 deg. is quite attained, ice gives evidence of a loosening of its crystalline texture. Indeed the unsoundness of ice at and near its melting point has been long known. Sir John Leslie, for example, states that ice at 32 deg. is _friable_; and every skater knows how rotten ice becomes before it thaws. M. Person has further shown that the latent heat of ice, that is to say, the quantity of heat necessary for its liquefaction, is not quite expressed by the quantity consumed in reducing ice at 32 deg. to the liquid state. The heat begins to be rendered latent, or in other words the change of aggregation commences, a little before the substance reaches 32 deg.,--a conclusion which is illustrated and confirmed by the deportment of melting ice under pressure.

[Sidenote: ROTTEN ICE AND SOFTENED WAX.]

In reference to the above result Professor Forbes writes as follows:--"I have now to refer to a fact ... established by a French experimenter, M. Person, who appears not to have had even remotely in his mind the theory of glaciers, when he announced the following facts, viz.--'That ice does not pass abruptly from the solid to the fluid state; that it begins to _soften_ at a temperature of 2 deg. Centigrade below its thawing point; that, consequently, between 28 deg. 4' and 32 deg. of Fahr. ice is actually passing through various degrees of plasticity within narrower limits, but in the same manner that wax, for example, softens before it melts.'" The "_softening_" here referred to is the "friability," of Sir J. Leslie, and what I have called a "loosening of the texture." Let us suppose the Serpentine covered by a sheet of pitch so smooth and hard as to enable a skater to glide over it; and which is afterwards gradually warmed until it begins to bend under his weight, and finally lets him through. A comparison of this deportment with that of a sheet of ice under the same circumstances enables us to decide whether ice "passes through various degrees of plasticity in the same manner as wax softens before it melts." M. Person concerned himself solely with the heat absorbed, and no doubt in both wax and ice that heat is expended in "interior work." In the one case, however, the body is so constituted that the absorbed heat is expended in rendering the substance viscous; and the question simply is, whether the heat absorbed by the ice gives its molecules a freedom of play which would entitle it also to be called viscous; whether, in short, "rotten ice" and softened wax present the same physical qualities?

FOOTNOTES:

[A] I borrow this term from Professor Clausius's excellent papers on the Dynamical Theory of Heat.

(20.)

There is one other point in connexion with the viscous theory which claims our attention. The announcement of that theory startled scientific men, and for two or three years after its first publication it formed the subject of keen discussion. This finally subsided, and afterwards Professor Forbes drew up an elaborate paper, which was presented in three parts to the Royal Society in 1845 and 1846, and subsequently published in the 'Philosophical Transactions.'

In the concluding portion of Part III. Professor Forbes states and answers the question, "How far a glacier is to be regarded as a plastic mass?" in these words:--"Were a glacier composed of a solid crystalline cake of ice, fitted or moulded to the mountain bed which it occupies, like a lake tranquilly frozen, it would seem impossible to admit such a flexibility or yielding of parts as should permit any comparison to a fluid or semifluid body, transmitting pressure horizontally, and whose parts might change their mutual positions so that one part should be pushed out whilst another remained behind. But we know, in point of fact, that a glacier is a body very differently constituted. It is clearly proved by the experiments of Agassiz and others that the glacier is not a mass of ice, but of ice and water, the latter percolating freely through the crevices of the former to all depths of the glacier; and it is a matter of ocular demonstration that these crevices, though very minute, communicate freely with one another to great distances; the water with which they are filled communicates force also to great distances, and exercises a tremendous hydrostatic pressure to move onwards in the direction in which gravity urges it, the vast porous mass of seemingly rigid ice in which it is as it were bound up."

[Sidenote: CAPILLARY HYPOTHESIS.]

"Now the water in the crevices," continues Professor Forbes, "does not constitute the glacier, but only the principal vehicle of the force which acts on it, and the slow irresistible energy with which the icy mass moves onwards from hour to hour with a continuous march, bespeaks of itself the presence of a fluid pressure. But if the ice were not in some degree ductile or plastic, this pressure could never produce any the least forward motion of the mass. The pressure in the capillaries of the glacier can only tend to separate one particle from another, and thus produce tensions and compressions _within the body of the glacier itself_, which yields, owing to its slightly ductile nature, in the direction of least resistance, retaining its continuity, or recovering it by reattachment after its parts have suffered a bruise, according to the violence of the action to which it has been exposed."

I will not pretend to say that I fully understand this passage, but, taking it and the former one together, I think it is clear that the water which is supposed to gorge the capillaries of the glacier is assumed to be essential to its motion. Indeed, an extreme degree of sensitiveness has been ascribed to the glacier as regards the changes of temperature by which the capillaries are affected. In three succeeding days, for example, Professor Forbes found the diurnal summer motion of a point upon the Mer de Glace to increase from 15.2 to 17.5 inches a day; a result which he says he is "persuaded" to be due to the increasing heat of the weather at the time. If, then, the glacier capillaries can be gorged so quickly as this experiment would indicate, it is fair to assume that they are emptied with corresponding speed when the supply is cut away.

[Sidenote: TEMPERATURE AT CHAMOUNI; WINTER 1859.]

The extraordinary coldness of the weather previous to the Christmas of 1859 is in the recollection of everybody: this lowness of temperature also extended to the Mer de Glace and its environs. I had last summer left with Auguste Balmat and the Abbe Vueillet thermometers with which observations were made daily during the cold weather referred to. I take the following from Balmat's register.

Minimum Date. temperature Centigrade. December 16 -15 deg. " 17 -20 " 18 -16-1/2 " 19 -9 " 20 -13 " 21 -20-1/2 " 22 -4-1/4 December 23 -4-1/2 deg. " 24 -6-1/2 " 25 -2 " 26 +2 " 27 -3 " 28 -10-1/2 " 29 -6

The temperature at the Montanvert during the above period may be assumed as generally some degrees lower, so that for a considerable period, previous to my winter observations, the portion of the Mer de Glace near the Montanvert had been exposed to a very low temperature. I reached the place after the weather had become warm, but during my stay there the maximum temperature did not exceed -4-1/2 deg. C. Considering therefore the long drain to which the glacier had been subjected previous to the 29th of December, it is not unreasonable to infer that the capillary supply assumed by Professor Forbes must by that time have been exhausted. Notwithstanding this, the motion of the glacier at the Montanvert amounted at the end of December to half its maximum summer motion.

[Sidenote: BALMAT'S MEASUREMENTS.]

The observations of Balmat which have been published by Professor Forbes[A] also militate, as far as they go, against the idea of proportionality between the capillary supply and the motion. If the temperatures recorded apply to the Mer de Glace during the periods of observation, it would follow that from the 19th of December 1846 to the 12th of April 1847 the temperature of the air was constantly under zero Centigrade, and hence, during this time, the gorging of the capillaries, which is due to superficial melting, must have ceased. Still, throughout this entire period of depletion the motion of the glacier steadily increased from twenty-four inches to thirty-four and a half inches a day. What has been here said of the Montanvert, and of the points lower down where Balmat's measurements were made, of course applies with greater force to the higher portions of the glacier, which are withdrawn from the operation of superficial melting for a longer period, and which, nevertheless, if I understand Professor Forbes aright, have their motion _least affected_ in winter. He records, for example, an observation of Mr. Bakewell's, by which the Glacier des Bossons is shown to be stationary at its end, while its upper portions are moving at the rate of a foot a day. This surely indicates that, at those places where the glacier is longest cut off from superficial supply, the motion is least reduced, which would be a most strange result if the motion depended, as affirmed, upon the gorging of the capillaries.

[Sidenote: BAKEWELL'S OBSERVATIONS.]

The perusal of the conclusion of Professor Forbes's last volume shows me that a thought similar to that expressed above occurred to Mr. Bakewell also. Speaking of a shallow glacier which moved when the alleged temperature was so enormously below the freezing point that Professor Forbes regards the observation as open to question (in which I agree with him), Mr. Bakewell asks, "Is it possible that infiltrated water can have any action whatever under such circumstances?" The reply of Professor Forbes contains these words:--"I have nowhere affirmed the presence of liquid water to be a _sine qua non_ to the plastic motion of glaciers." This statement, I confess, took me by surprise, which was not diminished by further reading. Speaking of the influence of temperature on the motion of the Mer de Glace, Professor Forbes says, the glacier "took no real start until the frost had given way, and the tumultuous course of the Arveiron showed that its veins were again filled with the circulating medium to which the glacier, like the organic frame, owes its moving energy."[B] And again:--"It is this fragility precisely which, yielding to the hydrostatic pressure of the unfrozen water contained in the countless capillaries of the glacier, produces the crushing action which shoves the ice over its neighbour particles."[C]

[Sidenote: HUXLEY'S OBSERVATIONS.]

After the perusal of the foregoing paragraphs the reader will probably be less interested in the question as to whether the assumed capillaries exist at all in the glacier. According to Mr. Huxley's observations, they do not.[D] During the summer of 1857 he carefully experimented with coloured liquids on the Mer de Glace and its tributaries, and in no case was he able to discover these fissures in the sound unweathered ice. I have myself seen the red liquid resting in an auger-hole, where it had lain for an hour without diffusing itself in any sensible degree. This cavity intersected both the white ice and the blue veins of the glacier; and Mr. Huxley, in my presence, cut away the ice until the walls of the cavity became extremely thin, still no trace of liquid passed through them. Experiments were also made upon the higher portions of the Mer de Glace, and also on the Glacier du Geant, with the same result. Thus the very existence of these capillaries is rendered so questionable, that no theory of glacier-motion which invokes their aid could be considered satisfactory.

FOOTNOTES:

[A] 'Occ. Pap.,' p. 224.

[B] 'Phil. Trans.,' 1846, p. 137, and 'Occ. Pap.,' p. 138.

[C] 'Occ. Pap.,' p. 47.

[D] 'Phil. Mag.,' 1857, vol. xiv., p. 241.

THOMSON'S THEORY.

(21.)

In the 'Transactions' of the Royal Society of Edinburgh for 1849 is published a very interesting paper by Prof. James Thomson of Queen's College, Belfast, wherein he deduces, as a consequence of a principle announced by the French philosopher Carnot, that water, when subjected to pressure, requires a greater cold to freeze it than when the pressure is removed. He inferred that the lowering of the freezing point for every atmosphere of pressure amounted to .0075 of a degree Centigrade. This deduction was afterwards submitted to the test of experiment by his distinguished brother Prof. Wm. Thomson, and proved correct. On the fact thus established is founded Mr. James Thomson's theory of the "Plasticity of Ice as manifested in Glaciers."

[Sidenote: STATEMENT OF THEORY.]

The theory is this:--Certain portions of the glacier are supposed first to be subjected to pressure. This pressure liquefies the ice, the water thus produced being squeezed through the glacier in the direction in which it can most easily escape. But cold has been evolved by the act of liquefaction, and, when the water has been relieved from the pressure, it freezes in a new position. The pressure being thus abolished at the place where it was first applied, new portions of the ice are subjected to the force; these in their turn liquefy, the water is dispersed as before, and re-frozen in some other place. To the succession of processes here assumed Mr. Thomson ascribes the changes of form observed in glaciers.

This theory was first communicated to the Royal Society through the author's brother, Prof. William Thomson, and is printed in the 'Proceedings' of the Society for May, 1857. It was afterwards communicated to the British Association in Dublin, in whose 'Reports' it is further published; and again it was communicated to the Belfast Literary and Philosophical Society, in whose 'Proceedings' it also finds a place.

On the 24th of November, 1859, Mr. James Thomson communicated to the Royal Society, through his brother, a second paper, in which he again draws attention to his theory. He offers it in substitution for my views as the best argument that he can adduce against them; he also controverts the explanations of regelation propounded by Prof. James D. Forbes and Prof. Faraday, believing that his own theory explains all the facts so well as to leave room for no other.

[Sidenote: DIFFICULTIES OF THEORY.]

But the passage in this paper which demands my chief attention is the following:--"Prof. Tyndall (writes Mr. Thomson), in papers and lectures subsequent to the publication of this theory, appears to adopt it to some extent, and to endeavour to make its principles co-operate with the views he had previously founded on Mr. Faraday's fact of regelation." I may say that Mr. Thomson's main thought was familiar to me long before his first communication on the plasticity of ice appeared; but it had little influence upon my convictions. Were the above passage correct, I should deserve censure for neglecting to express my obligations far more explicitly than I have hitherto done; but I confess that even now I do not understand the essential point of Mr. Thomson's theory,--that is to say, its application to the phenomena of glacier motion. Indeed, it was the obscurity in my mind in connexion with this point, and the hope that time might enable me to seize more clearly upon his meaning, which prevented me from giving that prominence to the theory of Mr. Thomson which, for aught I know, it may well deserve. I will here briefly state one or two of my difficulties, and shall feel very grateful to have them removed.

[Sidenote: IMPROBABLE DEDUCTION.]

Let us fix our attention on a vertical slice of ice transverse to the glacier, and to which the pressure is applied perpendicular to its surfaces. The ice liquefies, and, supposing the means of escape offered to the compressed water to be equal all round, it is plain that there will be as great a tendency to squeeze the water upwards as downwards; for the mere tendency to flow down by its own gravity becomes, in comparison to the forces here acting on the water, a vanishing quantity. But the fact is, that the ice above the slice is more permeable than that below it; for, as we descend a glacier, the ice becomes more compact. Hence the greater part of the dispersed water will be refrozen on that side of the slice which is turned towards the origin of the glacier; and the consequence is, that, according to Mr. Thomson's principle, the glacier ought to move up hill instead of down.

I would invite Mr. Thomson to imagine himself and me together upon the ice, desirous of examining this question in a philosophic spirit; and that we have taken our places beside a stake driven into the ice, and descending with the glacier. We watch the ice surrounding the stake, and find that every speck of dirt upon it retains its position; there is no liquefaction of the ice that bears the dirt, and consequently it rests on the glacier undisturbed. After twelve hours we find the stake fifteen inches distant from its first position: I would ask Mr. Thomson how did it get there? Or let us fix our attention on those six stakes which M. Agassiz drove into the glacier of the Aar in 1841, and found erect in 1842 at some hundreds of feet from their first position:--how did they get there? How, in fine, does the end of a glacier become its end? Has it been liquefied and re-frozen? If not, it must have been _pushed_ down by the very forces which Mr. Thomson invokes to produce his liquefaction. Both the liquefaction, as far as it exists, and the motion, are products of the same cause. In short, this theory, as it presents itself to my mind, is so powerless to account for the simplest fact of glacier-motion, that I feel disposed to continue to doubt my own competence to understand it rather than ascribe to Mr. Thomson an hypothesis apparently so irrelevant to the facts which it professes to explain.

Another difficulty is the following:--Mr. Thomson will have seen that I have recorded certain winter measurements made on the Mer de Glace, and that these measurements show not only that the ice moves at that period of the year, but that it exhibits those characteristics of motion from which its plasticity has been inferred; the velocity of the central portions of the glacier being in round numbers double the velocity of those near the sides. Had there been any necessity for it, this ratio might have been augmented by placing the side-stakes closer to the walls of the glacier. Considering the extreme coldness of the weather which preceded these measurements, it is a moderate estimate to set down the temperature of the ice in which my stakes were fixed at 5 deg. Cent. below zero.

[Sidenote: REQUISITE PRESSURE CALCULATED.]

Let us now endeavour to estimate the pressure existing at the portion of the glacier where these measurements were made. The height of the Montanvert above the sea-level is, according to Prof. Forbes, 6300 feet; that of the Col du Geant, which is the summit of the principal tributary of the Mer de Glace, is 11,146 feet: deducting the former from the latter, we find the height of the Col du Geant above the Montanvert to be 4846 feet.

Now, according to Mr. Thomson's theory and his brother's experiments, the melting point of ice is lowered .0075 deg. Centigrade for every atmosphere of pressure; and one atmosphere being equivalent to the pressure of about thirty-three feet of water, we shall not be over the truth if we take the height of an equivalent column of glacier-ice, of a compactness the mean of those which it exhibits upon the Col du Geant and at the Montanvert respectively, at forty feet. The compactness of glacier ice is, of course, affected by the air-bubbles contained within it.

[Sidenote: ACTUAL PRESSURE INSUFFICIENT.]

If, then, the pressure of forty feet of ice lower the melting point .0075 deg. Centigrade, it follows that the pressure of a column 4846 feet high will lower it nine-tenths of a degree Centigrade. Supposing, then, the _unimpeded thrust of the whole glacier, from the Col du Geant downwards_, to be exerted on the ice at the Montanvert; or, in other words, supposing the bed of the glacier to be absolutely smooth and every trace of friction abolished, the utmost the pressure thus obtained could perform would be to lower the melting point of the Montanvert ice by the quantity above mentioned. Taking into account the actual state of things, the friction of the glacier against its sides and bed, the opposition which the three tributaries encounter in the neck of the valley at Trelaporte, the resistance encountered in the sinuous valley through which it passes; and finally, bearing in mind the comparatively short length of the glacier, which has to bear the thrust, and oppose the latter by its own friction merely;--I think it will appear evident that the ice at the Montanvert cannot possibly have its melting point lowered by pressure more than a small fraction of a degree.

The ice in which my stakes were fixed being -5 deg. Centigrade, according to Mr. Thomson's calculation and his brother's experiments, it would require 667 atmospheres of pressure to liquefy it; in other words, it would require the unimpeded pressure of a column of glacier-ice 26,680 feet high. Did Mont Blanc rise to two and a half times its present height above the Montanvert, and were the latter place connected with the summit of the mountain by a continuous glacier with its bed absolutely smooth, the pressure at the Montanvert would be rather under that necessary to liquefy the ice on which my winter observations were made.

[Sidenote: MEASUREMENTS APPLY TO SURFACE.]

If it be urged that, though the temperature near the surface may be several degrees below the freezing point, the great body of the glacier does not share this temperature, but is, in all probability, near to 32 deg., my reply is simple. I did not measure the motion of the ice in the body of the glacier; nobody ever did; my measurements refer to the ice at and near the surface, and it is this ice which showed the plastic deportment which the measurements reveal.

Such, then, are some of the considerations which prevent me from accepting the theory of Mr. Thomson, and I trust they will acquit me of all desire, to make his theory co-operate with my views. I am, however, far from considering his deduction the less important because of its failing to account for the phenomena of glacier motion.

THE PRESSURE-THEORY OF GLACIER-MOTION.

(22.)

[Sidenote: POSSIBLE MOULDING OF ICE.]

Broadly considered, two classes of facts are presented to the glacier-observer; the one suggestive of viscosity, and the other of the reverse. The former are seen where _pressure_ comes into play, the latter where _tension_ is operative. By pressure ice can be moulded to any shape, while the same ice snaps sharply asunder if subjected to tension. Were the result worth the labour, ice might be moulded into vases or statuettes, bent into spiral bars, and, I doubt not, by the proper application of pressure, a _rope_ of ice might be formed and coiled into a _knot_. But not one of these experiments, though they might be a thousandfold more striking than any ever made upon a glacier, would in the least demonstrate that ice is really a viscous body.

I have here stated what I believe to be feasible. Let me now refer to the experiments which have been actually made in illustration of this point. Two pieces of seasoned box-wood had corresponding cavities hollowed in them, so that, when one was placed upon the other, a lenticular space was enclosed. A and B, Fig. 30, represent the pieces of box-wood with the cavities in plan: C represents their section when they are placed upon each other.

[Sidenote: ACTUAL MOULDING OF ICE.]

A _sphere_ of ice rather more than sufficient to fill the lenticular space was placed between the pieces of wood and subjected to the action of a small hydraulic press. The ice was crushed, but the crushed fragments soon reattached themselves, and, in a few seconds, a lens of compact ice was taken from the mould.

This lens was placed in a cylindrical cavity hollowed out in another piece of box-wood, and represented at C, Fig. 31; and a flat piece of the wood was placed over the lens as a cover, as at D. On subjecting the whole to pressure, the lens broke, as the sphere had done, but the crushed mass soon re-established its continuity, and in less than half a minute a compact cake of ice was taken from the mould.

In the following experiment the ice was subjected to a still severer test:--A hemispherical cavity was formed in one block of box-wood, and upon a second block a hemispherical protuberance was turned, smaller than the cavity, so that, when the latter was placed in the former, a space of a quarter of an inch existed between the two. Fig. 32 represents a section of the two pieces of box-wood; the brass pins _a_, _b_, fixed in the slab G H, and entering suitable apertures in the mould I K, being intended to keep the two surfaces concentric. A lump of ice being placed in the cavity, the protuberance was brought down upon it, and the mould subjected to hydraulic pressure: after a short interval the ice was taken from the mould as a smooth compact _cup_, its crushed particles having reunited, and established their continuity.

[Sidenote: ICE MOULDED TO CUPS AND RINGS.]

To make these results more applicable to the bending of glacier-ice, the following experiments were made:--A block of box-wood, M, Fig. 33, 4 inches long, 3 wide, and 3 deep, had its upper surface slightly curved, and a groove an inch wide, and about an inch deep, worked into it. A corresponding plate was prepared, having its under surface part of a convex cylinder, of the same curvature as the concave surface of the former piece. When the one slab was placed upon the other, they presented the appearance represented in section at N. A straight prism of ice 4 inches long, an inch wide, and a little more than an inch in depth, was placed in the groove; the upper slab was placed upon it, and the whole was subjected to the hydraulic press. The prism broke, but, the quantity of ice being rather more than sufficient to fill the groove, the pressure soon brought the fragments together and re-established the continuity of the ice. After a few seconds it was taken from the mould a bent bar of ice. This bar was afterwards passed through three other moulds of gradually augmenting curvature, and was taken from the last of them a _semi-ring_ of compact ice.

The ice, in changing its form from that of one mould to that of another, was in every instance broken and crushed by the pressure; but suppose that instead of three moulds three thousand had been used; or, better still, suppose the curvature of a single mould to change by extremely slow degrees; the ice would then so gradually change its form that no rude rupture would be apparent. Practically the ice would behave as a _plastic_ substance; and indeed this plasticity has been contended for by M. Agassiz, in opposition to the idea of viscosity. As already stated, the ice, bruised, and flattened, and bent in the above experiments, was incapable of being sensibly stretched; it was plastic to pressure but not to tension.

A quantity of water was always squeezed out of the crushed ice in the above experiments, and the bruised fragments were intermixed with this and with air. Minute quantities of both remained in the moulded ice, and thus rendered it in some degree turbid. Its character, however, as to continuity may be inferred from the fact that the ice-cup, moulded as described, held water without the slightest visible leakage.

[Sidenote: SOFTNESS OF ICE DEFINED.]

[Sidenote: PRESSURE AND TENSION.]

Ice at 32 deg. may, as already stated, be crushed with extreme facility, and glacier-ice with still more readiness than lake-ice: it may also be scraped with a knife with even greater facility than some kinds of chalk. In comparison with ice at 100 deg. below the freezing point, it might be popularly called _soft_. But its softness is not that of paste, or wax, or treacle, or lava, or honey, or tar. It is the softness of calcareous spar in comparison with that of rock-crystal; and although the latter is incomparably harder than the former, I think it will be conceded that the term viscous would be equally inapplicable to both. My object here is clearly to define terms, and not permit physical error to lurk beneath them. How far this ice, with a softness thus defined, when subjected to the gradual pressures exerted in a glacier, is bruised and broken, and how far the motion of its parts may approach to that of a truly viscous body under pressure, I do not know. The critical point here is that the ice changes its form, and preserves its continuity, during its motion, in virtue of _external_ force. It remains continuous whilst it moves, because its particles are kept in juxtaposition by pressure, and when this external prop is removed, and the ice, subjected to tension, has to depend solely upon the mobility of its own particles to preserve its continuity, the analogy with a viscous body instantly breaks down.[A]

FOOTNOTES:

[A] "Imagine," writes Professor Forbes, "a long narrow trough or canal, stopped at both ends and filled to a considerable depth with treacle, honey, tar, or any such viscid fluid. Imagine one end of the trough to give way, the bottom still remaining horizontal: if the friction of the fluid against the bottom be greater than the friction against its own particles, the upper strata will roll over the lower ones, and protrude in a convex slope, which will be propagated backwards towards the other or closed end of the trough. Had the matter been quite fluid the whole would have run out, and spread itself on a level: as it is, it assumes precisely the conditions which we suppose to exist in a glacier." This is perfectly definite, and my equally definite opinion is that no glacier ever exhibited the mechanical effects implied by this experiment.

REGELATION.

(23.)

[Sidenote: FARADAY'S FIRST EXPERIMENT.]

I was led to the foregoing results by reflecting on an experiment performed by Mr. Faraday, at a Friday evening meeting of the Royal Institution, on the 7th of June, 1850, and described in the 'Athenaeum' and 'Literary Gazette' for the same month. Mr. Faraday then showed that when two pieces of ice, with moistened surfaces, were placed in contact, they became cemented together by the freezing of the film of water between them, while, when the ice was below 32 deg. Fahr., and therefore _dry_, no effect of the kind could be produced. The freezing was also found to take place under water; and indeed it occurs even when the water in which the ice is plunged is as hot as the hand can bear.

A generalisation from this interesting fact led me to conclude that a bruised mass of ice, if closely confined, must re-cement itself when its particles are brought into contact by pressure; in fact, the whole of the experiments above recorded immediately suggested themselves to my mind as natural deductions from the principle established by Faraday. A rough preliminary experiment assured me that the deductions would stand testing; and the construction of the box-wood moulds was the consequence. We could doubtless mould many solid substances to any extent by suitable pressure, breaking the attachment of their particles, and re-establishing a certain continuity by the mere force of cohesion. With such substances, to which we should never think of applying the term viscous, we might also imitate the changes of form to which glaciers are subject: but, superadded to the mere cohesion which here comes into play, we have, in the case of ice, the actual regelation of the severed surfaces, and consequently a more perfect solid. In the Introduction to this book I have referred to the production of slaty cleavage by pressure; and at a future page I hope to show that the lamination of the ice of glaciers is due to the same cause; but, as justly observed by Mr. John Ball, there is no tendency to cleave in the _sound_ ice of glaciers; in fact, this tendency is obliterated by the perfect regelation of the severed surfaces.

[Sidenote: RECENT EXPERIMENTS OF FARADAY.]

Mr. Faraday has recently placed pieces of ice, in water, under the strain of forces tending to pull them apart. When two such pieces touch at a single point they adhere and move together as a rigid piece; but a little lateral force carefully applied breaks up this union with a crackling noise, and a new adhesion occurs which holds the pieces together in opposition to the force which tends to divide them. Mr. James Thomson had referred regelation to the cold produced by the liquefaction of the pressed ice; but in the above experiment all pressure is not only taken away, but is replaced by tension. Mr. Thomson also conceives that, when pieces of ice are simply placed together without intentional pressure, the capillary attraction brings the pressure of the atmosphere into play; but Mr. Faraday finds that regelation takes place _in vacuo_. A true viscidity on the part of ice Mr. Faraday never has observed, and he considers that his recent experiments support the view originally propounded by himself, namely, that a particle of water on a surface of ice becomes solid when placed between two surfaces, because of the increased influence due to their joint action.

CRYSTALLIZATION AND INTERNAL LIQUEFACTION.

(24.)

[Sidenote: HOW CRYSTALS ARE "NURSED."]

In the Introduction to this book I have briefly referred to the force of crystallization. To permit this force to exercise its full influence, it must have free and unimpeded action; a crystal, for instance, to be properly built, ought to be suspended in the middle of the crystallizing solution, so that the little architects can work all round it; or if placed upon the bottom of a vessel, it ought to be frequently turned, so that all its facets may be successively subjected to the building process. In this way crystals can be _nursed_ to an enormous size. But where other forces mingle with that of crystallization, this harmony of action is destroyed; the figures, for example, that we see upon a glass window, on a frosty morning, are due to an action compounded of the pure crystalline force and the cohesion of the liquid to the window-pane. A more regular effect is obtained when the freezing particles are suspended in still air, and here they build themselves into those wonderful figures which Dr. Scoresby has observed in the Polar Regions, Mr. Glaisher at Greenwich, and I myself on the summit of Monte Rosa and elsewhere.

Not only however in air, but in water also, figures of great beauty are sometimes formed. Harrison's excellent machine for the production of artificial ice is, I suppose, now well known; the freezing being effected by carrying brine, which had been cooled by the evaporation of ether, round a series of flat tin vessels containing water. The latter gradually freezes, and, on watching those vessels while the action was proceeding very slowly, I have seen little six-rayed stars of thin ice forming, and rising to the surface of the liquid. I believe the fact was never before observed, but it would be interesting to follow it up, and to develop experimentally this most interesting case of crystallization.

[Sidenote: DISSECTION OF ICE BY SUNBEAM.]

The surface of a freezing lake presents to the eye of the observer nothing which could lead him to suppose that a similar molecular architecture is going on there. Still the particles are undoubtedly related to each other in this way; they are arranged together on this starry type. And not only is this the case at the surface, but the largest blocks of ice which reach us from Norway and the Wenham Lake are wholly built up in this way. We can reveal the internal constitution of these masses by a reverse process to that which formed them; we can send an agent into the interior of a mass of ice which shall take down the atoms which the crystallizing forces had set up. This agent is a solar beam; with which it first occurred to me to make this simple experiment in the autumn of 1857. I placed a large converging lens in the sunbeams passing through a room, and observed the place where the rays were brought to a focus behind the lens; then shading the lens, I placed a clear cube of ice so that the point of convergence of the rays might fall within it. On removing the screen from the lens, a cone of sunlight went through the cube, and along the course of the cone the ice became studded with lustrous spots, evidently formed by the beam, as if minute reflectors had been suddenly established within the mass, from which the light flashed when it met them. On examining the cube afterwards I found that each of these spots was surrounded by a liquid flower of six petals; such flowers were distributed in hundreds through the ice, being usually clear and detached from each other, but sometimes crowded together into liquid bouquets, through which, however, the six-starred element could be plainly traced. At first the edges of the leaves were unbroken curves, but when the flowers expanded under a long-continued action, the edges became serrated. When the ice was held at a suitable angle to the solar beams, these liquid blossoms, with their central spots shining more intensely than burnished silver, presented an exhibition of beauty not easily described. I have given a sketch of their appearance in Fig. 34.

[Sidenote: LIQUID FLOWERS IN ICE.]

I have here to direct attention to an extremely curious fact. On sending the sunbeam through the transparent ice, I often noticed that the appearance of the lustrous spots was accompanied by an audible clink, as if the ice were ruptured inwardly. But there is no ground for assuming such rupture, and on the closest examination no flaw is exhibited by the ice. What then can be the cause of the noise? I believe the following considerations will answer the question:--

Water always holds a quantity of air in solution, the diffusion of which through the liquid, as proved by M. Donny, has an immense effect in weakening the cohesion of its particles; recent experiments of my own show that this is also the case in an eminent degree with many volatile liquids. M. Donny has proved that, if water be thoroughly purged of its air, a long glass tube filled with this liquid may be inverted, while the tenacity with which the water clings to the tube, and with which its particles cling to each other, is so great that it will remain securely suspended, though no external hindrance be offered to its descent. Owing to the same cause, water deprived of its air will not boil at 212 deg. Fahr., and may be raised to a temperature of nearly 300 deg. without boiling; but when this occurs the particles break their cohesion suddenly, and ebullition is converted into explosion.

Now, when ice is formed, every trace of the air which the water contained is squeezed out of it; the particles in crystallizing reject all extraneous matter, so that in ice we have a substance quite free from the air, which is never absent in the case of water; it therefore follows that if we could preserve the water derived from the melting of ice from contact with the atmosphere, we should have a liquid eminently calculated to show the effects described by M. Donny. Mr. Faraday has proved by actual experiment that this is the case.

[Sidenote: WATER DEPRIVED OF AIR SNAPS ASUNDER.]

Let us apply these facts to the explanation of the clink heard in my experiments. On sending a sunbeam through ice, liquid cavities are suddenly formed at various points within the mass, and these cavities are completely cut off from atmospheric contact. But the water formed by the melting ice is less in volume than the ice which produces it; the water of a cavity is not able to fill it, hence a vacuous space must be formed in the cell. I have no doubt that, for a time, the strong cohesion between the walls of the cell and the drop within it augments the volume of the latter a little, so as to compel it to fill the cell; but as the quantity of liquid becomes greater the shrinking force augments, until finally the particles snap asunder like a broken spring. At the same moment a lustrous spot appears, which is a vacuum, and simultaneously with the appearance of this vacuum the clink was always heard. Multitudes of such little explosions must be heard upon a glacier when the strong summer sun shines upon it, the aggregate of which must, I think, contribute to produce the "crepitation" noticed by M. Agassiz, and to which I have already referred.

[Sidenote: FIGURES IN ICE; VACUOUS SPOTS.]

In Plate VI. of the Atlas which accompanies the 'Systeme Glaciaire' of M. Agassiz, I notice drawings of figures like those I have described, which he has observed in glacier-ice, and which were doubtless produced by direct solar radiation. I have often myself observed figures of exquisite beauty formed in the ice on the surface of glacier-pools by the morning sun. In some cases the spaces between the leaves of the liquid flowers melt partially away, and leave the central spot surrounded by a crimped border; sometimes these spaces wholly disappear, and the entire space bounded by the lines drawn from point to point of the leaves becomes liquid, thus forming perfect hexagons. The crimped borders exhibit different degrees of serration, from the full leaves themselves to a gentle undulating line, which latter sometimes merges into a perfect circle. In the ice of glaciers, I have seen the internal liquefaction ramify itself like sprigs of myrtle; in the same ice, and particularly towards the extremities of the glacier, disks innumerable are also formed, consisting of flat round liquid spaces, a bright spot being usually associated with each. These spots have been hitherto mistaken for air-bubbles; but both they and the lustrous disks at the centres of the flowers are vacuous. I proved them to be so by plunging the ice containing them into hot water, and watching what occurred when the walls of the cells were dissolved, and a liquid connexion established between them and the atmosphere. In all cases they totally collapsed, and no trace of air rose to the surface of the warm water.

No matter in what direction a solar beam is sent through lake-ice, the liquid flowers are all formed parallel to the surface of freezing. The beam may be sent parallel, perpendicular, or oblique to this surface; the flowers are always formed in the same planes. Every line perpendicular to the surface of a frozen lake is in fact an axis of symmetry, round which the molecules so arrange themselves, that, when taken down by the delicate fingers of the sunbeam, the six-leaved liquid flowers are the result.

In the ice of glaciers we have no definite planes of freezing. It is first snow, which has been disturbed by winds while falling, and whirled and tossed about by the same agency after it has fallen, being often melted, saturated with its own water, and refrozen: it is cast in shattered fragments down cascades, and reconsolidated by pressure at the bottom. In ice so formed and subjected to such mutations, definite planes of freezing are, of course, out of the question.

[Sidenote: CONSTITUTION OF GLACIER-ICE.]

The flat round disks and vacuous spots to which I have referred come here to our aid, and furnish us with an entirely new means of analysing the internal constitution of a glacier. When we examine a mass of glacier-ice which contains these disks, we find them lying in all imaginable planes; not confusedly, however--closer examination shows us that the disks are arranged in groups, the members of each group being parallel to a common plane, but the parallelism ceases when different groups are compared. The effect is exactly what would be observed, supposing ordinary lake-ice to be broken up, shaken together, and the confused fragments regelated to a compact continuous mass. In such a jumble the original planes of freezing would lie in various directions; but no matter how compact or how transparent ice thus constituted might appear, a solar beam would at once reveal its internal constitution by developing the flowers parallel to the planes of freezing of the respective fragments. A sunbeam sent through glacier-ice always reveals the flowers in the planes of the disks, so that the latter alone at once informs us of its crystalline constitution.

[Sidenote: VACUOUS CELLS MISTAKEN FOR AIR-CELLS.]

Hitherto, as I have said, these disks have been mistaken for bubbles containing air, and their flattening has been ascribed to the pressure to which they have been subjected. M. Agassiz thus refers to them:--"The air-bubbles undergo no less curious modifications. In the neighbourhood of the _neve_, where they are most numerous, those which one sees on the surface are all spherical or ovoid, but by degrees they begin to be flattened, and near the end of the glacier there are some that are so flat _that they might be taken for fissures when seen in profile_. The drawing represents a piece of ice detached from the gallery of infiltration. All the bubbles are greatly flattened. But what is most extraordinary is, that, far from being uniform, _the flattening is different in each fragment_; so that the bubbles, according to the face which they offer, appear either very broad or very thin." This description of glacier-ice is correct: it agrees with the statements of all other observers. But there are two assumptions in the description which must henceforth be given up; first, the bubbles seen like fissures in profile are not air-bubbles at all, but vacuous spots, which the very constitution of ice renders a necessary concomitant of its inward melting; secondly, the assumption that the bubbles have been _flattened_ by pressure must be abandoned; for they are found, and may be developed at will, in lake-ice on which no pressure has been exerted.

[Sidenote: CELLS OF AIR AND WATER.]

But these remarks dispose only of a certain class of cells contained in glacier-ice. Besides the liquid disks and vacuous spots, there are innumerable true bubbles entangled in the mass. These have also been observed and described by M. Agassiz; and Mr. Huxley has also given us an accurate account of them. M. Agassiz frequently found air and water associated in the same cell. Mr. Huxley found no exception to the rule: in each case the bubble of air was enclosed in a cell which was also partially filled with water. He supposes that the water may be that of the originally-melted snow which has been carried down from the _neve_ unfrozen. This hypothesis is worthy of a great deal more consideration than I have had time to give to it, and I state it here in the hope that it will be duly examined.

My own experience of these associated air and water cells is derived almost exclusively from lake-ice, in which I have often observed them in considerable numbers. In examining whether the liquid contents had ever been frozen or not, I was guided by the following considerations. If the air be that originally entangled in the solid, it will have the ordinary atmospheric density at least; but if it be due to the melting of the walls of the cell, then the water so formed being only eight-ninths of that of the ice which produced it, _the air of the bubble must be rarefied_. I suppose I have made a hundred different experiments upon these bubbles to determine whether the air was rarefied or not, and in every case found it so. Ice containing the bubbles was immersed in warm water, and always, when the rigid envelope surrounding a bubble was melted away, the air suddenly collapsed to a fraction of its original dimensions. I think I may safely affirm that, in some cases, the collapse reduced the bubbles to the thousandth part of their original volume. From these experiments I should undoubtedly infer, that in lake-ice at least, the liquid of the cells is produced by the melting of the ice surrounding the bubbles of air.

But I have not subjected the bubbles of glacier-ice to the same searching examination. I have tried whether the insertion of a pin would produce the collapse of the bubbles, but it did not appear to do so. I also made a few experiments at Rosenlaui, with warm water, but the result was not satisfactory. That ice melts internally at the surfaces of the bubbles is, I think, rendered certain by my experiments, but whether the water-cells of glacier-ice are entirely due to such melting, subsequent observers will no doubt determine.

[Sidenote: "LIQUID LIBERTY."]

I have found these composite bubbles at all parts of glaciers; in the ice of the moraines, over which a protective covering had been thrown; in the ice of sand-cones, after the removal of the superincumbent debris; also in ice taken from the roofs of caverns formed in the glacier, and which the direct sunlight could hardly by any possibility attain. That ice should liquefy at the surface of a cavity is, I think, in conformity with all we know concerning the physical nature of heat. Regarding it as a motion of the particles, it is easy to see that this motion is less restrained at the surface of a cavity than in the solid itself, where the oscillation of each atom is controlled by the particles which surround it; hence _liquid liberty_, if I may use the term, is first attained at the surface. Indeed I have proved by experiment that ice may be melted internally by heat which has been conducted through its external portions without melting them. These facts are the exact complements of those of "regelation;" for here, two moist surfaces of ice being brought into close contact, their liquid liberty is destroyed and the surfaces freeze together.

THE MOULINS.

(25.)

[Sidenote: MOULIN OF GRINDELWALD GLACIER.]

[Sidenote: DEPTH OF THE SHAFT.]

The first time I had an opportunity of seeing these remarkable glacier-chimneys was in the summer of 1856, upon the lower glacier of Grindelwald. Mr. Huxley was my companion at the time, and on crossing the so-called Eismeer we heard a sound resembling the rumble of distant thunder, which proceeded from a perpendicular shaft formed in the ice, and into which a resounding cataract discharged itself. The tube in fact resembled a vast organ-pipe, whose thunder-notes were awakened by the concussion of the falling water, instead of by the gentle flow of a current of air. Beside the shaft our guide hewed steps, on which we stood in succession, and looked into the tremendous hole. Near the first shaft was a second and smaller one, the significance of which I did not then understand; it was not more than 20 feet deep, but seemed filled with a liquid of exquisite blue, the colour being really due to the magical shimmer from the walls of the moulin, which was quite empty. As far as we could see, the large shaft was vertical, but on dropping a stone into it a shock was soon heard, and after a succession of bumps, which occupied in all seven seconds, we heard the stone no more. The depth of the moulin could not be thus ascertained, but we soon found a second and still larger one which gave us better data. A stone dropped into this descended without interruption for four seconds, when a concussion was heard; and three seconds afterwards the final shock was audible: there was thus but a single interruption in the descent. Supposing all the acquired velocity to have been destroyed by the shock, by adding the space passed over by the stone in four and in three seconds respectively, and making allowance for the time required by the sound to ascend from the bottom, we find the depth of the shaft to be about 345 feet. There is, however, no reason to suppose that this measures the depth of the glacier at the place referred to. These shafts are to be found in almost all great glaciers; they are very numerous in the Unteraar Glacier, numbers of them however being empty. On the Mer de Glace they are always to be found in the region of Trelaporte, one of the shafts there being, _par excellence_, called the Grand Moulin. Many of them also occur on the Glacier de Lechaud.

As truly observed by M. Agassiz, these moulins occur only at those parts of the glacier which are not much rent by fissures, for only at such portions can the little rills produced by superficial melting collect to form streams of any magnitude. The valley of unbroken ice formed in the Mer de Glace near Trelaporte is peculiarly favourable for the collection of such streams; we see the little rills commencing, and enlarging by the contributions of others, the trunk-rill pouring its contents into a little stream which stretches out a hundred similar arms over the surface of the glacier. Several such streams join, and finally a considerable brook, which receives the superficial drainage of a large area, cuts its way through the ice.

[Sidenote: MOULINS EXPLAINED.]

But although this portion of the glacier is free from those long-continued and permanent strains which, having once rent the ice, tend subsequently to widen the rent and produce yawning crevasses, it is not free from local strains sufficient to produce _cracks_ which penetrate the glacier to a great depth. Imagine such a crack intersecting such a glacier-rivulet as we have described. The water rushes down it, and soon scoops a funnel large enough to engulf the entire stream. The moulin is thus formed, and, as the ice moves downward, the sides of the crack are squeezed together and regelated, the seam which marks the line of junction being in most cases distinctly visible. But as the motion continues, other portions of the glacier come into the same state of strain as that which produced the first crack; a second one is formed across the stream, the old shaft is forsaken, and a new one is hollowed out, in which for a season the cataract plays the thunderer. I have in some cases counted the forsaken shafts of six old moulins in advance of an active one. Not far from the Grand Moulin of the Mer de Glace in 1857 there was a second empty shaft, which evidently communicated by a subglacial duct with that into which the torrent was precipitated. Out of the old orifice issued a strong cold blast, the air being manifestly impelled through the duct by the falling water of the adjacent moulin.

These shafts are always found in the same locality; the portion of the Mer de Glace to which I have referred is never without them. Some of the guides affirm that they are motionless; and a statement of Prof. Forbes has led to the belief that this was also his opinion.[A] M. Agassiz, however, observed the motion of some of these shafts upon the glacier of the Aar; and when on the spot in 1857, I was anxious to decide the point by accurate measurements with the theodolite.

My friend Mr. Hirst took charge of the instrument, and on the 28th of July I fixed a single stake beside the Grand Moulin, in a straight line between a station at Trelaporte and a well-defined mark on the rock at the opposite side of the valley. On the 31st, the displacement of the stake amounted to 50 inches, and on the 1st of August it had moved 74-1/2 inches--the moulin, to all appearance, occupying throughout the same position with regard to the stake. To render this certain, moreover we subsequently drove two additional stakes into the ice, thus enclosing the mouth of the shaft in a triangle. On the 8th of August the displacements were measured and gave the following results:--

Total Motion. First (old) stake 198 inches. Second (new) do. 123 " Third 124 "

[Sidenote: MOTION OF THE MOULINS.]

The old stake had been fixed for 11 days, and its daily motion--_which was also that of the moulin_--averaged 18 inches a day. Hence the moulins share the general motion of the glacier, and their apparent permanence is not, as has been alleged, a proof of the semi-fluidity of the glacier, but is due to the breaking of the ice as it passes the place of local strain.

[Sidenote: DEPTH OF "GRAND MOULIN" SOUGHT.]

Wishing to obtain some estimate as to the depth of the ice, Mr. Hirst undertook the sounding of some of the moulins upon the Glacier de Lechaud, making use of a tin vessel filled with lumps of lead and iron as a weight. The cord gave way and he lost his plummet. To measure the depth of the Grand Moulin, we obtained fresh cord from Chamouni, to which we attached a four-pound weight. Into a cavity at the bottom of the weight we stuffed a quantity of butter, to indicate the nature of the bottom against which the weight might strike. The weight was dropped into the shaft, and the cord paid out until its slackening informed us that the weight had come to rest; by shaking the string, however, and walking round the edge of the shaft, the weight was liberated, and sank some distance further. The cord partially slackened a second time, but the strain still remaining was sufficient to render it doubtful whether it was the weight or the action of the falling water which produced it. We accordingly paid out the cord to the end, but, on withdrawing it, found that the greater part of it had been coiled and knotted up by the falling water. We uncoiled, and sounded again. At a depth of 132 feet the weight reached a ledge or protuberance of ice, and by shaking and lifting it, it was caused to descend 31 feet more. A depth of 163 feet was the utmost we could attain to. We sounded the old moulin to a depth of 90 feet; while a third little shaft, beside the large one, measured only 18 feet in depth. We could see the water escape from it through a lateral canal at its bottom, and doubtless the water of the Grand Moulin found a similar exit. There was no trace of dirt upon the butter, which might have indicated that we had reached the bed of the glacier.

FOOTNOTES:

[A] "Every year, and year after year, the watercourses follow the same lines of direction--their streams are precipitated into the heart of the glacier by vertical funnels, called 'moulins,' at the very same points."--Forbes's Fourth Letter upon Glaciers: 'Occ. Pap.,' p. 29.

DIRT-BANDS OF THE MER DE GLACE.

(26.)

[Sidenote: DIRT-BANDS FROM THE FLEGERE.]

These bands were first noticed by Prof. Forbes on the 24th of July, 1842, and were described by him in the following words:--"My eye was caught by a very peculiar appearance of the surface of the ice, which I was certain that I now saw for the first time. It consisted of nearly hyperbolic brownish bands on the glacier, the curves pointing downwards, and the two branches mingling indiscriminately with the moraines, presenting an appearance of a succession of waves some hundred feet apart."[A] From no single point of view hitherto attained can all the Dirt-Bands of the Mer de Glace be seen at once. To see those on the terminal portion of the glacier, a station ought to be chosen on the opposite range of the Brevent, a few hundred yards beyond the Croix de la Flegere, where we stand exactly in front of the glacier as it issues into the valley of Chamouni. The appearance of the bands upon the portion here seen is represented in Fig. 35.

It will be seen that the bands are confined to one side of the glacier, and either do not exist, or are obliterated by the debris, upon the other side. The cause of the accumulation of dirt on the right side of the glacier is, that no less than five moraines are crowded together at this side. In the upper portions of the Mer de Glace these moraines are distinct from each other; but in descending, the successive engulfments and disgorgings of the blocks and dirt have broken up the moraines; and at the place now before us the materials which composed them are strewn confusedly on the right side of the glacier. The portion of the ice on which the dirt-bands appear is derived from the Col du Geant. They do not quite extend to the end of the glacier, being obliterated by the dislocation of the ice upon the frozen cascade of Des Bois.

[Sidenote: DIRT-BANDS FROM LES CHARMOZ.]

Let us now proceed across the valley of Chamouni to the Montanvert; where, climbing the adjacent heights to an elevation of six or eight hundred feet above the hotel, we command a view of the Mer de Glace, from Trelaporte almost to the commencement of the Glacier des Bois. It was from this position that Professor Forbes first observed the bands. Fifteen, sixteen, and seventeen years later I observed them from the same position. The number of bands which Professor Forbes counted from this position was eighteen, with which my observations agree. The entire series of bands which I observed, with the exception of one or two, must have been the _successors_ of those observed by Professor Forbes; and my finding the same number after an interval of so many years proves that the bands must be due to some regularly recurrent cause. Fig. 36 represents the bands as seen from the heights adjacent to the Montanvert.

I would here direct attention to an analogy between a glacier and a river, which may be observed from the heights above the Montanvert, but to which no reference, as far as I know, has hitherto been made. When a river meets the buttress of a bridge, the water rises against it, and, on sweeping round it, forms an elevated ridge, between which and the pier a depression occurs which varies in depth with the force of the current. This effect is shown by the Mer de Glace on an exaggerated scale. Sweeping round Trelaporte, the ice pushes itself beyond the promontory in an elevated ridge, from which it drops by a gradual slope to the adjacent wall of the valley, thus forming a depression typified by that already alluded to. A similar effect is observed at the opposite side of the glacier on turning round the Echelets; and both combine to form a kind of skew surface. A careful inspection of the frontispiece will detect this peculiarity in the shape of the glacier.

[Sidenote: FROM THE CLEFT-STATION.]

From neither of the stations referred to do we obtain any clue to the origin of the dirt-bands. A stiff but pleasant climb will place us in that singular cleft in the cliffy mountain-ridge which is seen to the right of the frontispiece; and from it we easily attain the high platform of rock immediately to the left of it. We stand here high above the promontory of Trelaporte, and occupy the finest station from which the Mer de Glace and its tributaries can be viewed. From this station we trace the dirt-bands over most of the ice that we have already scanned, and have the further advantage of being able to follow them to their very source.

This source is the grand ice-cascade which descends in a succession of precipices from the plateau of the Col du Geant into the valley which the Glacier du Geant fills. We see from our present point of view that the bands _are confined to the portion of the glacier which has descended the cascade_. Fig. 37 represents the bands as seen from the Cleft-station above Trelaporte.

We are now however at such a height above the glacier and at such a distance from the base of the cascade, that we can form but an imperfect notion of the true contour of the surface. Let us therefore descend, and walk up the Glacier du Geant towards the cascade. At first our road is level, but we gradually find that at certain intervals we have to ascend slopes which follow each other in succession, each being separated from its neighbour by a space of comparatively level ice. The slopes increase in steepness as we ascend; they are steepest, moreover, on the right-hand side of the glacier, where it is bounded by that from the Periades, and at length we are unable to climb them without the aid of an axe. Soon afterwards the dislocation of the glacier becomes considerable; we are lost in the clefts and depressions of the ice, and are unable to obtain a view sufficiently commanding to subdue these local appearances and convey to us the general aspect. We have at all events satisfied ourselves as to the existence, on the upper portion of the glacier, of a succession of undulations which sweep transversely across it. The term "wrinkles," applied to them by Prof. Forbes, is highly suggestive of the appearance which they present.

[Sidenote: SNOW-BANDS ON THE GLACIER DU GEANT.]

From the Cleft-station bands of snow may also be seen partially crossing the glacier in correspondence with the undulations upon its surface. If the quantity deposited the winter previous be large, and the heat of summer not too great, these bands extend quite across the glacier. They were first observed by Professor Forbes in 1843. In his Fifth Letter is given an illustrative diagram, which, though erroneous as regards the position of the veined structure, is quite correct in limiting the snow-bands to the Glacier du Geant proper.

At the place where the three welded tributaries of the Mer de Glace squeeze themselves through the strait of Trelaporte, the bands undergo a considerable modification in shape. Near their origin they sweep across the Glacier du Geant in gentle curves, with their convexities directed downwards; but at Trelaporte these curves, the chords of which a short time previous measured a thousand yards in length, have to squeeze themselves through a space of four hundred and ninety-five yards wide; and as might be expected, they are here suddenly sharpened. The apex of each being thrust forward, they take the form of sharp hyperbolas, and preserve this character throughout the entire length of the Mer de Glace.

I would now conduct the reader to a point from which a good general view of the ice cascade of the Geant is attainable. From the old moraine near the lake of the Tacul we observe the ice, as it descends the fall, to be broken into a succession of precipices. It would appear as if the glacier had its back periodically broken at the summit of the fall, and formed a series of vast chasms separated from each other by cliffy ridges of corresponding size. These, as they approach the bottom of the fall, become more and more toned down by the action of sun and air, and at some distance below the base of the cascade they are subdued so as to form the transverse undulations already described. These undulations are more and more reduced as the glacier descends; and long before the Tacul is attained, every sensible trace of them has disappeared. The terraces of the ice-fall are referred to by Professor Forbes in his Thirteenth Letter, where he thus describes them:--"The ice-falls succeed one another at regulated intervals, which appear to correspond to the renewal of each summer's activity in those realms of almost perpetual frost, when a swifter motion occasions a more rapid and wholesale projection of the mass over the steep, thus forming curvilinear terraces like vast stairs, which appear afterwards by consolidation to form the remarkable protuberant wrinkles on the surface of the Glacier du Geant."

[Sidenote: FORBES'S EXPLANATION.]

With regard to the cause of the distribution of the dirt in bands, Professor Forbes writes thus in his Third Letter:--"I at length assured myself that it was entirely owing to the structure of the ice, which retains the dirt diffused by avalanches and the weather on those parts which are most porous, whilst the compacter portion is washed clean by the rain, so that those bands are nothing more than visible traces of the direction of the internal icy structure." Professor Forbes's theory, at that time, was that the glacier is composed throughout of a series of alternate segments of hard and porous ice, in the latter of which the dirt found a lodgment. I do not know whether he now retains his first opinion; but in his Fifteenth Letter he speaks of accounting for "the less compact structure of the ice beneath the dirt-band."

It appears to me that in the above explanation cause has been mistaken for effect. The ice on which the dirt-bands rest certainly appears to be of a spongier character than the cleaner intermediate ice; but instead of this being the cause of the dirt-bands, the latter, I imagine, by their more copious absorption of the sun's rays and the consequent greater disintegration of the ice, are the cause of the apparent porosity. I have not been able to detect any relative porosity in the "internal icy structure," nor am I able to find in the writings of Professor Forbes a description of the experiments whereby he satisfied himself that this assumed difference exists.

[Sidenote: TRANSVERSE UNDULATIONS.]

[Sidenote: INFLUENCE OF DIRECTION OF GLACIER.]

Several days of the summer of 1857 were devoted by me to the examination of these bands. I then found the bases and the frontal slopes of the undulations to which I have referred covered with a fine brown mud. These slopes were also, in some cases, covered with snow which the great heat of the weather had not been able entirely to remove. At places where the residue of snow was small its surface was exceedingly dirty--so dirty indeed that it appeared as if peat-mould had been strewn over it; its edges particularly were of a black brown. It was perfectly manifest that this snow formed a receptacle for the fine dirt transported by the innumerable little rills which trickled over the glacier. The snow gradually wasted, but it left its sediment behind, and thus each of the snowy bands observed by Professor Forbes in 1843, contributed to produce an appearance perfectly antithetical to its own. I have said that the frontal slopes of the undulations were thus covered; and it was on these, and not in the depressions, that the snow principally rested. The reason of this is to be found in the _bearing_ of the Glacier du Geant, which, looking downwards, is about fourteen degrees east of the meridian.[B] Hence the frontal slopes of the undulations have a _northern aspect_, and it is this circumstance which, in my opinion, causes the retention of the snow upon them. Irrespective of the snow, the mere tendency of the dirt to accumulate at the bases of the undulations would also produce bands, and indeed does so on many glaciers; but the precision and beauty of the dirt-bands of the Mer de Glace are, I think, to be mainly referred to the interception by the snow of the fine dark mud before referred to on the northern slopes of its undulations.

[Sidenote: BANDS DO NOT CROSS MORAINES.]

Were the statements of some writers upon this subject well founded, or were the dirt-bands as drawn upon the map of Professor Forbes correctly shown, this explanation could not stand a moment. It has been urged that the dirt-bands cannot thus belong to a single tributary of the Mer de Glace; for if they did, they would be confined to that tributary upon the trunk-glacier; whereas the fact is that they extend quite across the trunk, and intersect the moraines which divide the Glacier du Geant from its fellow-tributaries. From my first acquaintance with the Mer de Glace I had reason to believe that this statement was incorrect; but last year I climbed a third time to the Cleft-station for the purpose of once more inspecting the bands from this fine position. I was accompanied by Dr. Frankland and Auguste Balmat, and I drew the attention of both particularly to this point. Neither of them could discern, nor could I, the slightest trace of a dirt-band crossing any one of the moraines. Upon the trunk-stream they were just as much confined to the Glacier du Geant as ever. If the bands even existed east of the moraines, they could not be seen, the dirt on this part of the glacier being sufficient to mask them.

The following interesting fact may perhaps have contributed to the production of the error referred to. Opposite to Trelaporte the eastern arms of the dirt-bands run so obliquely into the moraine of La Noire that the latter appears to be a tangent to them. But this moraine runs along the Mer de Glace, not far from its centre, and consequently the point of contact of each dirt-band with the moraine moves more quickly than the point of contact of the western arm of the same band with the side of the valley. Hence there is a tendency to _straighten_ the bands; and at some distance down the glacier the effect of this is seen in the bands abutting against the moraine of La Noire at a larger angle than before. The branches thus abutting have, I believe, been ideally prolonged across the moraines.

On the map published by Prof. Forbes in 1843 the bands are shown crossing the medial moraines of the Mer de Glace; and they are also thus drawn on the map in Johnson's 'Physical Atlas' published in 1849. The text is also in accordance with the map:--"Opposite to the Montanvert, and beyond les Echelets, the curved loops (dirt-bands) extend _across the entire glacier_. They are single, and therefore _cut_ the medial moraine, though at a very slight angle."--'Travels,' p. 166. The italics here belong to Prof. Forbes. In order to help future observers to place this point beyond doubt, I annex, in Fig. 38, a portion of the map of the Mer de Glace taken from the Atlas referred to. If it be compared with Fig. 35 the difference between Prof. Forbes and myself will be clearly seen. The portion of the glacier represented in both diagrams may be viewed from the point near the Flegere already referred to.

[Sidenote: ANNUAL "RINGS."]

The explanation which I have given involves three considerations:--The transverse breaking of the glacier on the cascade, and the gradual accumulation of the dirt in the hollows between the ridges; the subsequent toning down of the ridges to gentle protuberances which sweep across the glacier; and the collection of the dirt upon the slopes and at the bases of these protuberances. Whether the periods of transverse fracture are annual or not--whether the "wrinkles" correspond to a yearly gush--and whether, consequently, the dirt-bands mark the growth of a glacier as the "annual rings" mark the growth of a tree, I do not know. It is a conjecture well worthy of consideration; but it is only a conjecture, which future observation may either ratify or refute.

FOOTNOTES:

[A] 'Travels,' page 162.

[B] In the large map of Professor Forbes the bearing of the valley is nearly sixty degrees west of the meridian; but this is caused by the true north being drawn on the wrong side of the magnetic north; thus making the declination easterly instead of westerly. In the map in Johnson's 'Physical Atlas' this mistake is corrected.

THE VEINED STRUCTURE OF GLACIERS.

(27.)

[Sidenote: GENERAL APPEARANCE.]

The general appearance of the veined structure may be thus briefly described:--The ice of glaciers, especially midway between their mountain-sources and their inferior extremities, is of a whitish hue, caused by the number of small air-bubbles which it contains, and which, no doubt, constitute the residue of the air originally entrapped in the interstices of the snow from which it has been derived. Through the general whitish mass, at some places, innumerable parallel veins of clearer ice are drawn, which usually present a beautiful blue colour, and give the ice a laminated appearance. The cause of the blueness is, that the air-bubbles, distributed so plentifully through the general mass, do not exist in the veins, or only in comparatively small numbers.

In different glaciers, and in different parts of the same glacier, these veins display various degrees of perfection. On the clean unweathered walls of some crevasses, and in the channels worn in the ice by glacier-streams, they are most distinctly seen, and are often exquisitely beautiful. They are not to be regarded as a partial phenomenon, or as affecting the constitution of glaciers to a small extent merely. A large portion of the ice of some glaciers is thus affected. The greater part, for example, of the Mer de Glace consists of this laminated ice; and the whole of the Glacier of the Rhone, from the base of the ice-cascade downwards, is composed of ice of the same description.

[Sidenote: GROOVES ON THE SURFACE OF GLACIERS.]

Those who have ascended Snowdon, or wandered among the hills of Cumberland, or even walked in the environs of Leeds, Blackburn, and other towns in Yorkshire and Lancashire, where the stratified sandstone of the district is used for building purposes, may have observed the weathered edges of the slate rocks or of the building-stone to be grooved and furrowed. Some laminae of such rocks withstand the action of the atmosphere better than others, and the more resistant ones stand out in ridges after the softer parts between them have been eaten away. An effect exactly similar is observed where the laminated ice of glaciers is exposed to the action of the sun and air. Little grooves and ridges are formed upon its surface, the more resistant plates protruding after the softer material between them has been melted away.

One consequence of this furrowing is, that the light dirt scattered by the winds over the surface of the glacier is gradually washed into the little grooves, thus forming fine lines resembling those produced by the passage of a rake over a sanded walk. These lines are a valuable index to some of the phenomena of motion. From a position on the ice of the Glacier du Geant a little higher up than Trelaporte a fine view of these superficial groovings is obtained; but the dirt-lines are not always straight. A slight power of independent motion is enjoyed by the separate parts into which a glacier is divided by its crevasses and dislocations, and hence it is, that, at the place alluded to, the dirt-lines are bent hither and thither, though the ruptures of continuity are too small to affect materially the general direction of the structure. On the glacier of the Talefre I found these groovings useful as indicating the character of the forces to which the ice near the summit of the fall is subjected. The ridges between the chasms are in many cases violently bent and twisted, while the adjacent groovings enable us to see the normal position of the mass.

[Sidenote: GUYOT'S OBSERVATIONS.]

The veined structure has been observed by different travellers; but it was probably first referred to by Sir David Brewster, who noticed the veins of the Mer de Glace on the 10th of September, 1814. It was also observed by General Sabine,[A] by Rendu, by Agassiz, and no doubt by many others; but the first clear description of it was given by M. Guyot, in a communication presented to the Geological Society of France in 1838. I quote the following passage from this paper:--"I saw under my feet the surface of the entire glacier covered with regular furrows from one to two inches wide, hollowed out in a half snowy mass, and separated by protruding plates of harder and more transparent ice. It was evident that the mass of the glacier here was composed of two sorts of ice, one that of the furrows, snowy and more easily melted; the other that of the plates, more perfect, crystalline, glassy, and resistant; and that the unequal resistance which the two kinds of ice presented to the atmosphere was the cause of the furrows and ridges. After having followed them for several hundreds of yards, I reached a fissure twenty or thirty feet wide, which, as it cut the plates and furrows at right angles, exposed the interior of the glacier to a depth of thirty or forty feet, and gave a beautiful transverse section of the structure. As far as my vision could reach I saw the mass of the glacier composed of layers of snowy ice, each two of which were separated by one of the plates of which I have spoken, the whole forming a regularly laminated mass, which resembled certain calcareous slates."

[Sidenote: FORBES'S RESEARCHES.]

Previous observers had mistaken the lamination for stratification; but M. Guyot not only clearly saw that they were different, but in the comparison which he makes he touches, I believe, on the true cause of the glacier-structure. He did not hazard an explanation of the phenomenon, and I believe his memoir remained unprinted. In 1841 the structure was noticed by Professor Forbes during his visit to M. Agassiz on the lower Aar Glacier, and described in a communication presented by him to the Royal Society of Edinburgh. He subsequently devoted much time to the subject, and his great merit in connexion with it consists in the significance which he ascribed to the phenomenon when he first observed it, and in the fact of his having proved it to be a constitutional feature of glaciers in general.

[Sidenote: FORBES'S THEORY.]

The first explanation given of those veins by Professor Forbes was, that they were small fissures formed in the ice by its motion; that these were filled with the water of the melted ice in summer, which froze in winter so as to form the blue veins. This is the explanation given in his 'Travels,' page 377; and in a letter published in the 'Edinburgh New Philosophical Journal,' October, 1844, it is re-affirmed in these words:--"With the abundance of blue bands before us in the direction in which the differential motion must take place (in this case sensibly parallel to the sides of the glacier), it is impossible to doubt that these infiltrated crevices (for such they undoubtedly are) have this origin." This theory was examined by Mr. Huxley and myself in our joint paper; but it has been since alleged that ours was unnecessary labour, Prof. Forbes himself having in his Thirteenth Letter renounced the theory, and substituted another in its place. The latter theory differs, so far as I can understand it, from the former in this particular, that the _freezing of the water_ in the fissures is discarded, their sides being now supposed to be united "by the simple effects of time and cohesion."[B] For a statement of the change which his opinions have undergone, I would refer to the Prefatory Note which precedes the volume of 'Occasional Papers' recently published by Prof. Forbes; but it would have diminished my difficulty had the author given, in connexion with his new volume, a more distinct statement of his present views regarding the veined structure. With many of his observations and remarks I should agree; with many others I cannot say whether I agree or not; and there are others still with which I do not think I should agree: but in hardly any case am I certain of his precise views, excepting, indeed, the cardinal one, wherein he and others agree in ascribing to the structure a different origin from stratification. Thus circumstanced, my proper course, I think, will be to state what I believe to be the cause of the structure, and leave it to the reader to decide how far our views harmonize; or to what extent either of them is a true interpretation of nature.

[Sidenote: USUAL ASPECT OF BLUE VEINS.]

Most of the earlier observers considered the structure to be due to the stratification of the mountain-snows--a view which has received later development at the hands of Mr. John Ball; and the practical difficulty of distinguishing the undoubted effects of _stratification_ from the phenomena presented by _structure_, entitles this view to the fullest consideration. The blue veins of glaciers are, however, not always, nor even generally, such as we should expect to result from stratification. The latter would furnish us with distinct planes extending parallel to each other for considerable distances through the glacier; but this, though sometimes the case, is by no means the general character of the structure. We observe blue streaks, from a few inches to several feet in length, upon the walls of the same crevasse, and varying from the fraction of an inch to several inches in thickness. In some cases the streaks are definitely bounded, giving rise to an appearance resembling the section of a lens, and hence called the "lenticular structure" by Mr. Huxley and myself; but more usually they fade away in pale washy streaks through the general mass of the whitish ice. In Fig. 39 I have given a representation of the structure as it is very commonly exhibited on the walls of crevasses. Its aspect is not that which we should expect from the consolidation of successive beds of mountain snow.

Further, at the bases of ice-cascades the structural laminae are usually _vertical_: below the cascade of the Talefre, of the Noire, of the Strahleck branch of the Lower Grindelwald Glacier, of the Rhone, and other ice-falls, this is the case; and it seems extremely difficult to conceive that a mass horizontally stratified at the summit of the fall, should, in its descent, contrive to turn its strata perfectly on end.

Again, we often find a very feebly-developed structure at the central portions of a glacier, while the lateral portions are very decidedly laminated. This is the case where the inclination of the glacier is nearly uniform throughout; and where no medial moraines occur to complicate the phenomenon. But if the veins mark the bedding, there seems to be no sufficient reason for their appearance at the lateral portions of the glacier, and their absence from the centre.

[Sidenote: ILLUSTRATIVE EXPERIMENTS.]

This leads me to the point at which what I consider to be the true cause of the structure may be referred to. The theoretic researches of Mr. Hopkins have taught us a good deal regarding the pressures and tensions consequent upon glacier-motion. Aided by this knowledge, and also by a mode of experiment first introduced by Professor Forbes, I will now endeavour to explain the significance of the fact referred to in the last paragraph. If a plastic substance, such as mud, flow down a sloping canal, the lateral portions, being held back by friction, will be outstripped by the central ones. When the flow is so regulated that the velocity of a point at the centre shall not vary throughout the entire length of the canal, a coloured circle stamped upon the centre of the mud stream, near its origin, will move along with the mud, and still retain its circular form; for, inasmuch as the velocity of all points along the centre is the same, there can be no elongation of the circle longitudinally or transversely by either strain or pressure. A similar absence of longitudinal pressure may exist in a glacier, and, where it exists throughout, no central structure can, in my opinion, be developed.

But let a circle be stamped upon the mud-stream near its side, then, when the mud flows, this circle will be distorted to an oval, with its major axis oblique to the direction of motion; the cause of this is that the portion of the circle farthest from the side of the canal moves more freely than that adjacent to the side. The mechanical effect of the slower lateral motion is to squeeze the circle in one direction, and draw it out in the perpendicular one.

[Sidenote: MARGINAL STRUCTURE.]

A glance at Fig. 40 will render all that I have said intelligible. The three circles are first stamped on the mud in the same transverse line; but after they have moved downwards they will be in the same straight line no longer. The central one will be the foremost; while the lateral ones have their forms changed from circles to ovals. In a glacier of the shape of this canal exactly similar effects are produced. Now the shorter axis _m n_ of each oval is a line of squeezing or pressure; the longer axis is a line of strain or tension; and the associated glacier-phenomena are as follows:--Across the line _m n_, or perpendicular to the pressure, we have the _veined structure_ developed, while across the line of tension the glacier usually breaks and forms _marginal crevasses_. Mr. Hopkins has shown that the lines of greatest pressure and of greatest strain are at right angles to each other, and that in valleys of a uniform width they enclose an angle of forty-five degrees with the side of the glacier. To the structure thus formed I have applied the term _marginal structure_. Here, then, we see that there are mechanical agencies at work near the side of such a glacier which are absent from the centre, and we have effects developed--I believe _by the pressure_--in the lateral ice, which are not produced in the central.

I have used the term "uniform inclination" in connexion with the marginal structure, and my reason for doing so will now appear. In many glaciers the structure, instead of being confined to the margins, sweeps quite across them. This is the case, for example, on the Glacier du Geant, the structure of which is prolonged into the Mer de Glace. In passing the strait at Trelaporte, however, the curves are squeezed and their apices bruised, so that the structure is thrown into a state of confusion; and thus upon the Mer de Glace we encounter difficulty in tracing it fairly from side to side. Now the key to this transverse structure I believe to be the following: Where the inclination of the glacier suddenly changes from a steep slope to a gentler, as at the bases of the "cascades,"--the ice to a certain depth must be thrown into a state of violent longitudinal compression; and along with this we have the resistance which the gentler slope throws athwart the ice descending from the steep one. At such places a structure is developed transverse to the axis of the glacier, and likewise transverse to the pressure. The quicker flow of the centre causes this structure to bend more and more, and after a time it sweeps in vast curves across the entire glacier.

[Sidenote: STRUCTURE OF GRINDELWALD GLACIER.]

In illustration of this point I will refer, in the first place, to that tributary of the Lower Glacier of Grindelwald which descends from the Strahleck. Walking up this tributary we come at length to the base of an ice-fall. Let the observer here leave the ice, and betake himself to either side of the flanking mountain. On attaining a point which commands a view both of the fall and of the glacier below it, an inspection of the glacier will, I imagine, solve to his satisfaction the case of structure now under consideration.

It is indeed a grand experiment which Nature here submits to our inspection. The glacier descending from its _neve_ reaches the summit of the cascade, and is broken transversely as it crosses the brow; it afterwards descends the fall in a succession of cliffy ice-ridges with transverse hollows between them. In these latter the broken ice and debris collect, thus partially choking the fissures formed in the first instance. Carrying the eye downwards along the fall, we see, as we approach the base, these sharp ridges toned down; and a little below the base they dwindle into rounded protuberances which sweep in curves quite across the glacier. At the base of the fall the structure begins to appear, feebly at first, but becoming gradually more pronounced, until, at a short distance below the base of the fall, the eye can follow the fine superficial groovings from side to side; while at the same time the ice underneath the surface has become laminated in the most beautiful manner.

It is difficult to convey by writing the force of the evidence which the actual observation of this natural experiment places before the mind. The ice at the base of the fall, retarded by the gentler inclination of the valley, has to bear the thrust of the descending mass, the sudden change of inclination producing powerful longitudinal compression. The protuberances are squeezed more closely together, the hollows between them appear to wrinkle up in submission to the pressure--in short, the entire aspect of the glacier suggests the powerful operations of the latter force. At the place where _it_ is exerted the veined structure makes its appearance; and being once formed, it moves downwards, and gives a character to other portions of the glacier which had no share in its formation.

[Sidenote: BASE OF CASCADE A "STRUCTURE-MILL."]

An illustration almost as good, and equally accessible, is furnished by the Glacier of the Rhone. I have examined the grand cascade of this glacier from both sides; and an ordinary mountaineer will find little difficulty in reaching a point from which the fall and the terminal portion of the glacier are both distinctly visible. Here also he will find the cliffy ridges separated from each other by transverse chasms, becoming more and more subdued at the bottom of the fall, and disappearing entirely lower down the glacier. As in the case of the Grindelwald Glacier the squeezing of the protuberances and of the spaces between them, is quite apparent, and where this squeezing commences the transverse structure makes its appearance. All the ice that forms the lower portion of this glacier has to pass through the _structure-mill_ at the bottom of the fall, and the consequence is that _it is all laminated_.

[Sidenote: STRUCTURE OF RHONE GLACIER.]

[Sidenote: TRANSVERSE STRUCTURE.]

This case of structural development will be better appreciated on reference to Figs. 41 and 42, the former of which is a plan, and the latter a section, of a part of the ice-fall and of the glacier below it; _a b e f_ is the gorge of the fall, _f b_ being the base. The transverse cliffy ice-ridges are shown crossing the cascade, being subdued at the base to protuberances which gradually disappear as they advance downwards. The structure sweeps over the glacier in the direction of the fine curved lines; and I have also endeavoured to show the direction of the radial crevasses, which, in the centre at least, are at right angles to the veins. To the manifestation of structure here considered I have, for the sake of convenient reference, applied the term _transverse structure_.

A third exhibition of the structure is now to be noticed. We sometimes find it in the _middle_ of a glacier and running _parallel_ to its length. On the centre of the ice-fall of the Talefre, for example, we have a structure of this kind which preserves itself parallel to the axis of the fall from top to bottom. But we discover its origin higher up. The structure here has been produced at the extremity of the Jardin, where the divided ice meets, and not only brings into partial parallelism the veins previously existing along the sides of the Jardin, but develops them still further by the mutual pressure of the portions of newly welded ice. Where two tributary glaciers unite, this is perhaps without exception the case. Underneath the moraine formed by the junction of the Talefre and Lechaud the structure is finely developed, and the veins run in the direction of the moraine. The same is true of the ice under the moraine formed by the junction of the Lechaud and Geant. These afterwards form the great medial moraines of the Mer de Glace, and hence the structure of the trunk-stream underneath these moraines is parallel to the direction of the glacier. This is also true of the system of moraines formed by the glaciers of Monte Rosa. It is true in an especial manner of the lower glacier of the Aar, whose medial moraine perhaps attains grander proportions than any other in the Alps, and underneath which the structure is finely developed.

[Sidenote: LONGITUDINAL STRUCTURE.]

The manner in which I have illustrated the production of this structure will be understood from Fig. 43. B B are two wooden boxes, communicating by sluice-fronts with two branch canals, which unite to a common trunk at G. They are intended to represent respectively the trunk and tributaries of the Unteraar Glacier, the part G being the Abschwung, where the Lauteraar and Finsteraar glaciers unite to form the Unteraar. The mud is first permitted to flow beneath the two sluices until it has covered the bottom of the trough for some distance, when it is arrested. The end of a glass tube is then dipped into a mixture of rouge and water, and small circles are stamped upon the mud. The two branches are thickly covered with these circles. The sluices being again raised, the mud in the branches moves downwards, carrying with it the circles stamped upon it; and the manner in which these circles are distorted enables us to infer the strains and pressures to which the mud is subjected during its descent. The figure represents approximately what takes place. The side-circles, as might be expected, are squeezed to oblique ovals, but it is at the junction of the branches that the chief effect of pressure is produced. Here, by the mutual thrust of the branches, the circles are not only changed to elongated ellipses, but even squeezed to straight lines. In the case of the glacier this is the region at which the structure receives its main development. To this manifestation of the veins I have applied the term _longitudinal structure_.

The three main sources of the blue veins are, I think, here noted; but besides these there are many local causes which influence their production. I have seen them well formed where a glacier is opposed by the sudden bend of a valley, or by a local promontory which presents an obstacle sufficient to bring the requisite pressure into play. In the glaciers of the Tyrol and of the Oberland I have seen examples of this kind; but the three principal sources of the veins are, I think, those stated above.

[Sidenote: EFFORTS TO SOLVE QUESTION.]

It was long before I cleared my mind of doubt regarding the origin of the lamination. When on the Mer de Glace in 1857 I spared neither risk nor labour to instruct myself regarding it. I explored the Talefre basin, its cascade, and the ice beneath it. Several days were spent amid the ice humps and cliffs at the lower portion of the fall. I suppose I traversed the Glacier du Geant twenty times, and passed eight or ten days amid the confusion of its great cascade. I visited those places where, it had been affirmed, the veins were produced. I endeavoured to satisfy myself of the mutability which had been ascribed to them; but a close examination reduced the value of each particular case so much that I quitted the glacier that year with nothing more than an _opinion_ that the structure and the stratification were two different things. I, however, drew up a statement of the facts observed, with the view of presenting it to the Royal Society; but I afterwards felt that in thus acting I should merely swell the literature of the subject without adding anything certain. I therefore withheld the paper, and resolved to devote another year to a search among the chief glaciers of the Oberland, of the Canton Valais, and of Savoy, for proofs which should relieve my mind of all doubt upon the subject.

[Sidenote: EXPEDITION FOR THIS PURPOSE.]

Accordingly in 1858 I visited the glaciers of Rosenlaui, Schwartzwald, Grindelwald, the Aar, the Rhone, and the Aletsch, to the examination of which latter I devoted more than a week. I afterwards went to Zermatt, and, taking up my quarters at the Riffelberg, devoted eleven days to the examination of the great system of glaciers of Monte Rosa. I explored the Goerner Glacier up almost to the Cima de Jazzi; and believed that in it I could trace the structure from portions of the glacier where it vanished, through various stages of perfection, up to its full development. I believe this still; but yet it is nothing but a belief, which the utmost labour that I could bestow did not raise to a certainty. The Western glacier of Monte Rosa, the Schwartze Glacier, the Trifti Glacier, the glacier of the little Mont Cervin, and of St. Theodule, were all examined in connexion with the great trunk-stream of the Goerner, to which they weld themselves; and though the more I pursued the subject the stronger my conviction became that pressure was the cause of the structure, a crucial case was still wanting.

In the phenomena of slaty cleavage, it is often, if not usually, found that the true cleavage _cuts_ the planes of stratification--sometimes at a very high angle. Had this not been proved by the observations of Sedgwick and others, geologists would not have been able to conclude that cleavage and bedding were two different things, and needed wholly different explanations. My aim, throughout the expedition of 1858, was to discover in the ice a parallel case to the above; to find a clear and undoubted instance where the veins and the stratification were simultaneously exhibited, cutting each other at an unmistakable angle. On the 6th of August, while engaged with Professor Ramsay upon the Great Aletsch Glacier, not far from its junction with the Middle Aletsch, I observed what appeared to me to be the lines of bedding running nearly horizontal along the wall of a great crevasse, while cutting them at a large angle was the true veined structure. I drew my friend's attention to the fact, and to him it appeared perfectly conclusive. It is from a sketch made by him at the place that Fig. 44 has been taken.

[Sidenote: CASE OF STRUCTURE ON THE ALETSCH.]

This was the only case of the kind which I observed upon the Aletsch Glacier; and as I afterwards spent day after day upon the Monte Rosa glaciers, vainly seeking a similar instance, the thought again haunted me that we might have been mistaken upon the Aletsch. In this state of mind I remained until the 18th of August, a day devoted to the examination of the Furgge Glacier, which lies at the base of the Mont Cervin.

[Sidenote: STRUCTURE OF THE FURGGE GLACIER.]

Crossing the valley of the Goerner Glacier, and taking a plunge as I passed into the Schwarze See, I reached, in good time, the object of my day's excursion. Walking up the glacier, I at length found myself opposed by a frozen cascade composed of four high terraces of ice. The highest of these was chiefly composed of ice-cliffs and _seracs_, many of which had fallen, and now stood like rocking-stones upon the edge of the second terrace. The glacier at the base of the cascade was strewn with broken ice, and some blocks two hundred cubic feet in volume had been cast to a considerable distance down the glacier.

Upon the faces of the terraces the stratification of the _neve_ was most beautifully shown, running in parallel and horizontal lines along the weathered surface. The snow-field above the cascade is a frozen plain, smooth almost as a sheltered lake. The successive snow-falls deposit themselves with great regularity, and at the summit of the cascade the sections of the _neve_ are for the first time exposed. Hence their peculiar beauty and definition.

[Sidenote: ICE TERRACE EXAMINED.]

Indeed the figure of a lake pouring itself over a rocky barrier which curves convexly upwards, thus causing the water to fall down it, not only longitudinally over the vertex of the curve, but laterally over its two arms, will convey a tolerably correct conception of the shape of the fall. Towards the centre the ice was powerfully squeezed laterally, the beds were bent, and their continuity often broken by faults. On inspecting the ice from a distance with my opera glass, I thought I saw structural groovings cutting the strata at almost a right angle. Had the question been an undisputed one, I should perhaps have felt so sure of this as not to incur the danger of pushing the inquiry further; but, under the circumstances, danger was a secondary point. Resigning, therefore, my glass to my guide, who was to watch the tottering blocks overhead, and give me warning should they move, I advanced to the base of the fall, removed with my hatchet the weathered surface of the ice, and found underneath it the true veined structure, cutting, at nearly a right angle, the planes of stratification. The superficial groovings were not uniformly distributed over the fall, but appeared most decided at those places where the ice appeared to have been most squeezed. I examined three or four of these places, and in each case found the true veins nearly vertical, while the bedding was horizontal. Having perfectly satisfied myself of these facts, I made a speedy retreat, for the ice-blocks seemed most threatening, and the sunny hour was that at which they fall most frequently.

I next tried the ascent of the glacier up a dislocated declivity to the right. The ice was much riven, but still practicable. My way for a time lay amid fissures which exposed magnificent sections, and every step I took added further demonstration to what I had observed below. The strata were perfectly distinct, the structure equally so, and one crossed the other at an angle of seventy or eighty degrees. Mr. Sorby has adduced a case of the crumpling of a bed of sandstone through which the cleavage passes: here on the glacier I had parallel cases; the beds were bent and crumpled, but the structure ran through the ice in sharp straight lines. This perhaps was the most pleasant day I ever spent upon the glaciers: my mind was relieved of a long brooding doubt, and the intellectual freedom thus obtained added a subjective grandeur to the noble scene before me. Climbing the cliffs near the base of the Matterhorn, I walked along the rocky spine which extends to the Hoernli, and afterwards descended by the valley of Zmutt to Zermatt.

A year after my return to England a remark contained in Professor Mousson's interesting little work 'Die Gletscher der Jetzzeit' caused me to refer to the atlas of M. Agassiz's 'Systeme Glaciaire,' from which I learned that this indefatigable observer had figured a case of stratification and structure cutting each other. If, however, I had seen this figure beforehand, it would not have changed my movements; for the case, as sketched, would not have convinced me. I have now no doubt that M. Agassiz has preceded me in this observation, and hence my results are to be taken as mere confirmations of his.

[Sidenote: LAMINATION AND STRATIFICATION.]

Fig. 45 represents a crumpled portion of the ice with the lines of lamination passing through the strata. Fig. 46 represents a case where a fault had occurred, the veins at both sides of the line of dislocation being inclined towards each other.

[Figs. 45 and 46 are from sketches made on the Furgge Glacier.--L. C. T.]

FOOTNOTES:

[A] In reply to a question in connexion with this subject, General Sabine has favoured me with the following note:--

"My dear Tyndall,

"It was in the summer of 1841, at the Lower Grindelwald Glacier, that I first saw, and was greatly impressed and interested by examining and endeavouring to understand (in which I did not succeed), the veined structure of the ice. I do not remember when I mentioned it to Forbes, but it must be before 1843, because it is noticed in his book, p. 29. I had never observed it in the glaciers of Spitzbergen or Baffin's Bay, or in the icebergs of the shores and straits of Davis or Barrow. I feel the more confident of this, because, when I first saw the veined structure in Switzerland, my Arctic experience was more fresh in my recollection, and I recollected nothing like it.

"_Veins_ are indeed not uncommon in icebergs, but they quite resemble veins in rocks, and are formed by water filling fissures and freezing into blue ice, finely contrasted with the white granular substance of the berg.

"The ice of the Grindelwald Glacier (where I examined the veined structure) was broken up into very large masses, which by pressure had been upturned, so that a very poor judgment would be formed of the direction of the veins as they existed in the glacier before it had broken up.

"Sincerely yours, "EDWARD SABINE.

"_Feb. 20, 1860_."

[B] In a letter to myself, published in the 17th volume of the 'Philosophical Magazine,' Professor Forbes writes as follows:--"In 1846, then, I abandoned no part of the theory of the veined structure, on which as you say so much labour had been expended, except the admission, always yielded with reluctance, and got rid of with satisfaction, that the congelation of water in the crevices of the glacier may extend in winter to a great depth."

THE VEINED STRUCTURE AND THE DIFFERENTIAL MOTION.

(28.)

[Sidenote: DIFFERENTIAL MOTION GREATEST AT EDGES.]

I have now to examine briefly the explanation of the structure which refers it to differential motion--to a sliding of the particles of ice past each other, which leaves the traces of its existence in the blue veins. The fact is emphatically dwelt upon by those who hold this view, that the structure is best developed nearest to the sides of the glacier, where the differential motion is greatest. Why the differential motion is at its maximum near to the sides is easily understood. Let A B, C D, Fig. 47, represent the two sides of a glacier, moving in the direction of the arrow, and let _m a b c n_ be a straight line of stakes set out across the glacier to-day. Six months hence this line, by the motion of the ice downwards, will be bent to the form _m a' b' c' n_: this curve will not be circular, it will be flattened in the middle; the points _a_ and _c_, at some distance on each side of the centre _b_, move in fact with nearly the same velocity as the centre itself. Not so with the sides:--_a'_ and _c'_ have moved considerably in advance of _m_ and _n_, and hence we say that the difference of motion, or the differential motion, of the particles of ice near to the side is a maximum.

During all this time the points _m a' b' c' n_ have been moving straight down the glacier; and hence it will be understood that the sliding of the parts past each other, or, in other words, the differential motion, _is parallel to the sides of the glacier_. This, indeed, is the only differential motion that experiment has ever established; and consequently, when we find the best blue veins referred to the sides of the glacier because the differential motion is there greatest, we naturally infer that the motion meant is parallel to the sides.

[Sidenote: STRUCTURE OBLIQUE TO SIDES.]

But the fact is, that this motion would not at all account for the blue veins, for they are not parallel to the sides, but _oblique_ to them. This difficulty revealed itself after a time to those who first propounded the theory of differential motion, and caused them to modify their explanation of the structure. Differential motion is still assumed to be the cause of the veins, but now a motion is meant oblique to the sides, and it is supposed to be obtained in the following way:--Through the quicker motion of the point _c'_ the ice between it and _n_ becomes distended; that is to say, the line _c' n_ is in a state of strain--there is a _drag_, it is said, oblique to the sides of the glacier; and it is therefore in this direction that the particles will be caused to slide past each other. Dr. Whewell, who advocates this view, thus expounds it. He supposes the case of an alpine valley filled with india-rubber which has been warmed until it has partially melted, or become viscous, and then asks, "What will now be the condition of the mass? The sides and bottom will still be held back by the friction; the middle and upper part will slide forwards, but not freely. This want of freedom in the motion (arising from the viscosity) will produce a drag towards the middle of the valley, where the motion is freest; hence the direction in which the filaments slide past each other will be obliquely directed towards the middle. The sliding will separate the mass according to such lines; and though new attachments will take place, the mass may be expected to retain the results of this separation in the traces of parallel fissures."[A] Nothing can be clearer than the image of the process thus placed before the mind's eye.

One fact of especial importance is to be borne in mind: the sliding of filaments which is thus supposed to take place oblique to the glacier has never been proved; it is wholly assumed. A moraine, it is admitted, will run parallel to the side of a glacier, or a block will move in the same direction from beginning to end, without being sensibly drawn towards the centre, but still it is supposed that the sliding of parts exists, though of a character so small as to render it insensible to measurement.

[Sidenote: STRUCTURE CROSSES LINES OF SLIDING.]

My chief difficulty as regards this theory may be expressed in a very few words. If the structure be produced by differential motion, why is the large and _real_ differential motion which experiments have established incompetent to produce it? And how can the veins run, as they are admitted to do, _across the lines of maximum sliding_ from their origin throughout the glacier to its end?

That a drag towards the centre of the glacier exists is undeniable, but that in consequence of the drag there is a sliding of filaments in this direction, is quite another thing. I have in another place[B] endeavoured to show experimentally that no such sliding takes place, that the drag on any point towards the centre expresses only half the conditions of the problem; being exactly neutralized by the thrust towards the sides. It has been, moreover, shown by Mr. Hopkins that the lines of maximum strain and of maximum sliding cannot coincide; indeed, if all the particles be urged by the same force, no matter how strong the pull may be, there will be no tendency of one to slide past the other.

FOOTNOTES:

[A] 'Philosophical Magazine,' Ser. III., vol. xxvi.

[B] 'Proceedings of the Royal Institution,' vol. ii. p. 324.

THE RIPPLE-THEORY OF THE VEINED STRUCTURE.

(29.)

[Sidenote: THEORY STATED.]

[Sidenote: THEORY EXAMINED.]

The assumption of oblique sliding, and the production thereby of the marginal structure, have, however, been fortified by considerations of an ingenious and very interesting kind. "How," I have asked, "can the oblique structure persist across the lines of greatest differential motion throughout the length of the glacier?" But here I am met by another question which at first sight might seem equally unanswerable--"How do ripple-marks on the surface of a flowing river, which are nothing else than lines of differential motion of a low order, cross the river from the sides obliquely, while the direction of greatest differential motion is parallel to the sides?" If I understand aright, this is the main argument of Professor Forbes in favour of his theory of the oblique marginal structure. It is first introduced in a note at page 378 of his 'Travels;' he alludes to it in a letter written the following year; in his paper in the 'Philosophical Transactions' he develops the theory. He there gives drawings of ripple-marks observed in smooth gutters after rain, and which he finds to be inclined to the course of the stream, exactly as the marginal structure is inclined to the side of the glacier. The explanation also embraces the case of an obstacle placed in the centre of a river. "A case," writes Professor Forbes, "parallel to the last mentioned, where a fixed obstacle cleaves a descending stream, and leaves its trace in a fan-shaped tail, is well known in several glaciers, as in that at Ferpecle, and the Glacier de Lys on the south side of Monte Rosa; particularly the last, where the veined structure follows the law just mentioned." In his Twelfth Letter he also refers to the ripples "as exactly corresponding to the position of the icy bands." In his letter to Dr. Whewell, published in the 'Occasional Papers,' page 58, he writes as follows:--"The same is remarkably shown in the case of a stream of water, for instance a mill-race. Although the movement of the water, as shown by floating bodies, is exceedingly nearly (for small velocities sensibly) parallel to the sides, yet the variation of the speed from the side to the centre of the stream occasions a _ripple_, or molecular discontinuity, which inclines forwards from the sides to the centre of the stream at an angle with the axis depending on the ratio of the central and lateral velocity. The veined structure of the ice corresponds to the ripple of the water, a molecular discontinuity whose measure is not comparable to the actual velocity of the ice; and therefore the general movement of the glacier, as indicated by the moraines, remains sensibly parallel to the sides." This theory opens up to us a series of interesting and novel considerations which I think will repay the reader's attention. If the ripples in the water and the veins in the ice be due to the same mechanical cause, when we develop clearly the origin of the former we are led directly to the explanation of the latter. I shall now endeavour to reduce the ripples to their mechanical elements.

The Messrs. Weber have described in their 'Wellenlehre' an effect of wave-motion which it is very easy to obtain. When a boat moves through perfectly smooth water, and the rower raises his oar out of the water, drops trickle from its blade, and each drop where it falls produces a system of concentric rings. The circular waves as they widen become depressed, and, if the drops succeed each other with sufficient speed, the rings cross each other at innumerable points. The effect of this is to blot out more or less completely all the circles, and to leave behind two straight divergent ripple-lines, which are tangents to all the external rings; being in fact formed by the intersections of the latter, as a caustic in optics is formed by the intersection of luminous rays. Fig. 48, which is virtually copied from M. Weber, will render this description at once intelligible. The boat is supposed to move in the direction of the arrow, and as it does so the rings which it leaves behind widen, and produce the divergence of the two straight resultant lines of ripple.

[Sidenote: RIPPLES DEDUCED FROM RINGS.]

The more quickly the drops succeed each other, the more frequent will be the intersections of the rings; but as the speed of succession augments we approach the case of _a continuous vein_ of liquid; and if we suppose the continuity to be perfectly established, the ripples will still be produced with a smooth space between them as before. This experiment may indeed be made with a well-wetted oar, which on its first emergence from the water sends into it a continuous liquid vein. The same effect is produced when we substitute for the stream of liquid a solid rod--a common walking-stick for example. A water-fowl swimming in calm water produces two divergent lines of ripples of a similar kind.

We have here supposed the water of the lake to be at rest, and the liquid vein or the solid rod to move through it; but precisely the same effect is produced if we suppose the rod at rest and the liquid in motion. Let a post, for example, be fixed in the middle of a flowing river; diverging from that post right and left we shall have lines of ripples exactly as if the liquid were at rest and the post moved through it with the velocity of the river. If the same post be placed close to the bank, so that _one_ of its edges only shall act upon the water, diverging from that edge we shall have a _single_ line of ripples which will cross the river obliquely towards its centre. It is manifest that any other obstacle will produce the same effect as our hypothetical post. In the words of Professor Forbes, "the slightest prominence of any kind in the wall of such a conduit, a bit of wood or a tuft of grass, is sufficient to produce a well-marked ripple-streak from the side towards the centre."

[Sidenote: MEASURE OF DIVERGENCE OF RIPPLES.]

The foregoing considerations show that the divergence of the two lines of ripples from the central post, and of the single line in the case of the lateral post, have their mechanical element, if I may use the term, in the experiment of the Messrs. Weber. In the case of a swimming duck the connexion between the diverging lines of ripples and the propagation of rings round a disturbed point is often very prettily shown. When the creature swims with vigour the little foot with which it strikes the water often comes sufficiently near to the surface to produce an elevation,--sometimes indeed emerging from the water altogether. Round the point thus disturbed rings are immediately propagated, and the widening of those rings is _the exact measure of the divergence of the ripple lines_. The rings never cross the lines;--the lines never retreat from the rings.

[Sidenote: RIPPLES AND VEINS DUE TO DIFFERENT CAUSES.]

If we compare the mechanical actions here traced out with those which take place upon a glacier, I think it will be seen that the analogy between the ripples and the veined structure is entirely superficial. How the structure ascribed to the Glacier de Lys is to be explained I do not know, for I have never seen it; but it seems impossible that it could be produced, as ripples are, by a fixed obstacle which "cleaves a descending stream." No one surely will affirm that glacier-ice so closely resembles a fluid as to be capable of transmitting undulations, as water propagates rings round a disturbed point. The difficulty of such a supposition would be augmented by taking into account the motion of the _individual liquid particles_ which go to form a ripple; for the Messrs. Weber have shown that these move in closed curves, describing orbits more or less circular. Can it be supposed that the particles of ice execute a motion of this kind? If so, their orbital motions may be easily calculated, being deducible from the motion of the glacier compounded with the inclination of the veins. If so important a result could be established, all glacier theories would vanish in comparison with it.

[Sidenote: POSITION OF RIPPLES NOT THAT OF STRUCTURE.]

There is another interesting point involved in the passage above quoted. Professor Forbes considers that the ripple is occasioned by the variation of speed from the side to the centre of the stream, and that its _inclination_ depends on the ratio of the central and lateral velocity. If I am correct in the above analysis, this cannot be the case. The inclination of the ripple depends solely on the ratio of the river's translatory motion to the velocity of its wave-motion. Were the lateral and central velocities alike, a momentary disturbance at the side would produce a _straight_ ripple-mark, whose inclination would be compounded of the two elements just mentioned. If the motion of the water vary from side to centre, the velocity of wave-propagation remaining constant, the inclination of the ripple will also vary, that is to say, we shall have a _curved_ ripple instead of a straight one. This, of course, is the case which we find in Nature, but the curvature of such ripples is totally different from that of the veined structure. Owing to the quicker translatory movement, the ripples, as they approach the centre, tend more to parallelism with the direction of the river; and after having passed the centre, and reached the slower water near the opposite side, their inclination to the axis gradually augments. Thus the ripples from the two sides form a pair of symmetric curves, which cross each other at the centre, and possess the form _a o b_, _c o d_, shown in Fig. 49. A similar pair of curves would be produced by the reflection of these. Knowing the variation of motion from side to centre, any competent mathematician could find the equation of the ripple-curves; but it would be out of place for me to attempt it here.

THE VEINED STRUCTURE AND PRESSURE.

(30.)

If a prism of glass be pressed by a sufficient weight, the particles in the line of pressure will be squeezed more closely together, while those at right angles to this line will be forced further apart. The existence of this state of strain may be demonstrated by the action of such squeezed glass upon polarised light. It gives rise to colours, and it is even possible to infer from the tint the precise amount of pressure to which the glass is subjected. M. Wertheim indeed has most ably applied these facts to the construction of a dynamometer, or instrument for measuring pressures, exceeding in accuracy any hitherto devised.

When the pressure applied becomes too great for the glass to sustain, it flies to pieces. But let us suppose the sides of the prism defended by an extremely strong jacket, in which the prism rests like a closely-fitting plug, and which yields only when a pressure more than sufficient to crush the glass is applied. Let the pressure be gradually augmented until this point is attained; afterwards both the glass and its jacket will shorten and widen; the jacket will yield laterally, being pushed out with extreme slowness by the glass within.

[Sidenote: POSSIBLE EXPERIMENT WITH GLASS PRISM.]

Now I believe that it would be possible to make this experiment in such a manner that the glass should be _flattened_, partly through rupture, and partly through lateral molecular yielding; the prism would change its form, and yet present a firmly coherent mass when removed from its jacket. I have never made the experiment; nobody has, as far as I know; but experiments of this kind are often made by Nature. In the Museum of the Government School of Mines, for example, we have a collection of quartz stones placed there by Mr. Salter, and which have been subjected to enormous pressure in the neighbourhood of a fault. These rigid pebbles have, in some cases, been squeezed against each other so as to produce mutual flattening and indentation. Some of them have yielded along planes passing through them, as if one half had slidden over the other; but the reattachment is very strong. Some of the larger stones, moreover, which have endured pressure at a particular point, are fissured radially around this point. In short, the whole collection is a most instructive example of the manner and extent to which one of the most rigid substances in Nature can yield on the application of a sufficient force.

[Sidenote: POSSIBLE EXPERIMENT WITH PRISM OF ICE.]

Let a prism of ice at 32 deg. be placed in a similar jacket to that which we have supposed to envelop the glass prism. The ice yields to the pressure with incomparably greater ease than the glass; and if the force be slowly applied, the lateral yielding will far more closely resemble that of a truly plastic body. Supposing such a piece of ice to be filled with numerous small air-bubbles, the tendency of the pressure would be to flatten these bubbles, and to squeeze them out of the ice. Were the substance perfectly homogeneous, this flattening and expulsion would take place uniformly throughout its entire mass; but I believe there is no such homogeneous substance in nature;--the ice will yield at different places, leaving between them spaces which are comparatively unaffected by the pressure. From the former spaces the air-bubbles will be more effectually expelled; and I have no doubt that the result of such pressure acting upon ice so protected would be to produce a laminated structure somewhat similar to that which it produces in those bodies which exhibit slaty cleavage.

[Sidenote: LAMINATION PRODUCED BY PRESSURE.]

[Sidenote: NO SLIDING OF FILAMENTS.]

I also think it certain that, in this lateral displacement of the particles, these must move past each other. This is an idea which I have long entertained, as the following passage taken from the paper published by Mr. Huxley and myself will prove:--"Three principal causes may operate in producing cleavage: first, the reducing of surfaces of weak cohesion to parallel planes; second, the flattening of minute cavities; and third, the weakening of cohesion by tangential action. The third action is exemplified by the state of the rails near a station where a break is habitually applied to a locomotive. In this case, while the weight of the train presses vertically, its motion tends to cause longitudinal sliding of the particles of the rail. Tangential action does not, however, necessarily imply a force of the latter kind. When a solid cylinder an inch in height is squeezed to a vertical cake a quarter of an inch in height, it is impossible, physically speaking, that the particles situated in the same vertical line shall move laterally with the same velocity; but if they do not, the cohesion between them will be weakened or ruptured. The pressure, however, will produce new contact; and if this have a cohesive value equal to that of the old contact, no cleavage from this cause can arise. The relative capacities of different substances for cleavage appear to depend in a great measure upon their different properties in this respect. In butter, for example, the new attachments are equal, or nearly so, to the old, and the cleavage is consequently indistinct; in wax this does not appear to be the case, and hence may arise in a great degree the perfection of its cleavage. The further examination of this subject promises interesting results." I would dwell upon this point the more distinctly as the advocates of differential motion may deem it to be in their favour; but it appears to me that the mechanical conceptions implied in the above passage are totally different from theirs. If they think otherwise, then it seems to me that they should change the expressions which refer the differential motion to a "drag" towards the centre, and the structure to the sliding of "filaments" past each other in consequence of this drag. Such filamentary sliding may take place in a truly viscous body, but it does not take place in ice.

In one particular the ice resembles the butter referred to in the above quotation; for its new attachments appear to be equal to the old, and this, I think, is to be ascribed to its perfect regelation. As justly pointed out by Mr. John Ball, the veined ice of a glacier, if unweathered, shows no tendency to cleave; for though the expulsion of the air-bubbles has taken place, the reattachment of the particles is so firm as to abolish all evidence of cleavage. When the ice, on the contrary, is weathered, the plates become detached, and I have often been able to split such ice into thin tablets having an area of two or three square feet.

In his Thirteenth Letter Professor Forbes throws out a new and possibly a pregnant thought in connexion with the veins. If I understand him aright--and I confess it is usually a matter of extreme difficulty with me to make sure of this--he there refers the veins, not to the expulsion of the air from the ice, but to its redistribution. The pressure produces "_lines of tearing_ in which the air is distributed in the form of regular globules." I do not know what might be made of this idea if it were developed, but at present I do not see how the supposed action could produce the blue bands; and I agree with Professor Wm. Thomson in regarding the explanation as improbable.[A]

FOOTNOTES:

[A] For an extremely ingenious view of the origin of the veined structure, I would refer to a paper by Professor Thomson, in the 'Proceedings of the Royal Society,' April, 1858.

THE VEINED STRUCTURE AND THE LIQUEFACTION OF ICE BY PRESSURE.

(31.)

I have already noticed an important fact for which we are indebted to Mr. James Thomson, and have referred to the original communications on the subject. I shall here place the physical circumstances connected with this fact before my reader in the manner which I deem most likely to interest him.

[Sidenote: INFLUENCE OF PRESSURE ON BOILING POINT.]

When a liquid is heated, the attraction of the molecules operates against the action of the heat, which tends to tear them asunder. At a certain point the force of heat triumphs, the cohesion is overcome, and the liquid boils. But supposing we assist the attraction of the molecules by applying an external pressure, the difficulty of tearing them asunder will be increased; more heat will be required for this purpose; and hence we say that the _boiling point_ of the liquid has been _elevated_ by the pressure.

[Sidenote: INFLUENCE OF PRESSURE ON FUSING POINT.]

If molten sulphur be poured into a bullet-mould, it will be found on cooling to contract, so as to leave a large hollow space in the middle of each sphere. Cast musket-bullets are thus always found to possess a small cavity within them produced by the contraction of the lead. Conceive the bullet placed within its mould and the latter heated; to produce fusion it is necessary that the sulphur or the lead should _swell_. Here, as in the case of the heated water, the tendency to expand is opposed by the attraction of the molecules; with a certain amount of heat however this attraction is overcome and the solid _melts_. But suppose we assist the molecular attraction by a suitable force applied externally, a greater amount of heat than before will be necessary to tear them asunder; and hence we say that the _fusing point_ has been _elevated_ by the pressure. This fact has been experimentally established by Messrs. Hopkins and Fairbairn, who applied to spermaceti and other substances pressures so great as to raise their points of fusion a considerable number of degrees.

Let us now consider the case of the metal bismuth. If the molten metal be poured into a bullet-mould it will _expand_ on solidifying. I have myself filled a strong cast-iron bottle with the metal, and found its expansion on cooling sufficiently great to split the bottle from neck to bottom. Hence, in order to fuse the bismuth the substance must _contract_; and it is manifest that an external pressure which tends to squeeze the molecules more closely together here _assists_ the heat instead of opposing it. Hence, to fuse bismuth under great pressure, a less amount of heat will be required than when the pressure is removed; or, in other words, the fusing point of bismuth is _lowered_ by the pressure. Now, in passing from the solid to the liquid state, _ice_, like bismuth, contracts, and if the contraction be promoted by external pressure, as shown by the Messrs. Thomson, a less amount of heat suffices to liquefy it.

[Sidenote: EXPERIMENTS.]

These remarks will enable us to understand a singular effect first obtained by myself at the close of 1856 or in January 1857, noticed at the time in the 'Proceedings of the Royal Society,' and afterwards fully described in a paper presented to the Society in December of that year. A cylinder of clear ice two inches high and an inch in diameter was placed between two slabs of box-wood, and subjected to a gradual pressure. I watched the ice in a direction perpendicular to its length, and saw cloudy lines drawing themselves across it. As the pressure continued, these lines augmented in numbers, until finally the prism presented the appearance of a crystal of gypsum whose planes of cleavage had been forced out of optical contact. When looked at obliquely it was found that the lines were merely the sections of flat dim surfaces, which lay like laminae one over the other throughout the length of the prism. Fig. 50 represents the prism as it appeared when looked at in a direction perpendicular to its axis; Fig. 51 shows the appearance when viewed obliquely.[A]

At first sight it might appear as if air had intruded itself between the separated surfaces of the ice, and to test this point I placed a cylinder two inches long and an inch wide upright in a copper vessel which was filled with ice-cold water. The ice cylinder rose about half an inch above the surface of the water. Placing the copper vessel on a slab of wood, and a second slab on the top of the cylinder of ice, the latter was subjected to the gradual action of a small hydraulic press. When the hazy surfaces were well developed in the portion of the ice above the water, the cylinder was removed and examined: the planes of rupture extended throughout the entire length of the cylinder, just as if it had been squeezed in air. I subsequently placed the ice in a stout vessel of glass, and squeezed it, as in the last experiment: the surfaces of discontinuity were seen forming _under the liquid_ quite as distinctly as in air.

To prove that the surfaces were due to compression and not to any tearing asunder of the mass by tension, the following experiment was made:--A cylindrical piece of ice, one of whose ends, however, was not parallel to the other, was placed between the slabs of wood, and subjected to pressure. Fig. 52 shows the disposition of the experiment. The effect upon the ice cylinder was that shown in Fig. 53, the surfaces being developed along that side which had suffered the pressure. On examining the surfaces by a pocket lens they resembled the effect produced upon a smooth cold surface by breathing on it.

[Sidenote: LIQUID LAYERS PRODUCED BY PRESSURE.]

The surfaces were always dim; and had the spaces been filled with air, or were they simply vacuous, the reflection of light from them would have been so copious as to render them much more brilliant than they were observed to be. To examine them more particularly I placed a concave mirror so as to throw the diffused daylight from a window full upon the cylinder. On applying the pressure dim spots were sometimes seen forming in the very middle of the ice, and these as they expanded laterally appeared to be in a state of intense motion, which followed closely the edge of each surface as it advanced through the solid ice. Once or twice I observed the hazy surfaces pioneered through the mass by dim offshoots, apparently liquid, and constituting a kind of decrystallisation. From the closest examination to which I was able to subject them, the surfaces appeared to me to be due to internal liquefaction; indeed, when the melting point of ice, having already a temperature of 32 deg., is lowered by pressure, its excess of heat must instantly be applied to produce this effect.

[Sidenote: APPLICATION TO THE VEINED STRUCTURE.]

I have already given a drawing (p. 386) showing the development of the veined structure at the base of the ice-cascade of the Rhone; and if we compare that diagram with Fig. 53 a striking similarity at once reveals itself. The ice of the glacier must undoubtedly be liquefied to some extent by the tremendous pressure to which it is here subjected. Surfaces of discontinuity will in all probability be formed, which facilitate the escape of the imprisoned air. The small quantity of water produced will be partly imbibed by the adjacent porous ice, and will be refrozen when relieved from the pressure. This action, associated with that ascribed to pressure in the last section, appears to me to furnish a complete physical explanation of the laminated structure of glacier-ice.

FOOTNOTES:

[A] This effect projected upon a screen is a most striking and instructive class experiment.

WHITE ICE-SEAMS IN THE GLACIER DU GEANT.

(32.)

[Sidenote: GENERAL APPEARANCE OF WHITE ICE-SEAMS.]

On the 28th of July, 1857, while engaged upon the Glacier du Geant, my attention was often attracted by protuberant ridges of what at first appeared to be pure white snow, but which on examination I found to be compact ice filled with innumerable round air-cells; and which, in virtue of its greater power of resistance to wasting, often rose to a height of three or four feet above the general level of the ice. As I stood amongst these ridges, they appeared detached and without order of arrangement, but looked at from a distance they were seen to sweep across the proper Glacier du Geant in a direction concentric with its dirt-bands and its veined structure. In some cases the seams were admirable indications of the relative displacement of two adjacent portions of the glacier, which were divided from each other by a crevasse. Usually the sections of a seam exposed on the opposite sides of a fissure accurately faced each other, and the direction of the seam on both sides was continuous; but at other places they demonstrated the existence of lateral faults, being shifted asunder laterally through spaces varying from a few inches to six or seven feet.

On the following day I was again upon the same glacier, and noticed in many cases the white ice-seams exquisitely honeycombed. The case was illustrative of the great difference between the absorptive power of the ice itself and of the objects which lie upon its surface. Deep cylindrical cells were produced by spots of black dirt which had been scattered upon the surface of the white ice, and which sank to a depth of several inches into the mass. I examined several sections of the veins, and in general I found that their deeper portions blended gradually with the ice on either side of them. But higher up the glacier I found that the veins penetrated only to a limited depth, and did not therefore form an integrant portion of the glacier. Figs. 54 and 55 show the sections of two of the seams which were exposed on the wall of a crevasse at some distance below the great ice-fall of the Glacier du Geant.

[Sidenote: SECTIONS OF SEAMS.]

It was at the base of the Talefre cascade that the explanation of these curious seams presented itself to me. In one of my earliest visits to this portion of the glacier I was struck by a singular disposition of the blue veins on the vertical wall of a crevasse. Fig. 56 will illustrate what I saw. The veins, within a short distance, dipped _backward_ and _forward_, like the junctions of stones used to turn an arch. In some cases I found this variation of the structure so great as to pass in a short distance from the vertical to the horizontal, as shown in Fig. 57.

[Sidenote: VARIATIONS IN "DIP" OF STRUCTURE.]

Further examination taught me that the glacier here is crumpled in a most singular manner; doubtless by the great pressure to which it is exposed. The following illustration will convey a notion of its aspect: Let one hand be laid flat upon a table, palm downwards, and let the fingers be bent until the space between the first joint and the ends of the fingers is vertical; one of the crumples to which I refer will then be represented. The ice seems bent like the fingers, and the crumples of the glacier are cut by crevasses, which are accurately typified by the spaces between the fingers. Let the second hand now be placed upon the first, as the latter is upon the table, so that the tops of the bent fingers of the second hand shall rest upon the roots of the first: two crumples would thus be formed; a series of such protuberances, with steep fronts, follow each other from the base of the Talefre cascade for some distance downwards.

On Saturday the 1st of August I ascended these rounded terraces in succession, and observed among them an extremely remarkable disposition of the structure. Fig. 58 is a section of a series of three of the crumples, on which the shading lines represent the direction of the blue veins. At the base of each protuberance I found a seam of white ice wedged firmly into the glacier, and _each of the seams marked a place of dislocation of the veins_. The white seams thinned off gradually, and finally vanished where the violent crumpling of the ice disappeared. In Fig. 59 I have sketched the wall of a crevasse, which represents what may be regarded as the incipient crumpling. The undulating line shows the contour of the surface, and the shading lines the veins. It will be observed that the direction of the veins yields in conformity with the undulation of the surface; and an augmentation of the effect would evidently result in the crumples shown in Fig. 58. The appearance of the white seams at those places where a dislocation occurred was, as far as I could observe, invariable; but in a few instances the seams were observed upon the platforms of the terraces, and also upon their slopes. The width of a seam was very irregular, varying from a few inches at some places to three or four feet at others.

[Sidenote: CRUMPLES OF THE TALEFRE.]

[Sidenote: MOULDS OF WHITE ICE-SEAMS.]

On the 3rd of August I was again at the base of the Talefre cascade, and observed a fact the significance of which had previously escaped me. The rills which ran down the ice-slopes collected at the base of each protuberance into a stream, which, at the time of my visit, had hollowed out for itself a deep channel in the ice. At some places the stream widened, at others its banks of ice approached each other, and rapids were produced; in fact, _the channels of such streams appeared to be the exact moulds of the seams of white ice_.

Instructed thus far, I ascended the Glacier du Geant on the 5th of August, and then observed on the wrinkles of this glacier the same leaning backwards and forwards of the blue veins as I had previously observed upon the Talefre. I also noticed on this day that a seam of white ice would sometimes open out into two branches, which, after remaining for some distance separate, would reunite and thus enclose a little glacier-island. At other places lateral branches were thrown off from the principal seam, thus suggesting the form of a glacier-rivulet which had been fed by tributary branches. On the 7th of August I hunted the seams still farther up the glacier; and found them at one place descending a steep ice-hill, being crossed by other similar bands, which however were far less white and compact. I followed these new bands to their origin, and found it to be a system of crevasses formed at the summit of the hill, some of which were filled with snow. Lower down the crevasses closed, and the snow thus jammed between their walls was converted into white ice. These seams, however, never attained the compactness and prominence of the larger ones which had their origin far higher up. I singled out one of the best of the latter, and traced it through all the dislocation and confusion of the ice, until I found it to terminate in a cavity filled with snow.

This was near the base of the _seracs_, and the streams here were abundant. Comparing the shapes of some of them with that of the ice-bands lower down the glacier, a striking resemblance was observed. Fig. 60 is the plan of a deep-cut channel through which a stream flowed on the day to which I now refer. Fig. 61 is the plan of a seam of white ice sketched on the same day, low down upon the glacier. Instances of this kind might be multiplied; and the result, I think, renders it certain that the white ice-seams referred to are due to the filling up of the channels of glacier-streams by snow during winter, and the subsequent compression of the mass to ice during the descent of the glacier. I have found such seams at the bases of all cascades that I have visited; and in all cases they appear to be due to the same cause. The depth to which they penetrate the glacier must be profound, or the _ablation_ of the ice must be less than what is generally supposed; for the seams formed so high up on the Glacier du Geant may be traced low down upon the trunk-stream of the Mer de Glace.[A]

[Sidenote: STREAMS AND SEAMS.]

[Sidenote: SCALING OFF BY PRESSURE.]

These observations on the white ice-seams enable us to add an important supplement to what has been stated regarding the origin of the dirt-bands of the Mer de Glace; The protuberances at the base of the cascade are due not only to the toning down of the ridges produced by the transverse fracture of the glacier at the summit of the fall, but they undergo modifications by the pressure locally exerted at its base. The state of things represented in Fig. 57 is plainly due to the partial pushing of one crumple over that next in advance of it. There seems to be a differential motion of the parts of the glacier in the same longitudinal line; showing that upon the general motion of the glacier smaller local motions are superposed. The occurrence of the seams upon the faces of the slopes seems also to prove that the pressure is competent, in some cases, to cause the bases of the protuberances to swell, so that what was once the base of a crumple may subsequently form a portion of its slope. Another interesting fact is also observed where the pressure is violent: the crumples _scale off_, bows of ice being thus formed which usually span the crumples over their most violently compressed portions. I have found this scaling off at the bases of all the cascades which I have visited, and it is plainly due to the pressure exerted at such places upon the ice.

FOOTNOTES:

[A] The more permanent seams may possibly be due to the filling of the profound crevasses of the cascade.

(33.)

[Sidenote: COMPRESSION OF GLACIER DU GEANT.]

Not only at the base of its great cascade, but throughout the greater part of its length, the Glacier du Geant is in a state of longitudinal compression. The meaning of this term will be readily understood: Let two points, for example, be marked upon the axis of the glacier; if these during its descent were drawn wider apart, it would show that the glacier was in a state of longitudinal strain or tension; if they remained at the same distance apart, it would indicate that neither strain nor pressure was exerted; whereas, if the two points approached each other, which could only be by the quicker motion of the hinder one, the existence of longitudinal compression would be thereby demonstrated.

Taking "Le Petit Balmat" with me, to carry my theodolite, I ascended the Glacier du Geant until I came near the place where it is joined by the Glacier des Periades, and whence I observed a patch of fresh green grass upon the otherwise rocky mountain-side. To this point I climbed, and made it the station for my instrument. Choosing a well-defined object at the opposite side of the glacier, I set, on the 9th of August, in the line between this object and the theodolite, three stakes, one in the centre of the glacier, and the other two at opposite sides of the centre and about 100 yards from it. This done, I descended for a quarter of a mile, when I again climbed the flanking rocks, placing my theodolite in a couloir, down which stones are frequently discharged from the end of a secondary glacier which hangs upon the heights above. Here, as before, I fixed three stakes, chiselled a mark upon the granite, so as to enable me to find the place, and regained the ice without accident. A day or two previously we had set out a third line at some distance lower down, and I was thus furnished with a succession of points along the glacier, the relative motions of which would decide whether it was _pressed_ or _stretched_ in the direction of its length. On the 10th of August Mr. Huxley joined us; and on the following day we all set out for the Glacier du Geant, to measure the progress of the stakes which I had fixed there. Hirst remained upon the glacier to measure the displacements; I shouldered the theodolite; and Huxley was my guide to the mountain-side, sounding in advance of me the treacherous-looking snow over which we had to pass.

Calling the central stake of the highest line No. 1, that of the middle line No. 2, and that of the line nearest the Tacul No. 3, the following are the spaces moved over by these three points in twenty-four hours:

Inches. Distances asunder.

No. 1 20.55 } 545 yards. No. 2 15.43 } 487 yards. No. 3 12.75

Here we have the fact which the aspect of the glacier suggested. The first stake moves five inches a day more than the second, and the second nearly three inches a day more than the third. As surmised, therefore, the glacier is in a state of longitudinal compression, whereby a portion of it 1000 yards in length is shortened at the rate of eight inches a day.

[Sidenote: STRUCTURE IN WHITE ICE-SEAMS.]

In accordance with this result, the transverse undulations of the Glacier du Geant, described in the chapter upon Dirt-Bands, _shorten_ as they descend. A series of three of them measured along the axis of the glacier on the 6th of August, 1857, gave the following respective lengths:--955 links, 855 links, 770 links, the shortest undulation being the farthest from the origin of the undulations. This glacier then constitutes a vast ice-press, and enables us to test the explanation which refers the veined structure of the ice to pressure. The glacier itself is transversely laminated, as already stated; and in many cases a structure of extreme definition and beauty is developed in the compressed snow, which constitutes the seams of white ice. In 1857 I discovered a well-developed lenticular structure in some of these seams. In 1858 I again examined them. Clearing away the superficial portions with my axe, I found, drawn through the body of the seams, long lines of blue ice of exquisite definition; in fact, I had never seen the structure so delicately exhibited. The seams, moreover, were developed in portions of the white ice which were near the _centre_ of the glacier, and where consequently filamentous sliding was entirely out of the question.

[Sidenote: PARTIAL SUMMARY.]

PARTIAL SUMMARY.

1. Glaciers are derived from mountain snow, which has been consolidated to ice by pressure.

2. That pressure is competent to convert snow into ice has been proved by experiment.

3. The power of yielding to pressure diminishes as the mass becomes more compact; but it does not cease even when the substance has attained the compactness which would entitle it to be called ice.

4. When a sufficient depth of snow collects upon the earth's surface, the lower portions are squeezed out by the pressure of the superincumbent mass. If it rests upon a slope it will yield principally in the direction of the slope, and move downwards.

5. In addition to this, the whole mass slides bodily along its inclined bed, and leaves the traces of its sliding on the rocks over which it passes, grinding off their asperities, and marking them with grooves and scratches in the direction of the motion.

6. In this way the deposit of consolidated and unconsolidated snow which covers the higher portions of lofty mountains moves slowly down into an adjacent valley, through which it descends as a true glacier, partly by sliding and partly by the yielding of the mass itself.

7. Several valleys thus filled may unite in a single valley, the tributary glaciers welding themselves together to form a trunk-glacier.

8. Both the main valley and its tributaries are often sinuous, and the tributaries must change their direction to form the trunk; the width of the valley often varies. The glacier is forced through narrow gorges, widening after it has passed them; the centre of the glacier moves more quickly than the sides, and the surface more quickly than the bottom; the point of swiftest motion follows the same law as that observed in the flow of rivers, shifting from one side of the centre to the other as the flexure of the valley changes.

9. These various effects may be reproduced by experiments on small masses of ice. The substance may moreover be moulded into vases and statuettes. Straight bars of it may be bent into rings, or even coiled into knots.

10. Ice, capable of being thus moulded, is practically incapable of being stretched. The condition essential to success is that the particles of the ice operated on shall be kept in close contact, so that when old attachments have been severed new ones may be established.

11. The nearer the ice is to its melting point in temperature, the more easily are the above results obtained; when ice is many degrees below its freezing point it is crushed by pressure to a white powder, and is not capable of being moulded as above.

12. Two pieces of ice at 32 deg. Fahr., with moist surfaces, when placed in contact freeze together to a rigid mass; this is called Regelation.

13. When the attachments of pressed ice are broken, the continuity of the mass is restored by the regelation of the new contiguous surfaces. Regelation also enables two tributary glaciers to weld themselves to form a continuous trunk; thus also the crevasses are mended, and the dislocations of the glacier consequent on descending cascades are repaired. This healing of ruptures extends to the smallest particles of the mass, and it enables us to account for the continued compactness of the ice during the descent of the glacier.

14. The quality of viscosity is practically absent in glacier-ice. Where pressure comes into play the phenomena are suggestive of viscosity, but where tension comes into play the analogy with a viscous body breaks down. When subjected to strain the glacier does not yield by stretching, but by breaking; this is the origin of the crevasses.

15. The crevasses are produced by the mechanical strains to which the glacier is subjected. They are divided into marginal, transverse, and longitudinal crevasses; the first produced by the oblique strain consequent on the quicker motion of the centre; the second by the passage of the glacier over the summit of an incline; the third by pressure from behind and resistance in front, which causes the mass to split at right angles to the pressure [strain?].

16. The moulins are formed by deep cracks intersecting glacier rivulets. The water in descending such cracks scoops out for itself a shaft, sometimes many feet wide, and some hundreds of feet deep, into which the cataract plunges with a sound like thunder. The supply of water is periodically cut off from the moulins by fresh cracks, in which new moulins are formed.

17. The lateral moraines are formed from the debris which loads the glacier along its edges; the medial moraines are formed on a trunk-glacier by the union of the lateral moraines of its tributaries; the terminal moraines are formed from the debris carried by the glacier to its terminus, and there deposited. The number of medial moraines on a trunk glacier is always one less than the number of tributaries.

18. When ordinary lake-ice is intersected by a strong sunbeam it liquefies so as to form flower-shaped figures within the mass; each flower consists of six petals with a vacuous space at the centre; the flowers are always formed parallel to the planes of freezing, and depend on the crystallization of the substance.

19. Innumerable liquid disks, with vacuous spots, are also formed by the solar beams in glacier-ice. These empty spaces have been hitherto mistaken for air-bubbles, the flat form of the disks being erroneously regarded as the result of pressure.

20. These disks are indicators of the intimate constitution of glacier-ice, and they teach us that it is composed of an aggregate of parts with surfaces of crystallization in all possible planes.

21. There are also innumerable small cells in glacier-ice holding air and water; such cells also occur in lake-ice; and here they are due to the melting of the ice in contact with the bubble of air. Experiments are needed on glacier-ice in reference to this point.

22. At a free surface within or without, ice melts with more ease than in the centre of a compact mass. The motion which we call heat is less controlled at a free surface, and it liberates the molecules from the solid condition sooner than when the atoms are surrounded on all sides by other atoms which impede the molecular motion. Regelation is the complementary effect to the above; for here the superficial portions of a mass of ice are made virtually central by the contact of a second mass.

23. The dirt-bands have their origin in the ice-cascades. The glacier, in passing the brow, is transversely fractured; ridges are formed with hollows between them; these transverse hollows are the principal receptacles of the fine debris scattered over the glacier; and after the ridges have been melted away, the dirt remains in successive stripes upon the glacier.

24. The ice of many glaciers is laminated, and when weathered may be cloven into thin plates. In the sound ice the lamination manifests itself in blue stripes drawn through the general whitish mass of the glacier; these blue veins representing portions of ice from which the air-bubbles have been more completely expelled. This is the veined structure of the ice. It is divided into marginal, transverse, and longitudinal structure; which may be regarded as complementary to marginal, longitudinal, and transverse crevasses. The latter are produced by tension, the former by pressure, which acts in two different ways: firstly, the pressure acts upon the ice as it has acted upon rocks which exhibit the lamination technically called cleavage; secondly, it produces partial liquefaction of the ice. The liquid spaces thus formed help the escape of the air from the glacier; and the water produced, being refrozen when the pressure is relieved, helps to form the blue veins.

APPENDIX.

COMPARATIVE VIEW OF THE CLEAVAGE OF CRYSTALS AND SLATE-ROCKS.

A LECTURE DELIVERED AT THE ROYAL INSTITUTION, ON FRIDAY EVENING THE 6TH OF JUNE, 1856.[A]

When the student of physical science has to investigate the character of any natural force, his first care must be to purify it from the mixture of other forces, and thus study its simple action. If, for example, he wishes to know how a mass of water would shape itself, supposing it to be at liberty to follow the bent of its own molecular forces, he must see that these forces have free and undisturbed exercise. We might perhaps refer him to the dew-drop for a solution of the question; but here we have to do, not only with the action of the molecules of the liquid upon each other, but also with the action of gravity upon the mass, which pulls the drop downwards and elongates it. If he would examine the problem in its purity, he must do as Plateau has done, withdraw the liquid mass from the action of gravity, and he would then find the shape of the mass to be perfectly spherical. Natural processes come to us in a mixed manner, and to the uninstructed mind are a mass of unintelligible confusion. Suppose half-a-dozen of the best musical performers to be placed in the same room, each playing his own instrument to perfection: though each individual instrument might be a well-spring of melody, still the mixture of all would produce mere noise. Thus it is with the processes of nature. In nature, mechanical and molecular laws mingle, and create apparent confusion. Their mixture constitutes what may be called the _noise_ of natural laws, and it is the vocation of the man of science to resolve this noise into its components, and thus to detect the "music" in which the foundations of nature are laid.

The necessity of this detachment of one force from all other forces is nowhere more strikingly exhibited than in the phenomena of crystallization. I have here a solution of sulphate of soda. Prolonging the mental vision beyond the boundaries of sense, we see the atoms of that liquid, like squadrons under the eye of an experienced general, arranging themselves into battalions, gathering round a central standard, and forming themselves into solid masses, which after a time assume the visible shape of the crystal which I here hold in my hand. I may, like an ignorant meddler wishing to hasten matters, introduce confusion into this order. I do so by plunging this glass rod into the vessel. The consequent action is not the pure expression of the crystalline forces; the atoms rush together with the confusion of an unorganized mob, and not with the steady accuracy of a disciplined host. Here, also, in this mass of bismuth we have an example of this confused crystallization; but in the crucible behind me a slower process is going on: here there is an architect at work "who makes no chips, no din," and who is now building the particles into crystals, similar in shape and structure to those beautiful masses which we see upon the table. By permitting alum to crystallize in this slow way, we obtain these perfect octahedrons; by allowing carbonate of lime to crystallize, nature produces these beautiful rhomboids; when silica crystallizes, we have formed these hexagonal prisms capped at the ends by pyramids; by allowing saltpetre to crystallize, we have these prismatic masses; and when carbon crystallizes, we have the diamond. If we wish to obtain a perfect crystal, we must allow the molecular forces free play: if the crystallizing mass be permitted to rest upon a surface it will be flattened, and to prevent this a small crystal must be so suspended as to be surrounded on all sides by the liquid, or, if it rest upon the surface, it must be turned daily so as to present all its faces in succession to the working builder. In this way the scientific man nurses these children of his intellect, watches over them with a care worthy of imitation, keeps all influences away which might possibly invade the strict morality of crystalline laws, and finally sees them developed into forms of symmetry and beauty which richly reward the care bestowed upon them.

In building up crystals, these little atomic bricks often arrange themselves into layers which are perfectly parallel to each other, and which can be separated by mechanical means; this is called the cleavage of the crystal. I have here a crystallized mass which has thus far escaped the abrading and disintegrating forces which, sooner or later, determine the fate of sugar-candy. If I am skilful enough, I shall discover that this crystal of sugar cleaves with peculiar facility in one direction. Here, again, I have a mass of rock-salt: I lay my knife upon it, and with a blow cleave it in this direction; but I find on further examining this substance that it cleaves in more directions than one. Laying my knife at right angles to its former position, the crystal cleaves again; and, finally placing the knife at right angles to the two former positions, the mass cleaves again. Thus rock-salt cleaves in three directions, and the resulting solid is this perfect cube, which may be broken up into any number of smaller cubes. Here is a mass of Iceland spar, which also cleaves in three directions, not at right angles, but obliquely to each other, the resulting solid being a rhomboid. In each of these cases the mass cleaves with equal facility in all three directions. For the sake of completeness, I may say that many substances cleave with unequal facility in different directions, and the heavy spar I hold in my hand presents an example of this kind of cleavage.

Turn we now to the consideration of some other phenomena to which the term cleavage may be applied. This piece of beech-wood cleaves with facility parallel to the fibre, and if our experiments were fine enough we should discover that the cleavage is most perfect when the edge of the axe is laid across the rings which mark the growth of the tree. The fibres of the wood lie side by side, and a comparatively small force is sufficient to separate them. If you look at this mass of hay severed from a rick, you will see a sort of cleavage developed in it also; the stalks lie in parallel planes, and only a small force is required to separate them laterally. But we cannot regard the cleavage of the tree as the same in character as the cleavage of the hayrick. In the one case it is the atoms arranging themselves according to organic laws which produce a cleavable structure; in the other case the easy separation in a certain direction is due to the mechanical arrangement of the coarse sensible masses of stalks of hay.

In like manner I find that this piece of sandstone cleaves parallel to the planes of bedding. This rock was once a powder, more or less coarse, held in mechanical suspension by water. The powder was composed of two distinct parts, fine grains of sand and small plates of mica. Imagine a wide strand covered by a tide which holds such powder in suspension:[B] how will it sink? The rounded grains of sand will reach the bottom first, the mica afterwards, and when the tide recedes we have the little plates shining like spangles upon the surface of the sand. Each successive tide brings its charge of mixed powder, deposits its duplex layer day after day, and finally masses of immense thickness are thus piled up, which, by preserving the alternations of sand and mica, tell the tale of their formation. I do not wish you to accept this without proof. Take the sand and mica, mix them together in water, and allow them to subside, they will arrange themselves in the manner I have indicated; and by repeating the process you can actually build up a sandstone mass which shall be the exact counterpart of that presented by nature, as I have done in this glass jar. Now this structure cleaves with readiness along the planes in which the particles of mica are strewn. Here is a mass of such a rock sent to me from Halifax: here are other masses from the quarries of Over Darwen in Lancashire. With a hammer and chisel you see I can cleave them into flags; indeed these flags are made use of for roofing purposes in the districts from which the specimens have come, and receive the name of "slate-stone." But you will discern, without a word from me, that this cleavage is not a crystalline cleavage any more than that of a hayrick is. It is not an arrangement produced by molecular forces; indeed it would be just as reasonable to suppose that in this jar of sand and mica the particles arranged themselves into layers by the forces of crystallization, instead of by the simple force of gravity, as to imagine that such a cleavage as this could be the product of crystallization.

This, so far as I am aware of, has never been imagined, and it has been agreed among geologists not to call such splitting as this cleavage at all, but to restrict the term to a class of phenomena which I shall now proceed to consider.

Those who have visited the slate quarries of Cumberland and North Wales will have witnessed the phenomena to which I refer. We have long drawn our supply of roofing-slates from such quarries; schoolboys ciphered on these slates, they were used for tombstones in churchyards, and for billiard-tables in the metropolis; but not until a comparatively late period did men begin to inquire how their wonderful structure was produced. What is the agency which enables us to split Honister Crag, or the cliffs of Snowdon, into laminae from crown to base? This question is at the present moment one of the greatest difficulties of geologists, and occupies their attention perhaps more than any other. You may wonder at this. Looking into the quarry of Penrhyn, you may be disposed to explain the question as I heard it explained two years ago. "These planes of cleavage," said a friend who stood beside me on the quarry's edge, "are the planes of stratification which have been lifted by some convulsion into an almost vertical position." But this was a great mistake, and indeed here lies the grand difficulty of the problem. These planes of cleavage stand in most cases at a high angle to the bedding. Thanks to Sir Roderick Murchison, who has kindly permitted me the use of specimens from the Museum of Practical Geology (and here I may be permitted to express my acknowledgments to the distinguished staff of that noble establishment, who, instead of considering me an intruder, have welcomed me as a brother), I am able to place the proof of this before you. Here is a mass of slate in which the planes of bedding are distinctly marked; here are the planes of cleavage, and you see that one of them makes a large angle with the other. The cleavage of slates is therefore not a question of stratification, and the problem which we have now to consider is, "By what cause has this cleavage been produced?"

In an able and elaborate essay on this subject in 1835, Professor Sedgwick proposed the theory that cleavage is produced by the action of crystalline or polar forces after the mass has been consolidated. "We may affirm," he says, "that no retreat of the parts, no contraction of dimensions in passing to a solid state can explain such phenomena. They appear to me only resolvable on the supposition that crystalline or polar forces acted upon the whole mass simultaneously in one direction and with adequate force." And again, in another place: "Crystalline forces have rearranged whole mountain-masses, producing a beautiful crystalline cleavage, passing alike through all the strata."[C] The utterance of such a man struck deep, as was natural, into the minds of geologists, and at the present day there are few who do not entertain this view either in whole or in part.[D] The magnificence of the theory, indeed, has in some cases caused speculation to run riot, and we have books published, aye and largely sold, on the action of polar forces and geologic magnetism, which rather astonish those who know something about the subject. According to the theory referred to, miles and miles of the districts of North Wales and Cumberland, comprising huge mountain-masses, are neither more nor less than the parts of a gigantic crystal. These masses of slate were originally fine mud; this mud is composed of the broken and abraded particles of older rocks. It contains silica, alumina, iron, potash, soda, and mica, mixed in sensible masses mechanically together. In the course of ages the mass became consolidated, and the theory before us assumes that afterwards a process of crystallization rearranged the particles and developed in the mass a single plane of crystalline cleavage. With reference to this hypothesis, I will only say that it is a bold stretch of analogies; but still it has done good service: it has drawn attention to the question; right or wrong, a theory thus thoughtfully uttered has its value; it is a dynamic power which operates against intellectual stagnation; and, even by provoking opposition, is eventually of service to the cause of truth. It would, however, have been remarkable, if, among the ranks of geologists themselves, men were not found to seek an explanation of the phenomena in question, which involved a less hardy spring on the part of the speculative faculty than the view to which I have just referred.

The first step in an inquiry of this kind is to put oneself into contact with nature, to seek facts. This has been done, and the labours of Sharpe (the late President of the Geological Society, who, to the loss of science and the sorrow of all who knew him, has so suddenly been taken away from us), Sorby, and others, have furnished us with a body of evidence which reveals to us certain important physical phenomena, associated with the appearance of slaty cleavage, if they have not produced it. The nature of this evidence we will now proceed to consider.

Fossil shells are found in these slate-rocks. I have here several specimens of such shells, occupying various positions with regard to the cleavage planes. They are squeezed, distorted, and crushed. In some cases a flattening of the convex shell occurs, in others the valves are pressed by a force which acted in the plane of their junction, but in all cases the distortion is such as leads to the inference that the rock which contains these shells has been subjected to enormous pressure in a direction at right angles to the planes of cleavage; the shells are all flattened and spread out upon these planes. I hold in my hand a fossil trilobite of normal proportions. Here is a series of fossils of the same creature which have suffered distortion. Some have lain across, some along, and some oblique to the cleavage of the slate in which they are found; in all cases the nature of the distortion is such as required for its production a compressing force acting at right angles to the planes of cleavage. As the creatures lay in the mud in the manner indicated, the jaws of a gigantic vice appear to have closed upon them and squeezed them into the shape you see. As further evidence of the exertion of pressure, let me introduce to your notice a case of contortion which has been adduced by Mr. Sorby. The bedding of the rock shown in this figure[E] was once horizontal; at A we have a deep layer of mud, and at _m n_ a layer of comparatively unyielding gritty material; below that again, at B, we have another layer of the fine mud of which slates are formed. This mass cleaves along the shading lines of the diagram; but look at the shape of the intermediate bed: it is contorted into a serpentine form, and leads irresistibly to the conclusion that the mass has been pressed together at right angles to the planes of cleavage. This action can be experimentally imitated, and I have here a piece of clay in which this is done and the same result produced on a small scale. The amount of compression, indeed, might be roughly estimated by supposing this contorted bed _m n_ to be stretched out, its length measured and compared with the distance _c d_; we find in this way that the yielding of the mass has been considerable.

Let me now direct your attention to another proof of pressure. You see the varying colours which indicate the bedding on this mass of slate. The dark portion, as I have stated, is gritty, and composed of comparatively coarse particles, which, owing to their size, shape, and gravity, sink first and constitute the bottom of each layer. Gradually from bottom to top the coarseness diminishes, and near the upper surface of each layer we have a mass of comparatively fine clean mud. Sometimes this fine mud forms distinct layers in a mass of slate-rock, and it is the mud thus consolidated from which are derived the German razor-stones, so much prized for the sharpening of surgical instruments. I have here an example of such a stone. When a bed is thin, the clean white mud is permitted to rest, as in this case, upon a slab of the coarser slate in contact with it: when the bed is thick, it is cut into slices which are cemented to pieces of ordinary slate, and thus rendered stronger. The mud thus deposited sometimes in layers is, as might be expected, often rolled up into nodular masses, carried forward, and deposited by the rivers from which the slate-mud has subsided. Here, indeed, are such nodules enclosed in sandstone. Everybody who has ciphered upon a school-slate must remember the whitish-green spots which sometimes dotted the surface of the slate; he will remember how his slate-pencil usually slid over such spots as if they were greasy. Now these spots are composed of the finer mud, and they could not, on account of their fineness, _bite_ the pencil like the surrounding gritty portions of the slate. Here is a beautiful example of the spots: you observe them on the cleavage surface in broad patches; but if this mass has been compressed at right angles to the planes of cleavage, ought we to expect the same marks when we look at the edge of the slab? The nodules will be flattened by such pressure, and we ought to see evidence of this flattening when we turn the slate edgeways. Here it is. The section of a nodule is a sharp ellipse with its major axis parallel to the cleavage. There are other examples of the same nature on the table; I have made excursions to the quarries of Wales and Cumberland, and to many of the slate-yards of London, but the same fact invariably appears, and thus we elevate a common experience of our boyhood into evidence of the highest significance as regards one of the most important problems of geology. In examining the magnetism of these slates, I was led to infer that these spots would contain a less amount of iron than the surrounding dark slate. The analysis was made for me by Mr. Hambly in the laboratory of Dr. Percy at the School of Mines. The result which is stated in this Table justifies the conclusion to which I have referred.

_Analysis of Slate._

Purple Slate. Two Analyses. 1. Percentage of iron 5.85 2. " " 6.13 Mean 5.99

Greenish Slate. 1. Percentage of iron 3.24 2. " " 3.12 Mean 3.18

The quantity of iron in the dark slate immediately adjacent to the greenish spot is, according to these analyses, nearly double of the quantity contained in the spot itself. This is about the proportion which the magnetic experiments suggested.

Let me now remind you that the facts which I have brought before you are typical facts--each is the representative of a class. We have seen shells crushed, the unhappy trilobites squeezed, beds contorted, nodules of greenish marl flattened; and all these sources of independent testimony point to one and the same conclusion, namely, that slate-rocks have been subjected to enormous pressure in a direction at right angles to the planes of cleavage.[F]

In reference to Mr. Sorby's contorted bed, I have said that by supposing it to be stretched out and its length measured, it would give us an idea of the amount of yielding of the mass above and below the bed. Such a measurement, however, would not quite give the amount of yielding; and here I would beg your attention to a point, the significance of which has, so far as I am aware of, hitherto escaped attention. I hold in my hand a specimen of slate, with its bedding marked upon it; the lower portions of each bed are composed of a comparatively coarse gritty material, something like what you may suppose this contorted bed to be composed of. Well, I find that the cleavage takes a bend in crossing these gritty portions, and that the tendency of these portions is to cleave more at right angles to the bedding. Look to this diagram: when the forces commenced to act, this intermediate bed, which though comparatively unyielding is not entirely so, suffered longitudinal pressure; as it bent, the pressure became gradually more lateral, and the direction of its cleavage is exactly such as you would infer from a force of this kind--it is neither quite across the bed, nor yet in the same direction as the cleavage of the slate above and below it, but intermediate between the two. Supposing the cleavage to be at right angles to the pressure, this is the direction which it ought to take across these more unyielding strata.

Thus we have established the concurrence of the phenomena of cleavage and pressure--that they accompany each other; but the question still remains, Is this pressure of itself sufficient to account for the cleavage? A single geologist, as far as I am aware, answers boldly in the affirmative. This geologist is Sorby, who has attacked the question in the true spirit of a physical investigator. You remember the cleavage of the flags of Halifax and Over Darwen, which is caused by the interposition of plates of mica between the layers. Mr. Sorby examines the structure of slate-rock, and finds plates of mica to be a constituent. He asks himself, what will be the effect of pressure upon a mass containing such plates confusedly mixed up in it? It will be, he argues--and he argues rightly--to place the plates with their flat surfaces more or less perpendicular to the direction in which the pressure is exerted. He takes scales of the oxide of iron, mixes them with a fine powder, and, on squeezing the mass, finds that the tendency of the scales is to set themselves at right angles to the line of pressure. Now the planes in which these plates arrange themselves will, he contends, be those along which the mass cleaves.

I could show you, by tests of a totally different character from those applied by Mr. Sorby, how true his conclusion is, that the effect of pressure on elongated particles or plates will be such as he describes it. Nevertheless, while knowing this fact, and admiring the ability with which Mr. Sorby has treated this question, I cannot accept his explanation of slate-cleavage. I believe that even if these plates of mica were wholly absent, the cleavage of slate-rocks would be much the same as it is at present.

I will not dwell here upon minor facts,--I will not urge that the perfection of the cleavage bears no relation to the quantity of mica present; but I will come at once to a case which to my mind completely upsets the notion that such plates are a necessary element in the production of cleavage.

Here is a mass of pure white wax: there are no mica particles here; there are no scales of iron, or anything analogous mixed up with the mass. Here is the self-same substance submitted to pressure. I would invite the attention of the eminent geologists whom I see before me to the structure of this mass. No slate ever exhibited so clean a cleavage; it splits into laminae of surpassing tenuity, and proves at a single stroke that pressure is sufficient to produce cleavage, and that this cleavage is independent of the intermixed plates of mica assumed in Mr. Sorby's theory. I have purposely mixed this wax with elongated particles, and am unable to say at the present moment that the cleavage is sensibly affected by their presence,--if anything, I should say they rather impair its fineness and clearness than promote it.

The finer the slate the more perfect will be the resemblance of its cleavage to that of the wax. Compare the surface of the wax with the surface of this slate from Borrodale in Cumberland. You have precisely the same features in both: you see flakes clinging to the surfaces of each, which have been partially torn away by the cleavage of the mass: I entertain the conviction that if any close observer compares these two effects, he will be led to the conclusion that they are the product of a common cause.[G]

But you will ask, how, according to my view, does pressure produce this remarkable result? This may be stated in a very few words.

Nature is everywhere imperfect! The eye is not perfectly achromatic, the colours of the rose and tulip are not pure colours, and the freshest air of our hills has a bit of poison in it. In like manner there is no such thing in nature as a body of perfectly homogeneous structure. I break this clay which seems so intimately mixed, and find that the fracture presents to my eyes innumerable surfaces along which it has given way, and it has yielded along these surfaces because in them the cohesion of the mass is less than elsewhere. I break this marble, and even this wax, and observe the same result: look at the mud at the bottom of a dried pond; look to some of the ungravelled walks in Kensington Gardens on drying after rain,--they are cracked and split, and other circumstances being equal, they crack and split where the cohesion of the mass is least. Take then a mass of partially consolidated mud. Assuredly such a mass is divided and subdivided by surfaces along which the cohesion is comparatively small. Penetrate the mass, and you will see it composed of numberless irregular nodules bounded by surfaces of weak cohesion. Figure to your mind's eye such a mass subjected to pressure,--the mass yields and spreads out in the direction of least resistance;[H] the little nodules become converted into laminae, separated from each other by surfaces of weak cohesion, and the infallible result will be that such a mass will cleave at right angles to the line in which the pressure is exerted.

Further, a mass of dried mud is full of cavities and fissures. If you break dried pipe-clay you see them in great numbers, and there are multitudes of them so small that you cannot see them. I have here a piece of glass in which a bubble was enclosed; by the compression of the glass the bubble is flattened, and the sides of the bubble approach each other so closely as to exhibit the colours of thin plates. A similar flattening of the cavities must take place in squeezed mud, and this must materially facilitate the cleavage of the mass in the direction already indicated.

Although the time at my disposal has not permitted me to develop this thought as far as I could wish, yet for the last twelve months the subject has presented itself to me almost daily under one aspect or another. I have never eaten a biscuit during this period in which an intellectual joy has not been superadded to the more sensual pleasure, for I have remarked in all such cases cleavage developed in the mass by the rolling-pin of the pastrycook or confectioner. I have only to break these cakes, and to look at the fracture, to see the laminated structure of the mass; nay, I have the means of pushing the analogy further: I have here some slate which was subjected to a high temperature during the conflagration of Mr. Scott Russell's premises. I invite you to compare this structure with that of a biscuit; air or vapour within the mass has caused it to swell, and the mechanical structure it reveals is precisely that of a biscuit. I have gone a little into the mysteries of baking while conducting my inquiries on this subject, and have received much instruction from a lady-friend in the manufacture of puff-paste. Here is some paste baked in this house under my own superintendence. The cleavage of our hills is accidental cleavage, but this is cleavage with intention. The volition of the pastrycook has entered into the formation of the mass, and it has been his aim to preserve a series of surfaces of structural weakness, along which the dough divides into layers. Puff-paste must not be handled too much, for then the continuity of the surfaces is broken; it ought to be rolled on a cold slab, to prevent the butter from melting and diffusing itself through the mass, thus rendering it more homogeneous and less liable to split. This is the whole philosophy of puff-paste; it is a grossly exaggerated case of slaty cleavage.

As time passed on, cases multiplied, illustrating the influence of pressure in producing lamination. Mr. Warren De la Rue informs me that he once wished to obtain white-lead in a fine granular state, and to accomplish this he first compressed the mass: the mould was conical, and permitted the mass to spread a little laterally under the pressure. The lamination was as perfect as that of slate, and quite defeated him in his effort to obtain a granular powder. Mr. Brodie, as you are aware, has recently discovered a new kind of graphite: here is the substance in powder, of exquisite fineness. This powder has the peculiarity of clinging together in little confederacies; it cannot be shaken asunder like lycopodium; and when the mass is squeezed, these groups of particles flatten, and a perfect cleavage is produced. Mr. Brodie himself has been kind enough to furnish me with specimens for this evening's lecture. I will cleave them before you: you see they split up into plates which are perpendicular to the line in which the pressure was exerted. This testimony is all the more valuable, as the facts were obtained without any reference whatever to the question of cleavage.

I have here a mass of that singular substance Boghead Cannel. This was once a mass of mud, more or less resembling this one, which I have obtained from a bog in Lancashire. I feel some hesitation in bringing this substance before you, for, as in other cases, so in regard to Boghead Cannel, science--not science, let me not libel it, but the quibbling, litigious, money-loving portion of human nature speaking through the mask of science--has so contrived to split hairs as to render the qualities of the substance somewhat mythical. I shall therefore content myself with showing you how it cleaves, and with expressing my conviction that pressure had a great share in the production of this cleavage.

The principle which I have enunciated is so simple as to be almost trivial; nevertheless, it embraces not only the cases I have mentioned, but, if time permitted, I think I could show you that it takes a much wider range. When iron is taken from the puddling furnace, it is a more or less spongy mass: it is at a welding heat, and at this temperature is submitted to the process of rolling: bright smooth bars such as this are the result of this rolling. But I have said that the mass is more or less spongy or nodular, and, notwithstanding the high heat, these nodules do not perfectly incorporate with their neighbours: what then? You would say that the process of rolling must draw the nodules into fibres--it does so; and here is a mass acted upon by dilute sulphuric acid, which exhibits in a striking manner this fibrous structure. The experiment was made by my friend Dr. Percy, without any reference to the question of cleavage.

Here are other cases of fibrous iron. This fibrous structure is the result of mechanical treatment. Break a mass of ordinary iron and you have a granular fracture; beat the mass, you elongate these granules, and finally render the mass fibrous. Here are pieces of rails along which the wheels of locomotives have slidden; the granules have yielded and become plates; they exfoliate or come off in leaves. All these effects belong, I believe, to the great class of phenomena of which slaty cleavage forms the most prominent example.[I]

Thus, ladies and gentlemen, we have reached the termination of our task. I commenced by exhibiting to you some of the phenomena of crystallization. I have placed before you the facts which are found to be associated with the cleavage of slate-rocks. These facts, as finely expressed by Helmholtz, are so many telescopes to our spiritual vision, by which we can see backward through the night of antiquity, and discern the forces which have been in operation upon the earth's surface

"Ere the lion roared, Or the eagle soared."

From evidence of the most independent and trustworthy character, we come to the conclusion that these slaty masses have been subjected to enormous pressure, and by the sure method of experiment we have shown--and this is the only really new point which has been brought before you--how the pressure is sufficient to produce the cleavage. Expanding our field of view, we find the self-same law, whose footsteps we trace amid the crags of Wales and Cumberland, stretching its ubiquitous fingers into the domain of the pastrycook and ironfounder; nay, a wheel cannot roll over the half-dried mud of our streets without revealing to us more or less of the features of this law. I would say, in conclusion, that the spirit in which this problem has been attacked by geologists indicates the dawning of a new day for their science. The great intellects who have laboured at geology, and who have raised it to its present pitch of grandeur, were compelled to deal with the subject in mass; they had no time to look after details. But the desire for more exact knowledge is increasing; facts are flowing in, which, while they leave untouched the intrinsic wonders of geology, are gradually supplanting by solid truths the uncertain speculations which beset the subject in its infancy. Geologists now aim to imitate, as far as possible, the conditions of nature, and to produce her results; they are approaching more and more to the domain of physics; and I trust the day will soon come when we shall interlace our friendly arms across the common boundary of our sciences, and pursue our respective tasks in a spirit of mutual helpfulness, encouragement, and good-will.

FOOTNOTES:

[A] Referred to in the Introduction.

[B] I merely use this as an illustration; the deposition may have really been due to sediment carried down by rivers. But the action must have been periodic, and the powder duplex.

[C] 'Transactions of the Geological Society,' Ser. ii. vol. iii. p. 477.

[D] In a letter to Sir Charles Lyell, dated from the Cape of Good Hope, February 20, 1836, Sir John Herschel writes as follows:--"If rocks have been so heated as to allow of a commencement of crystallization, that is to say, if they have been heated to a point at which the particles can begin to move amongst themselves, or at least on their own axes, some general law must then determine the position in which these particles will rest on cooling. Probably that position will have some relation to the direction in which the heat escapes. Now when all or a majority of particles of the same nature have a general tendency to one position, that must of course determine a cleavage plane."

[E] Omitted here.

[F] While to my mind the evidence in proof of pressure seems perfectly irresistible, I by no means assert that the manner in which I stated it is incapable of modification. All that I deem important is the fact that pressure has been exerted; and provided this remain firm, the fate of any minor portion of the evidence by which it is here established is of comparatively little moment.

[G] I have usually softened the wax by warming it, kneaded it with the fingers, and pressed it between thick plates of glass previously wetted. At the ordinary summer-temperature the wax is soft, and tears rather than cleaves; on this account I cool my compressed specimens in a mixture of pounded ice and salt, and when thus cooled they split beautifully.

[H] It is scarcely necessary to say that if the mass were squeezed equally in _all_ directions no laminated structure could be produced; it must have room to yield in a lateral direction.

[I] An eminent authority informs me that he believes these surfaces of weak cohesion to be due to the interposition of films of graphite, and not to any tendency of the iron itself to become fibrous: this of course does not in any way militate against the theory which I have ventured to propose. All that the theory requires is surfaces of weak cohesion, however produced, and a change of shape of such surfaces consequent on pressure or rolling.

INDEX.

AEggischhorn, 100, 105.

Agassiz on glacier motion, 270, 310.

Air-bubbles, 359, 376.

Aletsch Glacier, 101. -- --, bedding and structure observed on, 120, 391.

Aletschhorn, cloud iridescences on, 100, 238.

Allalein Glacier, 162.

Alpine climbers, suggestions to, 169.

Alps, winter temperature of, 168.

Altmann's theory of glacier motion, 296.

Ancient glaciers, action of, 99, 141.

Arveiron, arch of, 38, 217.

Atmosphere, permeability of, to radiant heat, 105, 243-247.

Atmospheric refraction, 35.

Avalanche at Saas, 164. --, sound of, explained, 12, 14.

Bakewell, Mr., on motion of Glacier des Bossons, 337.

Balmat, Auguste, 169, 188.

Bedding, lines of, 391.

Bennen, Johann Joseph, 104, 118.

Bergschrund, 98, 325.

"Blower," glacier, 87.

Blue colour of ice, 256. -- -- -- snow, 29, 83, 132, 203. -- -- -- water, 33, 253, 259-262.

Blueness of sky, 22, 174, 257-261.

Blue veins, 376, 381.

Boiling-point, influence of pressure on, 408. -- -- at different altitudes, 105, 106, 113, 120, 129, 175, 190.

Bois, Glacier des, 39, 275, 368.

Brevent, ascent of, 172.

Brocken, Spirit of the, 22, 238.

Bubbles, in ice, 44, 147, 359, 425. -- in snow, 18, 251.

Capillaries of glacier, 335-339.

Cave of ice, 135.

Cavities in ice, 163, 356, 424.

Cells in ice, 147, see Bubbles.

Chamouni, 37. --, difficulties at, 170, 192. -- in winter, 198, 336.

Charmoz, view from, 45, 68, 368.

Charpentier's theory of glacier motion, 296.

Chemical action, rays producing, 240.

Chromatic effects, 235.

Cleavage, 406. -- and stratification distinct, 2, 390, 431. -- caused by pressure, 6, 436. --, contortions of, 9, 59. -- of crystals and slate rocks, lecture on, 427. -- of glaciers, 26, 393, 425-426. -- -- ice, 352, 407. -- -- slate, &c., 1, 430.

"Cleft station," the, 47, 369.

Clouds, formation and dissipation of, 22, 97, 137, 146. --, iridescent, 100, 105, 147, 154, 238. -- on Mont Blanc, 82. -- on Monte Rosa, 124. --, winter, at Montanvert, 208.

Colour answers to pitch, 227.

Colours of sky, 257. --, subjective, 37.

Comet, discovery of, 186.

Compass affected by rocks, 140.

Crepitation of glaciers, 44, 357.

Crevasses, 315 (_marginal_, 318; _transverse_, 320; _longitudinal_, 322), 424. --, first opening of, 317, 327.

Crumples in ice, 174, 415, 419.

Crystallization of ice, 353.

Crystals, cleavage of, 3, 428. -- of snow, 130, 205, 212.

Deafness, artificial, 167.

Differential motion, 395. -- --, Dr. Whewell on, 396.

Diffraction, explanation of, 237.

Dirt-bands, 45, 46, 68, 95, 367, 373. -- --, maps of, 367, 368, 369. -- --, Forbes on, 371. -- --, source of, 369, 425.

Disks in ice, planes of, 163, 358, 425.

Dollfuss, M., hut of, 18, 112.

Dome du Gouter, 68, 75.

Donny, M., on cohesion of liquids, 355.

Echoes, theory of, 15.

Eismeer, the, 13, 362.

Expedition of 1856, Oberland and Tyrol, 9-32. -- -- 1857, Montanvert and Mer de Glace, 33-91. -- -- 1858, Oberland, Valais, and Monte Rosa district, 92-192. -- -- 1859, winter, Chamouni, and Mer de Glace, 195-219.

Faraday, Prof., on Regelation, 351.

Faulberg, cave of, 107.

Fee, glacier of, 165.

Fend, 32.

Finsteraarhorn, 104, 110. --, summit of, 112.

Flowers, liquid, in ice, 147, 354-358, 424.

Forbes, Prof., comparison of glacier to river, 306, 308. -- --, on glacier motion, 272, 304, 308. -- --, on magnetism of rocks, 145. -- --, on veined structure, 379. -- --, viscous theory, 311, 327, 333, 335.

Freezing, planes of, 163, 358, 424.

Frost-bites, 191.

Frozen flowers, 130, 212.

Furgge glacier, structure crossing strata on, 160, 392-394.

Gases, passage of heat through, 243.

Geant, Col du, 50, 173.

Geant, glacier du, 53-57, 280, 369-373. --, measurements on, 419-421. --, motion of, 281, 286. --, white ice seams of, 56, 413.

Gebatsch Alp, 23. --, glacier of, 24, 26.

Geneva, Lake of, 33, 259-262.

Glaciers, ancient, action of, 99, 163. -- "blower," 87. --, capillaries of, 335-339. --, crepitation of, 44, 357. -- d'ecoulement, 301. -- de Lechaud, see Lechaud. -- des Bois, 39, 275, 368. -- du Geant, see Geant. -- du Talefre, see Talefre. --, groovings on, 20, 56, 377. --, measurement of, 276. -- motion, 52, 269-295, 422. -- --, earlier theories of, 296-314. -- --, pressure theory of, 346. --, origin of, 248-252. -- reservoirs, 301. --, ridges on, 42, 55. --, structure of, 136, 148, see Veined structure. -- tables, 44, 265. --, veins of, 54, 376, 381. --, wrinkles on, 370.

Goethe's theory of colours, 258.

Goerner glacier, 120, 138.

Goerner grat, 137, 145.

Goernerhorn glacier, 147, 149.

Grand Plateau, 187.

Grands Mulets, 73, 185.

Graun, 29.

Grimsel, the, 18, 99.

Grindelwald, lower glacier of, 13, 92, 321, 384.

Groovings on glaciers, 20, 56, 377.

Gruener's theory of glacier motion, 296.

Guides of Chamouni, rules of, 60, 170, 192. -- lost in crevasse, 76.

Guyot, M., on veined structure, 378.

Hailstones, conical, 31. --, spherical, 65.

Handeck, waterfall of, 17.

Hasli, valley of, 17, 99.

Heat and light, 223, 239, 241. -- -- work, 328. --, luminous, 241-247. --, mechanical equivalent of, 329. --, obscure, 240. --, passage through gases, 243-245. --, radiant, 239. -- --, permeability of atmosphere to, 105, 243-247. --, radiated, 242. --, specific, 331.

Heisse Platte, the, 13.

Hirst, Mr., measurements on Mer de Glace, 38, 46, 275, 283, 289, 313, 420.

Hochjoch, 32.

Hoechste Spitze of Monte Rosa, 128.

Hopkins, Mr., on crevasses, 318, 383.

Hotel des Neufchatelois, 19, 112, 270.

Hugi on glacier motion, 270.

Huxley, Mr., on glacier capillaries, 338. -- --, on water-cells, 251, 359.

Hydrogen, effect on rays, 253.

Ice, blue colour of, 256. -- cascades, 94, 384, 391. -- cave, 135. -- cells, 147, see Bubbles. -- cones, 266. --, cracking of, 317, 326. --, crystallization of, 353. --, effects of pressure on, 405, 409. --, experiments on, 346. --, friability of, 333. --, liquefaction of, 353, 408. --, liquid flowers in, 354-358, 424. --, Thomson's theory of plasticity of, 340. --, softening of, 333. --, structure of, 136, 148. --, temperature of, 241, 332. --, white, seams of, 56, 413, 421.

Illumination of trees, &c., at sunrise and sunset, 178, 238.

Interference rings, 229. -- spectra, 76, 178, 235, 238.

Iridescent clouds, 100, 105, 147, 154, 238.

Jardin, the, 61, 174.

Joch, the passage of a, 28.

Joule, M., on heat and work, 328.

Jungfrau, the, 11. --, evening near, 106.

Laminated structure, 376, 378, 426.

Lechaud, glacier de, 53, 387. -- -- --, motion of, 60, 286-288.

Lenticular structure, 381.

Light and heat, 223, 239, 241. --, undulation theory of, 224.

Linth, M. Escher de la, 271.

Liquefaction of ice, 353, 408.

Liquid flowers, 147, 354-358, 424.

Magnetic force, 144.

Magnetism of rocks, 140, 143, 145.

Maerjelen See, 101, 119.

Mastic, Bruecke's solution of, 259.

Mattmark See, 162.

Maximum motion, locus of point of, 285, 323.

Mayenwand, summit of, 20, 100, 323.

Mayer, on connexion of heat with work, 328.

Measurement of glaciers, 276.

Mer de Glace, 42-67, 86-90, 173. -- -- --, dirt-bands of the, 367 (seen from Charmoz, 45, 368; from Cleft station, 47, 369; from the Flegere, 367). -- -- --, map of, 53, 264. -- -- --, motion of, 275-293. -- -- --, winter motion of, 294, 343. -- -- --, winter visit to, 195, 206-218.

Milk, cause of blueness of, 261.

Mirage, 36.

Montanvert, 40, 89, 173. -- in winter, 204.

Mont Blanc, first ascent of, 68. -- --, second ascent of, 177. -- --, summit of, 81, 189.

Monte Rosa, first ascent of, 122. -- --, second ascent of, 151. -- --, summit of, 128, 156. -- --, western glacier of, 138, 147. -- --, zones of colour, 154, 238.

Moraines, 263. -- of Talefre, 54, 63, 267, 387.

Motion of glaciers, 52, 269-295, 422.

Moulins, 362, 424. --, depth of, 365. --, motion of, 364.

Necker, letter from, 178.

Neufchatelois, Hotel des, 19, 112, 270.

Neve ice, 249, 251.

Oberland, the, visited, 9-22; 92-120; 390.

Oils, effect of films of, 236.

Person, M., on softening of ice, 333.

Pistol fired on summit of Mont Blanc, 82, 83, 224.

Pitch of musical sounds, 225.

Planes of freezing, 163, 358, 424.

Plasticity of ice, Thomson's theory of, 340.

Polar forces, 4.

Pressure and cleavage, see Cleavage. -- and liquefaction of ice, 340, 408. -- -- veined structure, 404; 147-149, 382-394, 412, 425-426. --, effects of, on boiling point, 408. -- -- -- -- ice, 405, 409. -- theory of glacier motion, 346.

Radiant heat, 105, 239.

Rays, calorific, 240. --, transmission of, 242.

Redness of sunset, 175.

Refraction on lake of Geneva, 35.

Regelation, 347, 351.

Reichenbach fall, 17.

Rendu, comparison of glacier to river, 306. --, measurements of glaciers, 304. --, notice of regelation, 301. -- on conversion of snow into ice, 301. -- on ductility, 298. -- on law of circulation, 300. -- on motion of glaciers, 305. -- on veined structure, 301. -- theory of glaciers, 299.

Rhone at lake of Geneva, 34, 261. -- glacier, 20, 100, 323, 386. -- --, chromatic effects, 21, 238.

Ridges on glaciers, 42, 55.

Riffelhorn, the, 133, 141-145.

Rings, interference, 229. -- round sun, 21, 238.

Ripples deduced from rings, 400.

Ripple theory, Forbes on, 398. -- -- of veined structure, 398. -- waves, movement of, 232.

River and glacier, analogies between, 281-285, 423; 368.

Rocks, magnetism of, 140, 143, 145.

Saas, avalanche at, 164.

Sabine, Gen., on veined structure, 378.

Sand-cones, 266.

Saussure's theory of glacier motion, 52, 296.

Scheuchzer's theory of glacier motion, 296.

Seams, white, in ice, 56, 88, 413, 421.

Sedgwick, Prof., on cleavage, 2-5, 390, 431.

Seracs, 51, 75.

Serpentine, boulders of, 161.

Shadows, coloured, 38.

Sharpe, on slaty cleavage, 5, 432.

Silberhorn, the, 11.

Sky, blueness of, 22, 174, 175. --, colours of, explained, 257.

Slate, cleavage of, 1, 430.

Snow, blue colour of, 29, 132, 203. -- crystals, 130, 205, 212. --, dry, 250. -- line, 29, 248. --, perpetual, 248. --, sound of breaking, 202. -- storm, sound through, 215. --, whiteness of, explained, 250.

Sorby, Mr., on slaty cleavage, 5, 435.

Sound in a vacuum, 224. --, intensity of, 83. --, rate of motion of, 226.

Spectra, interference, 76, 178, 235, 238.

Spectrum, rays of, 240.

Stars, twinkling of, 72, 238.

Stelvio, pass of, 29.

Storm on Grands Mulets, 185. -- -- Mer de Glace, 210.

Strahleck, glacier of, 94, 384. --, passage of, 93, 97.

Strata of ice, 136.

Stratification of neve, 392. -- -- slate, 1, 430.

Structure, doubts regarding, 44, 92, 389. -- of ice, 136, 148, see Veined structure.

Subjective colours, 37.

Summary of glacier theory, 422.

Sun, rings round, 21, 238.

Sunrise at Chamouni, 39. -- and sunset, illumination of trees, &c., at, 178, 238.

Sunset, gorgeous, 184.

Tables, glacier, 44, 265-266.

Tacul, motion of ice-wall at, 289.

Talefre, glacier of, 43, 61-62, 87. --, moraines of, 54, 63, 267, 387.

Temperature, winter, of Alps, 168.

Theodolite, use of, 275.

Theory of cleavage, 5.

Thermometer at Jardin, 174. -- buried on Mont Blanc, 190. -- on Finsteraarhorn, 113.

Thomson, Prof., theory of plasticity, 340. -- -- -- -- regelation, 352.

Twinkling of stars, 72, 238.

Tyrol, the, 23.

Undulation theory of light, 224.

Unteraar, glacier of, 18, 265, 388.

Vacuum in ice-cavities, 163, 356.

Veined structure, 376 (_marginal_, 383; _transverse_, 384; _longitudinal_, 387), 395, 404, 408. -- --, experiments on, 382, 388. -- -- caused by pressure, 147-149, 382-389, 412, 425-426. -- -- crossing strata, 389-394. -- --, Forbes on, 379. -- --, Gen. Sabine on, 378. -- --, M. Guyot on, 378. -- --, ripple theory of, 398.

Viesch, glacier of, 109, 118.

Viscosity, 312, 325, 334, 350, 423.

Water absorbs red rays, 254. --, blue colour of, 254; 33, 259, 261. --, rippling waves of, 232.

Waves, frozen, 43, 55. --, interference of, 231. -- motion, Weber on, 232, 399. -- of sound, 225.

Wengern Alp, 9.

Wetterhorn, echoes of, 15.

White ice, seams of, 56, 57, 88, 413, 421.

Whiteness of ice, 250, 268, 376.

Winter motion of Mer de Glace, 294.

Wrinkles on glacier, 370.

Young, Thomas, theory of light, 224.

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Transcriber's Notes.

The titles from the List of Illustrations were copied to the captions of the figures that otherwise had no caption, for the convenience of the reader.

The "sidenotes" in the main body of the text were originally page headers. They have been moved to a place more fitting for the flow, typically to the head of the appropriate paragraph.

Spelling variants where there was no obviously preferred choice were retained. These include: "Cleft-Station" and "Cleft Station," plus variants; "Cima di Jazzi" and "Cima de Jazzi;" "fanlike" and "fan-like;" "firewood" and "fire-wood;" "Flegere" and "Flegere;" "foreshorten(ed)" and "fore-shorten(ing);" "generalisation" and "generalization;" "judgment" and "judgement;" "Kumm" and "Kumme," which may be the same as "Kamm;" "lime light" and "lime-light;" "realize" and "realise(d);" "recognise" and "recognize(d);" "rearranged" and "re-arranged;" "refrozen" and "re-frozen;" "self-same" and "selfsame;" "semifluid" and "semi-fluid;" "sundial" and "sun-dial;" "Trift" and "Trifti," probably the same glacier; "weatherworn" and "weather-worn."

In the Latin-1 encoded text version, the oe-ligature was replaced by the two separate characters, "oe."

Changed "Hockjoch" to "Hochjoch" on page xi: "passage of the Hochjoch."

Changed "39" to "239" on page xvii, as the page number for chapter 2.

Changed "icefall" to "ice-fall" on page xxvi: "part of ice-fall."

Changed "havresack" to "haversack" on page 71: "my waterproof haversack."

Changed "affluent" to "affluent" on page 98: "Finsteraar affluent."

Changed "184 deg.92" to "184.92 deg." on page 129.

Changed "gulleys" to "gullies" on page 143: "fissures and gullies."

Changed "SNOWSTORM" to "SNOW-STORM" in the sidenote from page 215: "SOUND THROUGH THE SNOW-STORM."

Changed "neutralise" to "neutralize" on page 231: "oppose and neutralize."

Moved the semi-colon inside the double quotes on page 285, around: "corresponding points."

Changed "THOMPSON'S" to "THOMSON'S" in the chapter heading on page 340: "THOMSON'S THEORY."

Changed "last" to "least" in the footnote to page 292: "at least as anxious."

Changed "I" to "It" on page 377: "It was also."

"Die Gletscher der Jetzzeit" on page 393 should probably be "Die Gletscher der Jetztzeit," but was not changed.

Inserted a comma in the index entry for "Aletsch Glacier:" "-- --, bedding."

Inserted a comma in the index entry for "Dirt-bands:" "-- --, maps of."

Changed "Gouter" to "Gouter" in the index entry for "Dome du Gouter."

Changed "Hoch-joch" to "Hochjoch" in its index entry.

Inserted second em-dash in the index entry for "Mont Blanc:" "-- --, second ascent of."

Inserted a comma in the index entry for "Rays:" "--, transmission of."

Inserted a comma in the index entry for "Strahleck:" "--, passage of."

End of Project Gutenberg's The Glaciers of the Alps, by John Tyndall