CHAPTER XXXI.

THE PHYSICAL CAUSE OF THE MOTION OF GLACIERS.—THE MOLECULAR THEORY.

Present State of the Question.—Heat necessary to the Motion of a Glacier.—Ice does not shear in the Solid State.—Motion of a Glacier _molecular_.—How Heat is transmitted through Ice.—Momentary Loss of Shearing Force.—The _Rationale_ of Regelation.—The Origin of “Crevasses.”—Effects of Tension.—Modification of Theory.—Fluid Molecules crystallize in Interstices.—Expansive Force of crystallizing Molecules a Cause of Motion.—Internal molecular Pressure the chief Moving Power.—How Ice can excavate a Rock Basin.—How Ice can ascend a Slope.—How deep River Valleys are striated across.—A remarkable Example in the Valley of the Tay.—How Boulders can be carried from a lower to a higher Level.

The condition which the perplexing question of the cause of the descent of glaciers has now reached seems to be something like the following. The ice of a glacier is not in a soft and plastic state, but is solid, hard, brittle, and unyielding. It nevertheless behaves in some respects in a manner very like what a soft and plastic substance would do if placed in similar circumstances, inasmuch as it accommodates itself to all the inequalities of the channel in which it moves. The ice of the glacier, though hard and solid, moves with a differential motion; the particles of the ice are displaced over each other, or, in other words, the ice shears as it descends. It had been concluded that the mere weight of the glacier is sufficient to shear the ice. Canon Moseley has investigated this point, and shown that it is not. He has found that for a glacier to shear in the way that it is supposed to do, it would require a force some thirty or forty times as great as the weight of the glacier. Consequently, for the glacier to descend, a force in addition to that of gravitation is required. What, then, is this force? It is found that the rate at which the glacier descends depends upon the amount of heat which it is receiving. This shows that the motion of the glacier is in some way or other dependent upon heat. Is heat, then, the force we are in search of? The answer to this, of course, is, since heat is a force necessarily required, we have no right to assume any other till we see whether or not heat will suffice. In what way, then, does heat aid gravitation in the descent of the glacier? In what way does heat assist gravitation in the shearing of the ice? There are two ways whereby we may conceive the thing to be done: the heat may assist gravitation to shear, by pressing the ice forward, or it may assist gravitation by diminishing the cohesion of the particles, and thus allow gravitation to produce motion which it otherwise could not produce. Every attempt which has yet been made to explain how heat can act as a force in pushing the ice forward, has failed. The fact that heat cannot expand the ice of the glacier may be regarded as a sufficient proof that it does not act as a force impelling the glacier forward; and we are thus obliged to turn our attention to the other conception, viz., that heat assists gravitation to shear the ice, not by direct pressure, but by diminishing the cohesive force of the particles, so as to enable gravitation to push the one past the other. But how is this done? Does heat diminish the cohesion by acting as an expansive force in separating the particles? Heat cannot do this, because it cannot expand the ice of a glacier; and besides, were it to do this, it would destroy the solid and firm character of the ice, and the ice of the glacier would not then, as a mass, possess the great amount of shearing-force which observation and experiment show that it does. In short it is because the particles are so firmly fixed together at the time the glacier is descending, that we are obliged to call in the aid of some other force in addition to the weight of the glacier to shear the ice. Heat does not cause displacement of the particles by making the ice soft and plastic; for we know that the ice of the glacier is not soft and plastic, but hard and brittle. The shearing-force of the ice of the moving glacier is found to be by at least from thirty to forty times too great to permit of the ice being sheared by the mere force of gravitation; how, then, is it that gravitation, without the direct assistance of any other force, can manage to shear the ice? Or to put the question under another form: heat does not reduce the shearing-force of the ice of a glacier to something like 1·3193 lb. per square inch of surface, the unit required by Mr. Moseley to enable a glacier to shear by its weight; the shearing-force of the ice, notwithstanding all the heat received, still remains at about 75 lbs.; how, then, can the glacier shear without any other force than its own weight pushing it forward? _This is the fundamental question; and the true answer to it must reveal the mystery of glacier-motion._ We are compelled in the present state of the problem to admit that glaciers do descend with a differential motion without any other force than their own weight pushing them forward; and yet the shearing-force of the ice is actually found to be thirty or forty times the maximum that would permit of the glacier shearing by its weight only. _The explanation of this apparent paradox will remove all our difficulties in reference to the cause of the descent of glaciers._

There seems to be but one explanation (and it is a very obvious one), viz. that the motion of the glacier is _molecular_. The ice descends molecule by molecule. The ice of a glacier is in the hard crystalline state, but it does not descend in this state. Gravitation is a constantly acting force; if a particle of the ice lose its shearing-force, though but for the moment, it will descend by its weight alone. But a particle of the ice will lose its shearing-force for a moment if the particle loses its crystalline state for the moment. The passage of heat through ice, whether by conduction or by radiation, in all probability is a molecular process; that is, the form of energy termed heat is transmitted from molecule to molecule of the ice. A particle takes the energy from its neighbour A on the one side and hands it over to its neighbour B on the opposite side. But the particle must be in a different state at the moment it is in possession of the energy from what it was before it received it from A, and from what it will be after it has handed it over to B. Before it became possessed of the energy, it was in the crystalline state—it was ice; and after it loses possession of the energy it will be ice; but at the moment that it is in possession of the passing energy is it in the crystalline or icy state? If we assume that it is not, but that in becoming possessed of the energy, it loses its crystalline form and for the moment becomes water, all our difficulties regarding the cause of the motion of glaciers are removed. We know that the ice of a glacier in the mass cannot become possessed of energy in the form of heat without becoming fluid; _if it can be shown that the same thing holds true of the ice particle, we have the key to the mystery of glacier-motion_. A moment’s reflection will suffice to convince any one that if the glacier ice in the mass cannot receive energy in the form of heat without melting, the same must hold true of the ice particles, for it is inconceivable that the ice in the mass could melt and yet the ice particles themselves remain in the solid state. It is the solidity of the particles which constitutes the solidity of the mass. If the particles lose their solid form the mass loses its solid form, for the mass has no other solidity than that which is possessed by the particles.

The correctness of the conclusion, that the weight of the ice is not a sufficient cause, depends upon the truth of a certain element taken for granted in the reasoning, viz. that the _shearing-force_ of the molecules of the ice remains _constant_. If this force remains constant, then Canon Moseley’s conclusion is undoubtedly correct, but not otherwise; for if a molecule should lose its shearing-force, though it were but for a moment, if no obstacle stood in front of the molecule, it would descend in virtue of its weight.

The fact that the shearing-force of a mass of ice is found to be constant does not prove that the same is the case in regard to the individual molecules. If we take a mass of molecules in the aggregate, the shearing-force of the mass taken thus collectively may remain absolutely constant, while at the same time each individual molecule may be suffering repeated momentary losses of shearing-force. This is so obvious as to require no further elucidation. The whole matter, therefore, resolves itself into this one question, as to whether or not the shearing-force of a crystalline molecule of ice remains constant. In the case of ordinary solid bodies we have no reason to conclude that the shearing-force of the molecules ever disappears, but in regard to ice it is very different.

If we analyze the process by which heat is conducted through ice, we shall find that we have reason to believe _that while a molecule of ice is in the act of transmitting the energy received (say from a fire), it loses for the moment its shearing-force if the temperature of the ice be not under_ 32° F. If we apply heat to the end of a bar of iron, the molecules at the surface of the end have their temperatures raised. Molecule A at the surface, whose temperature has been raised, instantly commences to transfer to B a portion of the energy received. The tendency of this process is to lower the temperature of A and raise that of B. B then, with its temperature raised, begins to transfer the energy to C. The result here is the same; B tends to fall in temperature, and C to rise. This process goes on from molecule to molecule until the opposite end of the bar is reached. Here in this case the energy or heat applied to the end of the bar is transmitted from molecule to molecule under the form of _heat or temperature_. The energy applied to the bar does _not change its character; it passes right along from molecule to molecule under the form of heat or temperature_. But the nature of the process must be wholly different if the transferrence takes place through a bar of ice at the temperature of 32°. Suppose we apply the heat of the fire to the end of the bar of ice at 32°, the molecules of the ice cannot possibly have their temperatures raised in the least degree. How, then, can molecule A take on, _under the form of heat_, the energy received from the fire without being heated or having its _temperature_ raised? The thing is impossible. The energy of the fire must appear in A under a different form from that of heat. The same process of reasoning is equally applicable to B. The molecule B cannot accept of the energy from A under the form of heat; it must receive it under some other form. The same must hold equally true of all the other molecules till we reach the opposite end of the bar of ice. And yet, strange to say, the last molecule transmits in the form of heat its energy to the objects beyond; for we find that the heat applied to one side of a piece of ice will affect the thermal pile on the opposite side.

The question is susceptible of a clear and definite answer. When heat is applied to a molecule of ice at 32°, the heat applied does not raise the temperature of the molecule, it is consumed in work against the cohesive forces binding the atoms or particles together into the crystalline form. The energy then must exist in the dissolved crystalline molecule, under the statical form of an affinity—crystalline affinity, or whatever else we may call it. That is to say, the energy then exists in the particles as a power or tendency to rush together again into the crystalline form, and the moment they are allowed to do so they give out the energy that was expended upon them in their separation. This energy, when it is thus given out again, assumes the dynamical form of heat; in other words, the molecule gives out _heat_ in the act of freezing. The heat thus given out may be employed to melt the next adjoining molecule. The ice-molecules take on energy from a heated body by melting. That peculiar form of motion or energy called heat disappears in forcing the particles of the crystalline molecule separate, and for the time being exists in the form of a tendency in the separated particles to come together again into the crystalline form.

But it must be observed that although the crystalline molecule, when it is acting as a conductor, takes on energy under this form from the heated body, it only exists in the molecule under such a form during the moment of transmission; that is to say, the molecule is melted, but only for the moment. When B accepts of the energy from A, the molecule A instantly assumes the crystalline form. B is now melted; and when C accepts of the energy from B, then B also in turn assumes the solid state. This process goes on from molecule to molecule till the energy is transmitted through to the opposite side and the ice is left in its original solid state. This, as will be shown in the Appendix, is the _rationale_ of Faraday’s property of regelation.

This is no mere theory or hypothesis; it is a necessary consequence from known facts. We know that ice at 32° cannot take on energy from a heated body without melting; and we know also equally well that a slab of ice at 32°, notwithstanding this, still, as a mass, retains its solid state while the heat is being transmitted through it. This proves that every molecule resumes its crystalline form the moment after the energy is transferred to the adjoining molecule.

This point being established, every difficulty regarding the descent of the glacier entirely disappears; for a molecule the moment that it assumes the fluid state is completely freed from shearing-force, and can descend by virtue of its own weight without any impediment. All that the molecule requires is simply room or space to advance in. If the molecule were in absolute contact with the adjoining molecule below, it would not descend unless it could push that molecule before it, which it probably would not be able to do. But the molecule actually has room in which to advance; for in passing from the solid to the liquid state its volume is diminished by about 1/10, and it consequently can descend. True, when it again assumes the solid form it will regain its former volume; but the question is, will it go back to its old position? If we examine the matter thoroughly we shall find that it cannot. If there were only this one molecule affected by the heat, this molecule would certainly not descend; but all the molecules are similarly affected, although not all at the same moment of time.

Let us observe what takes place, say, at the lower end of the glacier. The molecule A at the lower end, say, of the surface, receives heat from the sun’s rays; it melts, and in melting not only loses its shearing-force and descends by its own weight, but it contracts also. B immediately above it is now, so far as A is concerned, at liberty to descend, and will do so the moment that it assumes the liquid state. A by this time has become solid, and again fixed by shearing-force; but it is not fixed in its old position, but a little below where it was before. If B has not already passed into the fluid state in consequence of heat derived from the sun, the additional supply which it will receive from the solidifying of A will melt it. The moment that B becomes fluid it will descend till it reaches A. B then is solidified a little below its former position. The same process of reasoning is in a similar manner applicable to every molecule of the glacier. Each molecule of the glacier consequently descends step by step as it melts and solidifies, and hence the glacier, considered as a mass, is in a state of constant motion downwards. The fact observed by Professor Tyndall that there are certain planes in the ice along which melting takes place more readily than others will perhaps favour the descent of the glacier.

We have in this theory a satisfactory explanation of the origin of “crevasses” in glaciers. Take, for example, the transverse crevasses formed at the point where an increase in the inclination of the glacier takes place. Suppose a change of inclination from, say, 4° to 8° in the bed of the glacier. The molecules on the slope of 8° will descend more rapidly than those above on the slope of 4°. A state of tension will therefore be induced at the point where the change of inclination occurs. The ice on the slope of 8° will tend to pull after it the mass of the glacier moving more slowly on the slope above. The pull being continued, the glacier will snap asunder the moment that the cohesion of the ice is overcome. The greater the change of inclination is, the more readily will the rupture of the ice take place. Every species of crevasse can be explained upon the same principle.[309]

This theory explains also why a glacier moves at a greater rate during summer than during winter; for as the supply of heat to the glacier is greater during the former season than during the latter, the molecules will pass oftener into the liquid state.

As regards the denuding power of glaciers, I may observe that, though a glacier descends molecule by molecule, it will grind the rocky bed over which it moves as effectually as it would do did it slide down in a rigid mass in the way generally supposed; for the grinding-effect is produced not by the ice of the glacier, but by the stones, sand, and other materials forced along under it. But if all the resistances opposing the descent of a glacier, internal and external, are overcome by the mere weight of the ice alone, it can be proved that in the case of one descending with a given velocity the amount of work performed in forcing the grinding materials lying under the ice forward must be as great, supposing the motion of the ice to be molecular in the way I have explained, as it would be supposing the ice descended in the manner generally supposed.

Of course, a glacier could not descend by means of its weight as rapidly in the latter case as in the former; for, in fact, as Canon Moseley has shown, it would not in the latter case descend at all; but assuming for the sake of argument the rate of descent in both cases to be the same, the conclusion I have stated would follow. Consequently whatever denuding effects may have been attributed to the glacier, according to the ordinary theory, must be equally attributable to it according to the present explanation.

This theory, however, explains, what has always hitherto excited astonishment, viz., why a glacier can descend a slope almost horizontal, or why the ice can move off the face of a continent perfectly level.

This is the form in which my explanation was first stated about half-a-dozen years ago.[310] There is, however another element which must be taken into account. It is one which will help to cast additional light on some obscure points connected with glacial phenomena.

Ice is evidently not absolutely solid throughout. It is composed of crystalline particles, which, though in contact with one another, are, however, not packed together so as to occupy the least possible space, and, even though they were, the particles would not fit so closely together as to exclude interstices. The crystalline particles are, however, united to one another at special points determined by their polarity, and on this account they require more space; and this in all probability is the reason, as Professor Tyndall remarks, why ice, volume for volume, is less dense than water.

“They (the molecules) like the magnets,” says Professor Tyndall, “are acted upon by two distinct forces; for a time, while the liquid is being cooled, they approach each other, in obedience to their general attraction for each other. But at a certain point new forces, some attractive some repulsive, _emanating from special points_ of the molecules, come into play. The attracted points close up, the repelled points retreat. Thus the molecules turn and rearrange themselves, demanding as they do so more space, and overcoming all ordinary resistance by the energy of their demand. This, in general terms, is an explanation of the expansion of water in solidifying.”[311]

It will be obvious, then, that when a crystalline molecule melts, it will not merely descend in the manner already described, but capillary attraction will cause it to flow into the interstices between the adjoining molecules. The moment that it parts with the heat received, it will of course resolidify, as has been shown, but it will not solidify so as to fit the cavity which it occupied when in the fluid state. For the liquid molecule in solidifying assumes the crystalline form, and of course there will be a definite proportion between the length, breadth, and thickness of the crystal; consequently it will always happen that the interstice in which it solidifies will be too narrow to contain it. The result will be that the fluid molecule in passing into the crystalline form will press the two adjoining molecules aside in order to make sufficient room for itself between them, and this it will do, no matter what amount of space it may possess in all other directions. The crystal will not form to suit the cavity, the cavity must be made to contain the crystal. And what holds true of one molecule, holds true of every molecule which melts and resolidifies. This process is therefore going on incessantly in every part of the glacier, and in proportion to the amount of heat which the glacier is receiving. This internal molecular pressure, resulting from the solidifying of the fluid molecules in the interstices of the ice, acts on the mass of the ice as an expansive force, tending to cause the glacier to widen out laterally in all directions.

Conceive a mass of ice lying on a flat horizontal surface, and receiving heat on its upper surface, say from the sun; as the heat passes downwards through the mass, the molecules, acting as conductors, melt and resolidify. Each fluid molecule solidifies in an interstice, which has to be widened in order to contain it. The pressure thus exerted by the continual resolidifying of the molecules will cause the mass to widen out laterally, and of course as the mass widens out it will grow thinner and thinner if it does not receive fresh acquisition on its surface. In the case of a glacier lying in a valley, motion, however, will only take place in one direction. The sides of the valley prevent the glacier from widening; and as gravitation opposes the motion of the ice up, and favours its motion down the valley, the path of least resistance to molecular pressure will always be down the slope, and consequently in this direction molecular displacement will take place. Molecular pressure will therefore produce motion in the same direction as that of gravity. In other words, it will tend to cause the glacier to descend the valley.

The lateral expansion of the ice from internal molecular pressure explains in a clear and satisfactory manner how rock-basins may be excavated by means of land-ice. It also removes the difficulties which have been felt in accounting for the ascent of ice up a steep slope. The main difficulty besetting the theory of the excavation of rock-basins by ice is to explain how the ice after entering the basin manages to get out again—how the ice at the bottom is made to ascend the sloping sides of the basin. Pressure acting from behind, it has been argued by some; but if the basin be deep and its sides steep, this will simply cause the ice lying above the level of the basin to move forward over the surface of the mass filling it. This conclusion is, however, incorrect. The ice filling the basin and the glacier overlying it are united in one solid mass, so that the latter cannot move over the former without shearing; and although the resistance to motion offered by the sloping sides of the basin may be much greater than the resistance to shear, still the ice will be slowly dragged out of the basin. However, in order to obviate this objection to which I refer, the advocates of the glacial origin of lake-basins point out that the length of those basins in proportion to their depth is so great that the slope up which the ice has to pass is in reality but small. This no doubt is true of lake-basins in general, but it does not hold universally true. But the theory does not demand that an ice-formed lake-basin cannot have steep sides. We have incontestable evidence that ice will pass up a steep slope; and, if ice can pass up a steep slope, it can excavate a basin with a steep slope. That ice will pass up a steep slope is proved by the fact that comparatively deep and narrow river valleys are often found striated across, while hills which stood directly in the path of the ice of the glacial epoch are sometimes found striated _upwards_ from their base to their summit. Some striking examples of striæ running up hill are given by Professor Geikie in his “Glacial Drift of Scotland.” I have myself seen a slope striated upwards so steep that one could not climb it.

A very good example of a river valley striated across came under my observation during the past summer. The Tay, between Cargill and Stanley (in the centre of the broad plain of Strathmore), has excavated, through the Old Red Sandstone, a channel between 200 and 300 feet in depth. The channel here runs at right angles to the path taken during the glacial epoch by the great mass of ice coming from the North-west Highlands. At a short distance below Cargill, the trap rising out of the bed of the river is beautifully ice-grooved and striated, at right angles to the stream. A trap-dyke, several miles in length, crosses the river about a mile above Stanley, forming a rapid, known as the Linn of Campsie. This dyke is _moutonnée_ and striated from near the Linn up the sloping bank to the level of the surrounding country, showing that the ice must have ascended a gradient of one in seven to a height of 300 feet.

From what has been already stated in reference to the resolidifying of the molecules in the interstices of the ice, the application of the molecular theory to the explanation of the effects under consideration will no doubt be apparent. Take the case of the passage of the ice-sheet across a river valley. As the upper surface of the ice-sheet is constantly receiving heat from the sun and the air in contact with it, there is consequently a transferrence of heat from above downwards to the bottom of the sheet. This transferrence of heat from molecule to molecule is accompanied by the melting and resolidifying of the successive molecules in the manner already detailed. As the fluid molecules tend to flow into adjoining interstices before solidifying and assuming the crystalline form, the interstices of the ice at the bottom of the valley are constantly being filled by fluid molecules from above. These molecules no sooner enter the interstices than they pass into the crystalline form, and become, of course, separated from their neighbours by fresh interstices, which new interstices become filled by fluid molecules, which, in turn, crystallize, forming fresh interstices, and so on. The ice at the bottom of the valley, so long as this process continues, is constantly receiving fresh additions from above. The ice must therefore expand laterally to make room for these additions, which it must do unless the resistance to lateral expansion be greater than the force exerted by the molecules in crystallizing. But a resistance sufficient to do this must be enormous. The ice at the bottom of the valley cannot expand laterally without passing up the sloping sides. In expanding it will take the path of least resistance, but the path of least resistance will always be on the side of the valley towards which the general mass of the ice above is flowing.

It has been shown (Chapter XXVII.) that the ice passing over Strathmore must have been over 2,000 feet in thickness. An ice-sheet 2,000 feet in thickness exerts on its bed a pressure of upwards of 51 tons per square foot. When we reflect that ice under so enormous a pressure, with grinding materials lying underneath, was forced by irresistible molecular energy up an incline of one in seven, it is not at all surprising that the hard trap should be ground down and striated.

We can also understand how the softer portions of the rocky surface over which the ice moved should have been excavated into hollow basins. We have also an explanation of the transport of boulders from a lower to a higher level, for if ice can move from a lower to a higher level, it of course can carry boulders along with it.

The bearing which the foregoing considerations of the manner in which heat is transmitted through ice have on the question of the cause of regelation will be considered in the Appendix.

APPENDIX.

I.

OPINIONS EXPRESSED PREVIOUS TO 1864 REGARDING THE INFLUENCE OF THE ECCENTRICITY OF THE EARTH’S ORBIT ON CLIMATE.[312]

M. DE MAIRAN.

M. de Mairan, in an article in the _Memoirs of the Royal Academy of France_[313] “On the General Cause of Heat in Summer and Cold in Winter, in so far as depends on the internal and permanent Heat of the Earth,” makes the following remarks on the influence of the difference of distance of the sun in apogee and perigee:—

“Cet élément est constant pour les deux solstices; tandis que les autres (height of the sun and obliquity of his rays) y varient à raison des latitudes locales; et il y a encore cela de particulier, qu’il tend à diminuer la valeur de notre été, et à augmenter celle de notre hiver dans l’hémisphère boréal où nous sommes, et tout au contraire dans l’austral. Remarquons cependant que de ces mêmes distances, qui constituent ce troisième élément, naît en partie un autre principe de chaleur tout opposé, et qui semble devoir tempérer les effets du précédent; sçavoir, la lenteur et la vitesse réciproques du mouvement annuel apparent, en vertu duquel et du réel qui s’y mêle, le soleil emploie 8 jours de plus à parcourir les signes septentrionaux. C’est-à-dire, que le soleil passe 186½ jours dans notre hémisphère, et seulement 178½ dans l’hémisphère opposé. Ce qui, en général, ne peut manquer de répandre un pen plus de chaleur sur l’été du premier, et un peu moins sur son hiver.”

MR. RICHARD KIRWAN.

“Œpinus,[314] reasoning on astronomical principles, attributes the inferior temperature of the southern hemisphere to the shorter abode of the sun in the southern tropic, shorter by seven days, which produces a difference of fourteen days in favour of the northern hemisphere, during which more heat is accumulated, and hence he infers that the temperature of the northern hemisphere is to that of the southern, as 189·5 to 175·5, or as 14 to 13.”—_Trans. of the Royal Irish Academy_, vol. viii., p. 417. 1802.

SIR CHARLES LYELL.

“Before the amount of difference between the temperature of the two hemispheres was ascertained, it was referred by astronomers to the acceleration of the earth’s motion in its perihelion; in consequence of which the spring and summer of the southern hemisphere are shorter by nearly eight days than those seasons north of the equator. A sensible effect is probably produced by this source of disturbance, but it is quite inadequate to explain the whole phenomena. It is, however, of importance to the geologist to bear in mind that in consequence of the precession of the equinoxes, the two hemispheres receive alternately, each for a period of upwards of 10,000 years, a greater share of solar light and heat. This cause may sometimes tend to counterbalance inequalities resulting from other circumstances of a far more influential nature; but, on the other hand, it must sometimes tend to increase the extreme of deviation, which certain combinations of causes produce at distant epochs.”—_Principles_, First Edition, 1830, p. 110, vol. i.

SIR JOHN F. HERSCHEL, BART.

The following, in so far as it relates to the effects of eccentricity, is a copy of Sir John Herschel’s memoir, “On the Astronomical Causes which may influence Geological Phenomena,” read before the Geological Society, Dec. 15th, 1830.—_Trans. Geol. Soc._, vol. iii., p. 293, Second Series:—

“... Let us next consider the changes arising in the orbit of the earth itself about the sun, from the disturbing action of the planets. In so doing it will be obviously unnecessary to consider the effect produced on the solar tides, to which the above reasoning applies much more forcibly than in the case of the lunar. It is, therefore, only the variations in the supply of light and heat received from the sun that we have now to consider.

“Geometers having demonstrated the absolute invariability of the _mean_ distance of the earth from the sun, it would seem to follow that the mean annual supply of light and heat derived from that luminary would be alike invariable; but a closer consideration of the subject will show that this would not be a legitimate conclusion, but that, on the contrary, the _mean_ amount of solar radiation is dependent on the eccentricity of the orbit, and therefore liable to variation. Without going at present into any geometrical investigations, it will be sufficient for the purpose here to state it as a theorem, of which any one may easily satisfy himself by no very abstruse geometrical reasoning, that ‘_the eccentricity of the orbit varying, the_ total _quantity of heat received by the earth from the sun in one revolution is inversely proportional to the_ minor _axis of the orbit_.’ Now since the major axis is, as above observed, invariable, and therefore, of course, the absolute length of the year, it will follow that the _mean annual_ average of heat will also be in the same inverse ratio of the _minor_ axis; and thus we see that the very circumstance which on a cursory view we should have regarded as demonstrative of the constancy of our supply of solar heat, forms an essential link in the chain of strict reasoning by which its variability is proved.

“The eccentricity of the earth’s orbits is actually diminishing, and has been so for ages, beyond the records of history. In consequence, the ellipse is in a state of approach to a circle, and its minor axis being, therefore, on the increase, the annual average of solar radiation is actually on the _decrease_.

“So far this is in accordance with the testimony of geological evidence, which indicates a general refrigeration of climate; but when we come to consider the amount of diminution which the eccentricity must be supposed to have undergone to render an account of the variation which has taken place, we have to consider that, in the first place, a great diminution of the eccentricity is required to produce any sensible increase of the minor axis. This is a purely geometrical conclusion, and is best shown by the following table:—

Eccentricity. Minor Axis. Reciprocal or Ratio of Heat received. 0·00 1·000 1·000 0·05 0·999 1·002 0·10 0·995 1·005 0·15 0·989 1·011 0·20 0·980 1·021 0·25 0·968 1·032 0·30 0·954 1·048

By this it appears that a variation of the eccentricity of the orbit from the circular form to that of an ellipse, having an eccentricity of one-fourth of the major axis, would produce only a variation of 3 per cent. on the _mean_ annual amount of solar radiation, and this variation takes in the whole range of the planetary eccentricities, from that of Pallas and Juno downwards.

“I am not aware that the limit of increase of the eccentricity of the earth’s orbit has ever been determined. That it has a limit has been satisfactorily proved; but the celebrated theorem of Laplace, which is usually cited as demonstrating that none of the planetary orbits can ever deviate materially from the circular form, leads to no such conclusion, except in the case of the great preponderant planets Jupiter and Saturn, while for anything that theorem proves to the contrary, the orbit of the earth may become elliptic to any amount.

“In the absence of calculations which though practicable have, I believe, never been made,[315] and would be no slight undertaking, we may assume that eccentricities which exist in the orbits of planets, both interior and exterior to that of the earth, may _possibly_ have been attained, and may be attained again by that of the earth itself. It is clear that such eccentricities _existing_ they cannot be incompatible with the stability of the system generally, and that, therefore, the question of the possibility of such an amount in the particular case of the earth’s orbit will depend on the particular data belonging to that case, and can only be determined by executing the calculations alluded to, having regard to the simultaneous effects of at least the four most influential planets, Venus, Mars, Jupiter, and Saturn, _not only on the orbit of the earth, but on those of each other_. The principles of this calculation are detailed in the article of Laplace’s work cited. But before entering on a work of so much labour, it is quite necessary to inquire what prospect of advantage there is to induce any one to undertake it.

“Now it certainly at first sight seems clear that a variation of 3 per cent. only in the mean annual amount of solar radiation, and that arising from an extreme supposition, does _not_ hold out such a prospect. Yet it might be argued that the effects of the sun’s heat is to maintain the temperature of the earth’s surface at its actual mean height, not above the zero of Fahrenheit’s or any other thermometer, but above the temperature of the celestial spaces, out of the reach of the sun’s influence, and what that temperature is may be a matter of much discussion. M. Fourier has considered it as demonstrated that it is not greatly inferior to that of the polar regions of our own globe, but the grounds of this decision appear to me open to considerable objection.[316] If those regions be really void of matter, their temperature can only arise, according to M. Fourier’s own view of the subject, from the radiation of the stars. It ought, therefore, to be as much inferior to that due to solar radiation, as the light of a starlight night is to that of the brightest noon day, in other words it should be very nearly a total privation of heat—almost the _absolute zero_ respecting which so much difference of opinion exists, some placing it at 1,000°, some at 5,000° of Fahrenheit below the freezing-point, and some still lower, in which case a single unit per cent. in the mean annual amount of radiation would suffice to produce a change of climate fully commensurate to the demands of geologists.[317]

“Without attempting, however, to enter further into the perplexing difficulties in which this point is involved, which are far greater than appear on a cursory view, let us next consider, not the _mean_, but the _extreme_ effects which a variation in the eccentricity of the earth’s orbit may be expected to produce in the summer and winter climates in particular regions of its surface, and under the influence of circumstances favouring a difference of effect. And here, if I mistake not, it will appear that an amount of variation, which we need not hesitate to admit (at least, provisionally) as a possible one, may be productive of considerable diversity of climate, and may operate during great periods of time either to mitigate or to exaggerate the difference of winter and summer temperatures, so as to produce alternately, in the same latitude of either hemisphere, a perpetual spring, or the extreme vicissitudes of a burning summer and a rigorous winter.

“To show this, let us at once take the extreme case of an orbit as eccentric as that of Juno or Pallas, in which the greatest and least distances of the sun are to each other as 5 to 3, and consequently the radiations at those distances as 25 to 9, or very nearly as 3 to 1. To conceive what would be the _extreme_ effects of this great variation of the heat received at different periods of the year, let us first imagine in our latitude the place of the perigee of the sun to coincide with the summer solstice. In that case, the difference between the summer and winter temperature would be exaggerated in the same degree as if three suns were placed side by side in the heavens in the former season and only one in the latter, which would produce a climate perfectly intolerable. On the other hand, were the perigee situated in the winter solstice our three suns would combine to warm us in the winter, and would afford such an excess of winter radiation as would probably more than counteract the effect of short days and oblique sunshine, and throw the summer season into the winter months.

“The actual diminution of the eccentricity is so slow, that the transition from a state of the orbit such as we have assumed to the present nearly circular figure would occupy upwards of 600,000 years, supposing it uniformly changeable—this, of course, would not be the case; when near the maximum, however, it would vary slower still, so that at that point it is evident a period of 10,000 years would elapse without any perceptible change in the state of the data of the case we are considering.

“Now this adopting the very ingenious idea of Mr. Lyell[318] would suffice, by reason of the combined effect of the precession of the equinoxes and the motion of the apsides of the orbit itself, to transfer the perigee from the summer to the winter solstice, and thus to produce a transition from the one to the other species of climate in a period sufficiently great to give room for a material change in the botanical character of country.

“The supposition above made is an extreme, but it is not demonstrated to be an impossible one, and should even an approach to such a state of things be possible, the same consequences, in a mitigated degree, would follow. But if, on executing the calculations, it should appear that the limits of the eccentricity of the earth’s orbit are really narrow, and if, on a full discussion of the very difficult and delicate point of the actual effect of solar radiation, it should appear that the mean, as well as the extreme, temperature of our climates would _not_ be materially affected,—it will be at least satisfactory to _know_ that the causes of the phenomena in question are to be sought elsewhere than in the relations of our planet to the system to which it belongs, since there does not appear to exist any other conceivable connections between these relations and the facts of geology than those we have enumerated, the obliquity of the ecliptic being, as we know, confined within too narrow limits for its variation to have any sensible influence.”—_J. F. W. Herschel._

The influence which this paper might have had on the question as to whether eccentricity may be regarded as a cause of changes in geological climate appears to have been completely neutralized by the following, which appeared shortly afterwards both in his “Treatise” and “Outlines of Astronomy,” showing evidently that he had changed his mind on the subject.

“It appears, therefore, from what has been shown, the supplies of heat received from the sun will be equal in the two segments, in whatever direction the line PTQ be drawn. They will, indeed, be described in unequal times: that in which the perihelion A lies in a shorter, and the other in a longer, in proportion to their unequal area; but the greater proximity of the sun in the smaller segment compensates exactly for its more rapid description, and thus an equilibrium of heat is, as it were, maintained.

“Were it not for this the eccentricity of the orbit would materially influence the transition of seasons. The fluctuation of distance amounts to nearly 1/30th of the mean quantity, and, consequently, the fluctuation of the sun’s direct heating power to double this, or 1/15th of the whole.... Were it not for the compensation we have just described, the effect would be to exaggerate the difference of summer and winter in the southern hemisphere, and to moderate it in the northern; thus producing a more violent alternation of climate in the one hemisphere and an approach to perpetual spring in the other. _As it is, however, no such inequality subsists_, but an equal and impartial distribution of heat and light is accorded to both.”—“_Treatise of Astronomy_,” _Cabinet Cyclopædia_, § 315; _Outlines of Astronomy_, § 368.

“The fact of a great change in the general climate of large tracts of the globe, if not of the whole earth, and of a diminution of general temperature, having been recognised by geologists, from their examination of the remains of animals and vegetables of former ages enclosed in the strata, various causes for such diminution of temperature have been assigned.... It is evident that the _mean_ temperature of the whole surface of the globe, in so far as it is maintained by the action of the sun at a higher degree than it would have were the sun extinguished, must depend on the mean quantity of the sun’s rays which it receives, or, which comes to the same thing, on the _total_ quantity received in a given invariable time; and the length of the year being unchangeable in all the fluctuations of the planetary system, it follows that the total _annual_ amount of solar radiation will determine, _cæteris paribus_, the general climate of the earth. Now, it is not difficult to show that this amount is inversely proportional to the minor axis of the ellipse described by the earth about the sun, regarded as slowly variable; and that, therefore, the major axis remaining, as we know it to be, constant, and the orbit being actually in a state of approach to a circle, and consequently the minor axis being on the _increase_, the mean annual amount of solar radiation received by the whole earth must be actually on the _decrease_. We have here, therefore, an evident real cause of sufficient universality, and acting _in the right direction_, to account for the phenomenon. Its adequacy is another consideration.”[319]—_Discourse on the Study of Natural Philosophy_, pp. 145−147 (1830).

SIR CHARLES LYELL, BART.

“_Astronomical Causes of Fluctuations in Climate._—Sir John Herschel has lately inquired, whether there are any astronomical causes which may offer a possible explanation of the difference between the actual climate of the earth’s surface, and those which formerly appear to have prevailed. He has entered upon this subject, he says, ‘impressed with the magnificence of that view of geological revolutions, which regards them rather as regular and necessary effects of great and general causes, than as resulting from a series of convulsions and catastrophes, regulated by no laws, and reducible to no fixed principles.’ Geometers, he adds, have demonstrated the absolute invariability of the mean distance of the earth from the sun; whence it would seem to follow that the mean annual supply of light and heat derived from that luminary would be alike invariable; but a closer consideration of the subject will show that this would not be a legitimate conclusion, but that, on the contrary, the _mean_ amount of solar radiation is dependent on the eccentricity of the earth’s orbit, and, therefore, liable to variation.

“Now, the eccentricity of the orbit, he continues, is actually diminishing, and has been so for ages beyond the records of history. In consequence, the ellipse is in a state of approach to a circle, and the annual average of solar heat radiated to the earth is actually on the _decrease_. So far, this is in accordance with geological evidence, which indicates a general refrigeration of climate; but the question remains, whether the amount of diminution which the eccentricity may have ever undergone can be supposed sufficient to account for any sensible refrigeration.[320] The calculations necessary to determine this point, though practicable, have never yet been made, and would be extremely laborious; for they must embrace all the perturbations which the most influential planets, Venus, Mars, Jupiter, and Saturn, would cause in the earth’s orbit and in each other’s movements round the sun.

“The problem is also very complicated, inasmuch as it depends not merely on the ellipticity of the earth’s orbit, but on the assumed temperature of the celestial spaces beyond the earth’s atmosphere; a matter still open to discussion, and on which M. Fourier and Sir J. Herschel have arrived at very different opinions. But if, says Herschel, we suppose an extreme case, as if the earth’s orbit should ever become as eccentric as that of the planet Juno or Pallas, a great change of climate might be conceived to result, the winter and summer temperatures being sometimes mitigated and at others exaggerated, in the same latitudes.

“It is much to be desired that the calculations alluded to were executed, as even if they should demonstrate, as M. Arago thinks highly probable, that the mean of solar radiation can never be materially affected by irregularities in the earth’s motion, it would still be satisfactory to ascertain the point.”—_Principles of Geology_, Ninth Edition, 1853, p. 127.

M. ARAGO.

“_Can the variations which certain astronomical elements undergo sensibly modify terrestrial climates?_

“The sun is not always equally distant from the earth. At this time its least distance is observed in the first days of January, and the greatest, six months after, or in the first days of July. But, on the other hand, a time will come when the _minimum_ will occur in July, and the _maximum_ in January. Here, then, this interesting question presents itself,—Should a summer such as those we now have, in which the _maximum_ corresponds to the solar distance, differ sensibly, from a summer with which the _minimum_ of this distance should coincide?

“At first sight every one probably would answer in the affirmative; for, between the _maximum_ and the _minimum_ of the sun’s distance from the earth there is a remarkable difference, a difference in round numbers of a thirtieth of the whole. Let, however, the consideration of the velocities be introduced into the problem, elements which cannot fairly be neglected, and the result will be on the side opposite to that we originally imagined.

“The part of the orbit where the sun is found nearest the earth, is, at the same time, the point where the luminary moves most rapidly along. The demi-orbit, or, in other words, the 180° comprehended betwixt the two equinoxes of spring-time and autumn, will then be traversed in the least possible time, when, in moving from the one of the extremities of this arc to the other, the sun shall pass, near the middle of this course of six months, at the point of the smallest distance. To resume—the hypothesis we have just adopted would give, on account of the lesser distance, a spring-time and summer hotter than they are in our days; but on account of the greater rapidity, the sum of the two seasons would be shorter by about seven days. Thus, then, all things considered, the compensation is mathematically exact. After this it is superfluous to add, that the point of the sun’s orbit corresponding to the earth’s least distance changes very gradually; and that since the most distant periods, the luminary has always passed by this point, either at the end of autumn or beginning of winter.

“We have thus seen that the changes which take place in the _position_ of the solar orbit, _have no power in modifying the climate of our globe_. We may now inquire, if it be the same concerning the variations which this orbit experiences in its _form_....

“Herschel, who has recently been occupying himself with this problem, in the hope of discovering the explanation of several geological phenomena, allows that the succession of ages might bring the eccentricity of the terrestrial orbit to the proportion of that of the planet Pallas, that is to say, to be the 25/100 of a semi-greater axis. It is exceedingly improbable that in these periodical changes the eccentricity of our orbit should ever experience such enormous variations, and even then these twenty-five hundredth parts (25/100), would not augment the _mean_ annual solar radiation except by about one hundredth part (1/100). To repeat, an eccentricity of 25/100 _would not alter in any appreciated manner the mean thermometrical state of the globe_....

“The changes of the form, and of the position, of the terrestrial orbit are mathematically inoperative, or, at most, their influence is so minute that it is not indicated by the most delicate instruments. For the explanation of the changes of climates, then, there only remains to us either the local circumstances, or some alteration in the heating or illuminating power of the sun. But of these two causes, we may continue to reject the last. And thus, in fact, all the changes would come to be attributed to agricultural operations, to the clearing of plains and mountains from wood, the draining of morasses, &c.

“Thus, at one swoop, to confine, the whole earth, the variations of climates, past and future, within the limits of the naturally very narrow influence which the labour of man can effect, would be a meteorological result of the very last importance.”—pp. 221−224, _Memoir on the “Thermometrical State of the Terrestrial Globe,” in the Edinburgh New Philosophical Journal_, vol. xvi., 1834.

BARON HUMBOLDT.

“The question,” he says, “has been raised as to whether the increasing value of this ellipticity is capable during thousands of years of modifying to any considerable extent the temperature of the earth, in reference to the daily and annual quantity and distribution of heat? Whether a partial solution of the great geological problem of the imbedding of tropical vegetable and animal remains in the now cold zones may not be found in these _astronomical_ causes proceeding regularly in accordance with eternal laws?... It might at the first glance be supposed that the occurrence of the perihelion at an opposite time of the year (instead of the winter, as, is now the case, in summer) must necessarily produce great climatic variations; but, on the above supposition, the sun will no longer remain seven days longer in the northern hemisphere; no longer, as is now the case, traverse that part of the ecliptic from the autumnal equinox to the vernal equinox, in a space of time which is one week shorter than that in which it traverses the other half of its orbit from the vernal to the autumnal equinox.

“The difference of temperature which is considered as the consequence to be apprehended from the turning of the major axis, _will on the whole disappear_, principally from the circumstance that the point of our planet’s orbit in which it is nearest to the sun is at the same time always that over which it passes with the greatest velocity....

“As the altered position of the major axis is capable of exerting only a very _slight influence upon the temperature of the earth;_ so likewise the _limit_ of the probable changes in the elliptical form of the earth’s orbit are, according to Arago and Poisson, so narrow that these changes could _only very slightly_ modify the climates of the individual zones, and that in very long periods.”[321]—_Cosmos_, vol. iv., pp. 458, 459. Bohn’s Edition. 1852.

SIR HENRY T. DE LA BECHE.

“Mr. Herschel, viewing this subject with the eye of an astronomer, considers that a diminution of the surface-temperature might arise from a change in ellipticity of the earth’s orbit, which, though slowly, gradually becomes more circular. No calculations having yet been made as to the probable amount of decreased temperature from this cause, it can at present be only considered as a possible explanation of those geological phenomena which point to considerable alterations in climates.”—_Geological Manual_. Third Edition. 1833. p. 8.

PROFESSOR PHILLIPS.

“_Temperature of the Globe._—_Influence of the Sun._—No proposition is more certain than the fundamental dependence of the temperature of the surface of the globe on the solar influence.

“It is, therefore, very important for geologists to inquire whether this be variable or constant; whether the amount of solar heat communicated to the earth is and has always been the same in every annual period, or what latitude the laws of planetary movements permit in this respect.

“Sir John Herschel has examined this question in a satisfactory manner, in a paper read to the Geological Society of London. The total amount of solar radiation which determines the general climate of the earth, the year being of invariable length, is inversely proportional to the minor axis of the ellipse described by the earth about the sun, regarded as slowly variable; the major axis remaining constant and the orbit being actually in a state of approach to a circle, and, consequently, the minor axis being on the increase, it follows that the mean annual amount of solar radiation received by the whole earth must be actually on the decrease. The limits of the variation in the eccentricity of the earth’s orbit are not known. It is, therefore, impossible to say accurately what may have been in former periods of time, the amount of solar radiation; it is, however, certain that if the ellipticity has ever been so great as that of the orbit of Mercury or Pallas, the temperature of the earth must have been sensibly higher than it is at present. But the difference of a few degrees of temperature thus occasioned, is of too small an order to be employed in explaining the growth of tropical plants and corals in the polar or temperate zones, and other great phenomena of Geology.”—_From A Treatise on Geology_, p. 11, _forming the article under that head in the seventh edition of the Encyclopædia Britannica_. 1837.

MR. ROBERT BAKEWELL.

“A change in the form of the earth’s orbit, if considerable, might change the temperature of the earth, by bringing it nearer to the sun in one part of its course. The orbit of the earth is an ellipsis approaching nearly to a circle; the distance from the centre of the orbit to either focus of the ellipsis is called by astronomers ‘the eccentricity of the orbit.’ This eccentricity has been for ages slowly decreasing, or, in other words, the orbit of the earth has been approaching nearer to the form of a perfect circle; after a long period it will again increase, and the possible extent of the variation has not been yet ascertained. From what is known respecting the orbits of Jupiter and Saturn, it appears highly probable that the eccentricity of the earth’s orbit is confined within limits that preclude the belief of any great change in the mean annual temperature of the globe ever having been occasioned by this cause.”—_Introduction to Geology_, p. 600. 1838. Fifth Edition.

MRS. SOMERVILLE.

“Sir John Herschel has shown that the elliptical form of the earth’s orbit has but a trifling share in producing the variation of temperature corresponding to the difference of the seasons.”—_Physical Geography_, vol. ii., p. 20. Third Edition.

MR. L. W. MEECH, A.M.

“Let us, then, look back to that primeval epoch when the earth was in aphelion at midsummer, and the eccentricity at its maximum value—assigned by Leverrier near to ·0777. Without entering into elaborate computation, it is easy to see that the extreme values of diurnal intensity, in Section IV., would be altered as by the multiplier ((1 ± _e_)/(1 ± _e′_))^2, that is 1 - 0·11 in summer, and 1 + 0·11 in winter. This would diminish the midsummer intensity by about 9°, and increase the midwinter intensity by 3° or 4°; the temperature of spring and autumn being nearly unchanged. But this does not appear to be of itself adequate to the geological effects in question.

“It is not our purpose, here, to enter into the inquiry whether the atmosphere was once more dense than now, whether the earth’s axis had once a different inclination to the orbit, or the sun a greater emissive power of heat and light. Neither shall we attempt to speculate upon the primitive heat of the earth, nor of planetary space, nor of the supposed connection of terrestrial heat and magnetism; nor inquire how far the existence of coal-fields in this latitude, of fossils, and other geological remains, have depended upon existing causes. The preceding discussion seems to prove simply that, under the present system of physical astronomy, the sun’s intensity could never have been materially different from what is manifested upon the earth at the present day. _The causes of notable geological changes must be other than the relative position of the sun and earth, under their present laws of motion._”—_“On the Relative Intensity of the Heat and Light of the Sun.” Smithsonian Contributions to Knowledge_, vol. ix.

M. JEAN REYNAUD.

“La révolution qui pourrait y causer les plus grands changements thermométriques, celle qui porte l’orbite à s’élargir et à se rétrécir alternativement et, par suite, la planète à passer, aux époques de périhélie, plus ou moins près du soleil, embrasse une période de plus de cent mille années terrestres et demeure comprise dans de si étroites limites que les habitants doivent être à peine avertis que la chaleur décroît, par cette raison, depuis une haute antiquité et décroîtra encore pendant des siècles en variant en même temps dans sa répartition selon les diverses époques de l’année.... Enfin, le tournoiement de l’axe du globe s’empreint également d’une manière particulière sur l’ètablissement des saisons qui, à tour de rôle, dans chacun des deux hémisphères, deviennent graduellement, durant une période d’environ vingt-cinq mille ans, de plus en plus uniformes, ou, à l’inverse, de plus en plus dissemblables. C’est actuellement dans l’hémisphère boréal que règne l’uniformité, et quoique les étés et les hivers y tendent, dès à présent, à se trancher de plus en plus, il ne paraît pas douteux que la modération des saisons n’y produise, pendant longtemps encore, des effets appréciables. En résumé, de tous ces changements il n’en est donc aucun ni qui suive un cours précipité, ni qui s’élève jamais à des valeurs considérables; ils se règlent tous sur un mode de développement presque insensible, et il s’ensuit que les années de la terre, malgré leur complexité virtuelle, se distinguent par le constance de leurs caractères non-seulement de ce qui peut avoir lieu, en vertu des mêmes principes, dans les autres systèmes planétaires de l’univers, mais même de ce qui s’observe dans plusieurs des mondes qui composent le nôtre.”—_Philosophie Religieuse: Terre et Ciel._

M. ADHÉMAR.

Adhémar does not consider the effects which ought to result from a change in the eccentricity of the earth’s orbit; he only concerns himself with those which, in his opinion, arise from the present amount of such eccentricity. He admits, of course, that both hemispheres receive from the sun equal quantities of heat per annum; but, as the southern hemisphere has a winter longer by 168 hours than the corresponding season in the northern hemisphere, an accumulation of heat necessarily takes place in the latter, and an accumulation of cold in the former. Adhémar also measures the loss of heat sustained by the southern hemisphere in a year by the number of hours by which the southern exceeds the northern winter. “The south pole,” he says, “loses in one year more heat than it receives, because the total duration of its nights surpasses that of the days by 168 hours; and the contrary takes place for the north pole. If, for example, we take for unity the mean quantity of heat which the sun sends off in one hour, the heat accumulated at the end of the year at the north pole will be expressed by 168, while the heat lost by the south pole will be equal to 168 times what the radiation lessens it by in one hour; so that at the end of the year the difference in the heat of the two hemispheres will be represented by 336 times what the earth receives from the sun or loses in an hour by radiation,”[322] and at the end of 100 years the difference will be 33,600 times, and at the end of 1,000 years 336,000 times, or equal to what the earth receives from the sun in 38½ years, and so on during the 10,000 years that the southern winter exceeds in length the northern. This, in his opinion, is all that is required to melt the ice off the arctic regions, and cover the antarctic regions with an enormous ice-cap. He further supposes that in about 10,000 years, when our northern winter will occur in aphelion and the southern in perihelion, the climatic conditions of the two hemispheres will be reversed; that is to say, the ice will melt at the south pole, and the northern hemisphere will become enveloped in one continuous mass of ice, leagues in thickness, extending down to temperate regions.

This theory, as shown in Chapter V., is based upon a misconception regarding the laws of radiant heat. The loss of heat sustained by the southern hemisphere from radiation, resulting from the greater length of the southern winter, is vastly over-estimated by M. Adhémar, and could not possibly produce the effects which he supposes. But I need not enter into this subject here, as the reader will find the whole question discussed at length in the chapter above referred to. By far the most important part of Adhemar’s theory, however, is his conception of the submergence of the land by means of a polar ice-cap. He appears to have been the first to put forth the idea that a mass of ice placed on the globe, say, for example, at the south pole, will shift the earth’s centre of gravity a little to the south of its former position, and thus, as a physical consequence, cause the sea to sink at the north pole and to rise at the south. According to Adhémar, as the one hemisphere cools and the other grows warmer, the ice at the pole of the former will increase in thickness and that at the pole of the latter diminish.

The sea, as a consequence, will sink on the warm hemisphere where the ice is decreasing and rise on the cold hemisphere where the ice is increasing. And, again, in 10,000 years, when the climatic conditions of the two hemispheres are reversed, the sea will sink on the hemisphere where it formerly rose, and rise on the hemisphere where it formerly sank, and so on in like manner through indefinite ages.

Adhémar, however, acknowledges to have derived the grand conception of a submergence of the land from the shifting of the earth’s centre of gravity from the following wild speculation of one Bertrand, of Hamburgh:—

“Bertrand de Hambourg, dans un ouvrage imprimé en 1799 et qui a pour titre: _Renouvellement périodique des Continents_, avait déjà émis cette idée, que la masse des eaux pouvait être alternativement entraînée d’un hémisphère à l’autre par le déplacement du centre de gravité du globe. Or, pour expliquer ce déplacement, il supposait que la terre était creuse et qu’il y avait dans son intérieur un gros noyau d’aimant auquel les comètes par leur attraction communiquaient un mouvement de va-et-vient analogue à celui du pendule.”—_Révolutions de la Mer_, p. 41.

The somewhat extravagant notions which Adhémar has advanced in connection with his theory of submergence have very much retarded its acceptance. Amongst other remarkable views he supposes the polar ice-cap to rest on the bottom of the ocean, and to rise out of the water to the enormous height of twenty leagues. Again, he holds that on the winter approaching perihelion and the hemisphere becoming warm the ice waxes soft and rotten from the accumulated heat, and the sea now beginning to eat into the base of the cap, this is so undermined as, at last, to be left standing upon a kind of gigantic pedestal. This disintegrating process goes on till the fatal moment at length arrives, when the whole mass tumbles down into the sea in huge fragments which become floating icebergs. The attraction of the opposite ice-cap, which has by this time nearly reached its maximum thickness, becomes now predominant. The earth’s centre of gravity suddenly crosses the plain of the equator, dragging the ocean along with it, and carrying death and destruction to everything on the surface of the globe. And these catastrophes, he asserts, occur alternately on the two hemispheres every 10,000 years.—_Révolutions de la Mer_, pp. 316−328.

Adhémar’s theory has been advocated by M. Le Hon, of Brussels, in a work entitled _Périodicité des Grands Déluges_. Bruxelles et Leipzig, 1858.

II.

ON THE NATURE OF HEAT-VIBRATIONS.[323]

From the _Philosophical Magazine_ for May, 1864.

In a most interesting paper on “Radiant Heat,” by Professor Tyndall, read before the Royal Society in March last, it is shown conclusively that the _period_ of heat-vibrations is not affected by the state of aggregation of the molecules of the heated body; that is to say, whether the substance be in the gaseous, the liquid, or, perhaps, the solid condition, the tendency of its molecules to vibrate according to a given period remains unchanged. The force of cohesion binding the molecules together exercises no effect on the rapidity of vibration.

I had arrived at the same conclusion from theoretical considerations several years ago, and had also deduced some further conclusions regarding the nature of heat-vibrations, which seem to be in a measure confirmed by the experimental results of Professor Tyndall. One of these conclusions was, that the heat-vibration does not consist in a motion of an aggregate mass of molecules, but in a motion of the individual molecules themselves. Each molecule, or rather we should say each atom, acts as if there were no other in existence but itself. Whether the atom stands by itself as in the gaseous state, or is bound to other atoms as in the liquid or the solid state, it behaves in exactly the same manner. The deeper question then suggested itself, viz., what is the nature of that mysterious motion called heat assumed by the atom? Does it consist in excursions across centres of equilibrium external to the atom itself? It is the generally received opinion among physicists that it does. But I think that the experimental results arrived at by Professor Tyndall, as well as some others which will presently be noticed, are entirely hostile to such an opinion. The relation of an atom to its centre of equilibrium depends entirely on the state of aggregation. Now if heat-vibrations consist in excursions to and fro across these centres, then the _period_ ought to be affected by the state of aggregation. The higher the _tension_ of the atom in regard to the centre, the more rapid ought its movement to be. This is the case in regard to the vibrations constituting sound. The harder a body becomes, or, in other words, the more firmly its molecules are bound together, the higher is the _pitch_. Two harp-cords struck with equal force will vibrate with equal force, however much they may differ in the rapidity of their vibrations. The _vis viva_ of vibration depends upon the force of the stroke; but the rapidity depends, not on the stroke, but upon the tension of the cord.

That heat-vibrations do not consist in excursions of the molecules or atoms across centres of equilibrium, follows also as a necessary consequence from the fact that the real specific heat of a body remains unchanged under all conditions. All changes in the specific heat of a body are due to differences in the amount of heat consumed in molecular work against cohesion or other forces binding the molecules together. Or, in other words, to produce in a body no other effect than a given rise of temperature, requires the same amount of force, whatever may be the physical condition of the body. Whether the body be in the solid, the fluid, or the gaseous condition, the same rise of temperature always indicates the same quantity of force consumed in the simple production of the rise. Now, if heat-vibrations consist in excursions of the atom to and fro across a centre of equilibrium _external to itself_, as is generally supposed, then the _real_ specific heat of a solid body, for example, _ought to decrease with the hardness of the body_, because an increase in the strength of the force binding the molecules together would in such a case tend to favour the rise in the rapidity of the vibrations.

These conclusions not only afford us an insight into the hidden nature of heat-vibrations, but they also appear to cast some light on the physical constitution of the atom itself. They seem to lead to the conclusion that the ultimate atom itself is _essentially elastic_.[324] For if heat-vibrations do not consist in excursions of the atom, then it must consist in alternate expansions and contractions of the atom itself. This again is opposed to the ordinary idea that the atom is essentially solid and impenetrable. But it favours the modern idea, that matter consists of forces of resistance acting from a centre.

Professor Tyndall in a memoir read before the Royal Society “On a new Series of Chemical Reactions produced by Light,” has subsequently arrived at a similar conclusion in reference to the atomic nature of heat-vibrations. The following are his views on the subject:—

“A question of extreme importance in molecular physics here arises:—What is the real mechanism of this absorption, and where is its seat?

“I figure, as others do, a molecule as a group of atoms, held together by their mutual forces, but still capable of motion among themselves. The vapour of the nitrite of amyl is to be regarded as an assemblage of such molecules. The question now before us is this:—In the act of absorption, is it the _molecules_ that are effective, or is it their constituent _atoms?_ Is the _vis viva_ of the intercepted waves transferred to the molecule as a whole, or to its constituent parts?

“The molecule, as a whole, can only vibrate in virtue of the forces exerted between it and its neighbour molecules. The intensity of these forces, and consequently the rate of vibration, would, in this case, be a function of the distance between the molecules. Now the identical absorption of the liquid and of the vaporous nitrite of amyl indicates an identical vibrating period on the part of liquid and vapour, and this, to my mind, amounts to an experimental demonstration that the absorption occurs in the main _within_ the molecule. For it can hardly be supposed, if the absorption were the act of the molecule as a whole, that it could continue to affect waves of the same period after the substance had passed from the vaporous to the liquid state.”—_Proc. of Roy. Soc._, No. 105. 1868.

Professor W. A. Norton, in his memoir on “Molecular Physics,”[325] has also arrived at results somewhat similar in reference to the nature of heat-vibrations. “It will be seen,” he says, “that these (Mr. Croll’s) ideas are in accordance with the conception of the constitution of a molecule adopted at the beginning of the present memoir (p. 193), and with the theory of heat-vibrations or heat-pulses deduced therefrom (p. 196).”[326]

III.

ON THE REASON WHY THE DIFFERENCE OF READING BETWEEN A THERMOMETER EXPOSED TO DIRECT SUNSHINE AND ONE SHADED DIMINISHES AS WE ASCEND IN THE ATMOSPHERE.[327]

From the _Philosophical Magazine_ for March, 1867.

The remarkable fact was observed by Mr. Glaisher, that the difference of reading between a black-bulb thermometer exposed to the direct rays of the sun and one shaded diminishes as we ascend in the atmosphere. On viewing the matter under the light of Professor Tyndall’s important discovery regarding the influence of aqueous vapour on radiant heat, the fact stated by Mr. Glaisher appears to be in perfect harmony with theory. The following considerations will perhaps make this plain.

The shaded thermometer marks the temperature of the surrounding air; but the exposed thermometer marks not the temperature of the air, but that of the bulb heated by the direct rays of the sun. The temperature of the bulb depends upon two elements: (1) the rate at which it receives heat by _direct radiation_ from the sun above, the earth beneath, and all surrounding objects, and by _contact_ with the air; (2) the rate at which it loses heat by radiation and by contact with the air. As regards the heat gained and lost by contact with the surrounding air, both thermometers are under the same conditions, or nearly so. We therefore require only to consider the element of radiation.

We begin by comparing the two thermometers at the earth’s surface, and we find that they differ by a very considerable number of degrees. We now ascend some miles into the air, and on again comparing the thermometers we find that the difference between them has greatly diminished. It has been often proved, by direct observation, that the intensity of the sun’s rays increases as we rise in the atmosphere. How then does the exposed thermometer sink more rapidly than the shaded one as we ascend? The reason is obviously this. The temperature of the thermometers depends as much upon the rate at which they are losing their heat as upon the rate at which they are gaining it. The higher temperature of the exposed thermometer is the result of _direct radiation_ from the sun. Now, although this thermometer receives by radiation more heat from the sun at the upper position than at the lower, it does not necessarily follow on this account that its temperature ought to be higher. Suppose that at the upper position it should receive one-fourth more heat from the sun than at the lower, yet if the rate at which it loses its heat by radiation into space be, say, one-third greater at the upper position than at the lower, the temperature of the bulb would sink to a considerable extent, notwithstanding the extra amount of heat received. Let us now reflect on how matters stand in this respect in regard to the actual case under our consideration. When the exposed thermometer is at the higher position, it receives more heat from the sun than at the lower, but it receives less from the earth; for a considerable part of the radiation from the earth is cut off by the screen of aqueous vapour intervening between the thermometer and the earth. But, on the whole, it is probable that the total quantity of radiant heat reaching the thermometer is greater in the higher position than in the lower. Compare now the two positions in regard to the rate at which the thermometer loses its heat by radiation. When the thermometer is at the lower position, it has the warm surface of the ground against which to radiate its heat downwards. The high temperature of the ground thus tends to diminish the rate of radiation. Above, there is a screen of aqueous vapour throwing back upon the thermometer a very considerable part of the heat which the instrument is radiating upwards. This, of course, tends greatly to diminish the loss from radiation. But at the upper position this very screen, which prevented the thermometer from throwing off its heat into the cold space above, now affects the instrument in an opposite manner; for the thermometer has now to radiate its heat downwards, not upon the warm surface of the ground as before, but upon the cold upper surface of the aqueous screen intervening between the instrument and the earth. This of course tends to lower the mercury. We are now in a great measure above the aqueous screen, with nothing to protect the thermometer from the influence of cold stellar space. It is true that the air above is at a temperature little below that of the thermometer itself; but then the air is dry, and, owing to its diathermancy, it does not absorb the heat radiated from the thermometer, and consequently the instrument radiates its heat directly into the cold stellar space above, some hundreds of degrees below zero, almost the same as it would do were the air entirely removed. The enormous loss of heat which the thermometer now sustains causes it to fall in temperature to a great extent. The molecules of the comparatively dry air at this elevation, being very bad radiators, do not throw off their heat into space so rapidly as the bulb of the exposed thermometer; consequently their temperature does not (for this reason) tend to sink so rapidly as that of the bulb. Hence the shaded thermometer, which indicates the temperature of those molecules, is not affected to such an extent as the exposed one. Hence also the difference of reading between the two instruments must diminish as we rise in the atmosphere.

This difference between the temperature of the two thermometers evidently does not go on diminishing to an indefinite extent. Were we able to continue our ascent in the atmosphere, we should certainly find that a point would be reached beyond which the difference of reading would begin to increase, and would continue to do so till the outer limits of the atmosphere were reached. The difference between the temperatures of the two thermometers beyond the limits of the atmosphere would certainly be enormous. The thermometer exposed to the direct rays of the sun would no doubt be much colder than it had been when at the earth’s surface; but the shaded thermometer would now indicate the temperature of space, which, according to Sir John Herschel and M. Pouillet, is more than 200° Fahrenheit below zero.

It follows also, from what has been stated, that even under direct sunshine the removal of the earth’s atmosphere would tend to lower the temperature of the earth’s surface to a great extent. This conclusion also follows as an immediate inference from the fact that the earth’s atmosphere, as it exists at present charged with aqueous vapour, affects terrestrial radiation more than it does radiation from the sun; for the removal of the atmosphere would increase the rate at which the earth throws off its heat into space more than it would increase the rate at which it receives heat from the sun; therefore its temperature would necessarily fall until the rate of radiation _from_ the earth’s surface exactly equalled the rate of radiation _to_ the surface. Let the atmosphere again envelope the earth, and terrestrial radiation would instantly be diminished; the temperature of the earth’s surface would therefore necessarily begin to rise, and would continue to do so till the rate of radiation from the surface would equal the rate of radiation received by the surface. Equilibrium being thus restored, the temperature would remain stationary. It is perfectly obvious that if we envelope the earth with a substance such as our atmosphere, that offers more resistance to terrestrial radiation than to solar, the temperature of the earth’s surface must necessarily rise until the heat which is being radiated off equals that which is being received from the sun. Remove the air and thus get quit of the resistance, and the temperature of the surface would fall, because in this case a lower temperature would maintain equilibrium.

It follows, therefore, that the moon, which has no atmosphere, must be much colder than our earth, even on the side exposed to the sun. Were our earth with its atmosphere as it exists at present removed to the orbit of Venus or Mars, for example, it certainly would not be habitable, owing to the great change of temperature that would result. But a change in the physical constitution of the atmospheric envelope is really all that would be necessary to retain the earth’s surface at its present temperature in either position.

IV.

REMARKS ON MR. J. Y. BUCHANAN’S THEORY OF THE VERTICAL DISTRIBUTION OF TEMPERATURE OF THE OCEAN.[328]

Since the foregoing was in type, a paper on the “Vertical Distribution of Temperature of the Ocean,” by Mr. J. Y. Buchanan, chemist on board the _Challenger_, has been read before the Royal Society.[329] In that paper Mr. Buchanan endeavours to account for the great depth of warm water in the middle of the North Atlantic compared with that at the equator, without referring it to horizontal circulation of any kind.

The following is the theory as stated by Mr. Buchanan:—

“Let us assume the winter temperature of the surface-water to be 60° F. and the summer temperature to be 70° F. If we start from midwinter, we find that, as summer approaches, the surface-water must get gradually warmer, and that the temperature of the layers below the surface must decrease at a very rapid rate, until the stratum of winter temperature, or 60° F., is reached; in the language of the isothermal charts, the isothermal line for degrees between 70° F. (if we suppose that we have arrived at midsummer) and 60° F. open out or increase their distance from each other as the depth increases. Let us now consider the conditions after the summer heat has begun to waver. During the whole period of heating, the water, from its increasing temperature, has been always becoming lighter, so that heat communication by convection with the water below has been entirely suspended during the whole period. The heating of the surface-water has, however, had another effect, besides increasing its volume; it has, by evaporation, rendered it denser than it was before, at the same temperature. Keeping in view this double effect of the summer heat upon the surface-water, let us consider the effect of the winter cold upon it. The superficial water having assumed the atmospheric temperature of, say 60° F., will sink through the warmer water below it, until it reaches the stratum of water having the same temperature as itself. Arrived here, however, although it has the same temperature as the surrounding water, the two are no longer in equilibrium, for the water which has come from the surface, has a greater density than that below at the same temperature. It will therefore not be arrested at the stratum of the same temperature, as would have been the case with fresh water; but it will continue to sink, carrying of course its higher temperature with it, and distributing it among the lower layers of colder water. At the end of the winter, therefore, and just before the summer heating recommences, we shall have at the surface a more or less thick stratum of water having a nearly uniform temperature of 60° F., and below this the temperature decreasing at a considerable but less rapid rate than at the termination of the summer heating. If we distinguish between _surface-water_, the temperature of which rises with the atmospheric temperature (following thus, in direction at least, the variation of the seasons), and _subsurface_-water, or the stratum immediately below it, we have for the latter the, at first sight, paradoxical effect of summer cooling and winter heating. The effect of this agency is to diffuse the same heat to a greater depth in the ocean, the greater the yearly range of atmospheric temperature at the surface. This effect is well shown in the chart of isothermals, on a vertical section, between Madeira and a position in lat. 3° 8′ N., long. 14° 49′ W. The isothermal line for 45° F. rises from a depth of 740 fathoms at Madeira to 240 fathoms at the above-mentioned position. In equatorial regions there is hardly any variation in the surface-temperature of the sea; consequently we find cold water very close to the surface all along the line. On referring to the temperature section between the position lat. 3° 8′ N., long. 14° 49′ W., and St. Paul’s Rocks, it will be seen that, with a surface-temperature of from 75° F. to 79° F., water at 55° F. is reached at distances of less than 100 fathoms from the surface. Midway between the Azores and Bermuda, with a surface-temperature of 70° F., it is only at a depth of 400 fathoms that we reach water of 55° F.”

What Mr. Buchanan states will explain why the mean annual temperature of the water at the surface extends to a greater depth in the middle of the North Atlantic than at the equator. It also explains why the temperature from the surface downwards decreases more rapidly at the equator than in the middle of the North Atlantic; but, if I rightly understand the theory, it does not explain (and this is the point at issue) why at a given depth the temperature of the water in the North Atlantic should be higher than the temperature at a corresponding depth at the equator. Were there no horizontal circulation the greatest thickness of warm water would certainly be found at the equator and the least at the poles. The isothermals would in such a case gradually slope downwards from the poles to the equator. The slope might not be uniform, but still it would be a continuous downward slope.

V.

ON THE CAUSE OF THE COOLING EFFECT PRODUCED ON SOLIDS BY TENSION.[330]

From the _Philosophical Magazine_ for May, 1864.

From a series of experiments made by Dr. Joule with his usual accuracy, he found that when bodies are subjected to tension, a cooling effect takes place. “The quantity of cold,” he says, “produced by the application of tension was sensibly equal to the heat evolved by its removal; and further, that the thermal effects were proportional to the weight employed.”[331] He found that when a weight was applied to compress a body, a certain amount of heat was evolved; but the same weight, if applied to stretch the body, produced a corresponding amount of cold.

This, although it does not appear to have been remarked, is a most singular result. If we employ a force to compress a body, and then ask what has become of the force applied, it is quite a satisfactory answer to be told that the force is converted into heat, and reappears in the molecules of the body as such; but if the same force be employed to stretch the body, it will be no answer to be told that the force is converted into cold. Cold cannot be the force under another form, for cold is a privation of force. If a body, for example, is compressed by a weight, the _vis viva_ of the descending weight is transmitted to the molecules of the body and reappears under that form of force called heat; but if the same weight is applied so as to stretch or expand the body, not only does the force of the weight disappear without producing heat, but the molecules which receive the force lose part of that which they already possessed. Not only does the force of the weight disappear, but along with it a portion of the force previously existing in the molecules under the form of heat. We have therefore to inquire, not merely into what becomes of the force imparted by the weight, but also what becomes of the force in the form of heat which disappears from the molecules of the body itself. That the _vis viva_ of the descending weight should disappear without increasing the heat of the molecules is not so surprising, because it may be transformed into some other form of force different from that of heat. For it is by no means evident _à priori_ that heat should be the only form under which it may exist. But it is somewhat strange that it should cause the force previously existing in the molecules in the form of heat also to change into some other form.

When a weight, for example, is employed to stretch a solid body, it is evident that the force exerted by the weight is consumed in work against the cohesion of the particles, for the entire force is exerted so as to pull them separate from each other. But the cooling effect which takes place shows that more force disappears than simply what is exerted by the weight; for the cooling effect is caused by the disappearance of force in the shape of heat from the body itself. The force exerted by the weight disappears in performing work against the cohesion of the particles of the body stretched. But what becomes of the energy in the form of heat which disappears from the body at the same time? It must be consumed in performing work of some kind or other. The force exerted by the weight cannot be the cause of the cooling effect. The transferrence of force from the weight to the body may be the cause of a heating effect—an increase of force in the body; but this transferrence of force to the body cannot be the cause of a decrease of force in the body. If a decrease of force actually follows the application of tension, the weight can only be the occasion, not the cause of the decrease.

In what manner, then, does the stretching of the body by the weight become the occasion of its losing energy in the shape of heat? Or, in other words, what is the cause of the cooling effects which result from tension? The probable explanation of the phenomenon seems to be this: if the molecules of a body are held together by any force, of whatever nature it may be, which prevents any further separation taking place, then the entire heat applied to such a body will appear as temperature; but if this binding force becomes lessened so as to allow further expansion, then a portion of the heat applied will be lost in producing expansion. All solids at any given temperature expand until the expansive force of their heat exactly balances the cohesive force of their molecules, after which no further expansion at the same temperature can possibly take place while the cohesive force of the molecules remains unchanged. But if, by some means or other, the cohesive force of the molecules become reduced, then instantly the body will expand under the heat which it possesses, and of course a portion of the heat will be consumed in expansion, and a cooling effect will result. Now tension, although it does not actually lessen the cohesive force of the molecules of the stretched body, yet produces, by counteracting this force, the same effect; for it allows the molecules an opportunity of performing work of expansion, and a cooling effect is the consequence. If the piston of a steam-engine, for example, be loaded to such an extent that the steam is unable to move it, the steam in the interior of the cylinder will not lose any of its heat; but if the piston be raised by some external force, the molecules of the steam will assist this force, and consequently will suffer loss of heat in proportion to the amount of work which they perform. The very same occurs when tension is applied to a solid. Previous to the application of tension, the heat existing in the molecules is unable to produce any expansion against the force of cohesion. But when the influence of cohesion is partly counteracted by the tension applied, the heat then becomes enabled to perform work of expansion, and a cooling effect is the result.

VI.

THE CAUSE OF REGELATION.[332]

There are two theories which have been advanced to explain Regelation, the one by Professor Faraday, and the other by Professor James Thomson.

According to Professor James Thomson, pressure is the cause of regelation. Pressure applied to ice tends to lower the melting-point, and thus to produce liquefaction; but the water which results is colder than the ice, and refreezes the moment it is relieved from pressure. When two pieces of ice are pressed together, a melting takes place at the points in contact, resulting from the lowering of the melting-point; the water formed, re-freezing, joins the two pieces together.

The objection which has been urged against this theory is that regelation will take place under circumstances where it is difficult to conceive how pressure can be regarded as the cause. Two pieces of ice, for example, suspended by silken threads in an atmosphere above the melting-point, if but simply allowed to touch each other, will freeze together. Professor J. Thomson, however, attributes the freezing to the pressure resulting from the capillary attraction of the two moist surfaces in contact. But when we reflect that it requires the pressure of a mile of ice—135 tons on the square foot—to lower the melting-point one degree, it must be obvious that the lowering effect resulting from capillary attraction in the case under consideration must be infinitesimal indeed.

The following clear and concise account of Faraday’s theory, I quote from Professor Tyndall’s “Forms of Water:”—

“Faraday concluded that _in the interior_ of any body, whether solid or liquid, where every particle is grasped, so to speak, by the surrounding particles, and grasps them in turn, the bond of cohesion is so strong as to require a higher temperature to change the state of aggregation than is necessary _at the surface_. At the surface of a piece of ice, for example, the molecules are free on one side from the control of other molecules; and they therefore yield to heat more readily than in the interior. The bubble of air or steam in overheated water also frees the molecules on one side; hence the ebullition consequent upon its introduction. Practically speaking, then, the point of liquefaction of the interior ice is higher than that of the superficial ice....

“When the surfaces of two pieces of ice, covered with a film of the water of liquefaction, are brought together, the covering film is transferred from the surface to the centre of the ice, where the point of liquefaction, as before shown, is higher than at the surface. The special solidifying power of ice upon water is now brought into play _on both sides of the film_. Under these circumstances, Faraday held that the film would congeal, and freeze the two surfaces together.”—_The Forms of Water_, p. 173.

The following appears to be a more simple explanation of the phenomena than either of the preceding:—

The freezing-point of water, and the melting-point of ice, as Professor Tyndall remarks, touch each other as it were at this temperature. At a hair’s-breadth lower water freezes; at a hair’s-breadth higher ice melts. Now if we wish, for example, to freeze water, already just about the freezing-point, or to melt a piece of ice already just about the melting-point, we can do this either by a change of temperature or by a change of the melting-point. But it will be always much easier to effect this by the former than by the latter means. Take the case already referred to, of the two pieces of ice suspended in an atmosphere above the melting-point. The pieces at their surfaces are in a melting condition, and are surrounded by a thin film of water just an infinitesimal degree above the freezing-point. The film has on the one side solid ice at the freezing-point, and on the other a warm atmosphere considerably above the freezing-point. The tendency of the ice is to lower the temperature of the film, while that of the air is to raise its temperature. When the two pieces are brought into contact the two films unite and form one film separating the two pieces of ice. This film is not like the former in contact with ice on the one side and warm air on the other. It is surrounded on both sides by solid ice. The tendency of the ice, of course, is to lower the film to the same temperature as the ice itself, and thus to produce solidification. It is evident that the film must either melt the ice or the ice must freeze the film, if the two are to assume the same temperature. But the power of the ice to produce solidification, owing to its greater mass, is enormously greater than the power of the film to produce fluidity, consequently regelation is the result.

VII.

LIST OF PAPERS WHICH HAVE APPEARED IN DR. A. PETERMANN’S _GEOGRAPHISCHE MITTHEILUNGEN_ RELATING TO THE GULF-STREAM AND THERMAL CONDITION OF THE ARCTIC REGIONS.

The most important memoir which we have on the Gulf-stream and its influence on the climate of the arctic regions is the one by Dr. A. Petermann, entitled “Der Golfstrom und Standpunkt der thermometrischen Kenntniss des nord-atlantischen Oceans und Landgebiets im Jahre 1870.” _Geographische Mittheilungen_, Band XVI. 1870.

Dr. Petermann has, in this memoir, by a different line of argument from that which I have pursued in this volume, shown in the most clear and convincing manner that the abnormally high temperature of the north-western shores of Europe and the seas around Spitzbergen is owing entirely to the Gulf-stream, and not to any general circulation such as that advocated by Dr. Carpenter. From a series of no fewer than 100,000 observations of temperature in the North Atlantic and in the arctic seas, he has been enabled to trace with accuracy on his charts the very footsteps of the heat in its passage from the Gulf of Mexico up to the shores of Spitzbergen.

The following is a list of the more important papers bearing on the subject which have recently appeared in Dr. Petermann’s _Geogr. Mittheilungen_:—

An English translation of Dr. Petermann’s Memoir, and of a few more in the subjoined list, has been published in a volume, with supplements, by the Hydrographic Department of the United States, under the superintendence of Commodore R. H. Wyman.

The papers whose titles are in English have appeared in the American volume. In that volume the principal English papers on the subject, in as far as they relate to the north-eastern extension of the Gulf-stream, have also been reprinted.

The System of Oceanic Currents in the Circumpolar Basin of the Northern Hemisphere. By Dr. A. Mühry. Vol. XIII., Part II. 1867.

The Scientific Results of the first German North Polar Expedition. By Dr. W. von Freeden. Vol. XV., Part VI. 1869.

The Gulf-stream, and the Knowledge of the Thermal Properties of the North Atlantic Ocean and its Continental Borders, up to 1870. By Dr. A. Petermann. _Geographische Mittheilungen_, Vol. XVI., Part VI. 1870.

The Temperature of the North Atlantic Ocean and the Gulf-stream. By Rear-Admiral C. Irminger. Vol. XVI., Part VI. 1870.

Meteorological Observations during a Winter Stay on Bear Island, 1865−1866. By Sievert Tobilson. Vol. XVI., Part VII. 1870.

Die Temperatur-verhältnisse in den arktischen Regionen. Von Dr. Petermann. Band XVI., Heft VII. 1870.

Preliminary Reports of the Second German North Polar Expedition, and of minor Expeditions, in 1870. Vol. XVII.

Preliminary Report of the Expedition for the Exploration of the Nova-Zembla Sea (the sea between Spitzbergen and Nova Zembla), by Lieutenants Weyprecht and Payer, June to September, 1871. By Dr. A. Petermann. Vol. XVII. 1871.

Der Golfstrom ostwärts vom Nordkap. Von A. Middendorff. Band XVII., Heft I. 1871.

Kapitän E. H. Johannesen’s Umfahrung von Nowaja Semlä im Sommer 1870, und norwegischer Finwalfang östlich vom Nordkap. Von Th. v. Heuglin. Band XVII., Heft I. 1871.

Die Nordpol-Expeditionen, das sagenhafte Gillis-land und der Golfstrom im Polarmeere. Von Dr. A. Petermann. 5 Nov. 1870.

Th. v. Heuglin’s Aufnahmen in Ost-Spitzbergen. Begleitworte zur neuen Karte dieses Gebiets. Tafel 9. 1870. Band XVII., Heft V. 1871.

Die zweite deutsche Nordpolar-Expedition, 1869−70. Schlittenreise an der Küste Grönlands nach Norden, 8 März−27 April, 1870. Von Ober-Lieutenant Julius Payer. Band XVII., Heft V. 1871.

Die Entdeckung des Kaiser Franz Josef-Fjordes in Ost-Grönland, August, 1870. Von Ober-Lieutenant Julius Payer. Band XVII., Heft V. 1871.

Die Erschliessung eines Theiles des nördlichen Eismeeres durch die Fahrten und Beobachtungen der norwegischen Seefahrer Torkildsen, Ulve, Mack Qvale, und Nedrevaag im karischen Meere, 1870. Von Dr. A. Petermann. Band XVII., Heft III. 1871.

Die zweite deutsche Nordpolar-Expedition, 1869−70. Schlittenreise nach Ardencaple Inlet, 8−29 Mai, 1870. Von Ober-Lieutenant Julius Payer. Band XVII., Heft XI. 1871.

Ein Winter unter dem Polarkreise. Von Ober-Lieutenant Julius Payer. Band XVII., Heft XI. 1871.

Die Entdeckung eines offenen Polarmeeres durch Payer und Weyprecht im September, 1871. Von Dr. A. Petermann. Band XVII., Heft XI. 1871.

James Lamont’s Nordfahrt, Mai-August, 1871. Die Entdeckungen von Weyprecht, Payer, Tobiesen, Mack, Carlsen, Ulve, und Smyth im Sommer, 1871.

Stand der Nordpolarfrage zu Ende des Jahres 1871. Von Dr. A. Petermann. Band XVII., Heft XII. 1871.

Das Innere von Grönland. Von Dr. Robert Brown. Band XVII., Heft X. 1871.

Captain T. Torkildsen’s Cruise from Tromsö to Spitzbergen, July 26 to September 26, 1871. Vol. XVIII. 1872.

The Sea north of Spitzbergen, and the most northern Meteorological Observations. Vol. XVIII. 1872.

Results of the Observations of the Deep-sea Temperature in the Sea between Greenland, Northern Europe, and Spitzbergen. By Professor H. Möhn. Vol. XVIII. 1872.

The Norwegian Cruises to Nova Zembla and the Kara Sea in 1871. Vol. XVIII. 1872.

The Cruises in the Polar Sea in 1872. Vol. XVIII. 1872.

The Cruise of Smyth and Ulve, June 19 to September 27, 1871. Vol. XVIII. 1872.

Die fünfmonatliche Schiffbarkeit des sibirischen Eismeeres um Nowaja Semlja, erwiesen durch die norwegischen Seefahrer in 1869 und 1870, ganz besonders aber in 1871. Von Dr. A. Petermann. Band XVIII., Heft X. 1872.

Die neuen norwegischen Aufnahmen des nordöstlichen Theiles von Nowaja Semlja durch Mack, Dörma, Carlsen, u. A., 1871. Von Dr. Petermann. Band XVIII., Heft X. 1872.

Nachrichten über die sieben zurückgekehrten Expeditionen unter Graf Wiltschek, Altmann, Johnsen, Nilsen, Smith, Gray, Whymper; die drei Überwinterungs-Expeditionen; die Amerikanische, Schwedische, Österreichisch-Ungarische; und die zwei neuen: die norwegische Winter-Expedition und diejenige unter Kapitän Mack. Von Dr. A. Petermann. Band XVIII., Heft XII. 1872.

Konig Karl-Land im Osten von Spitzbergen und seine Erreichung und Aufnahme durch norwegische Schiffer im Sommer 1872. Von Professor H. Möhn. Band XIX., Heft IV. 1873.

Resultate der Beobachtungen angestellt auf der Fahrt des Dampfers “Albert” nach Spitzbergen im November und Dezember, 1872. Von Professor Möhn. Band XIX., Heft VII. 1873.

Die amerikanische Nordpolar-Expedition unter C. F. Hall, 1871−3. Von Dr. A. Petermann. Band XIX., Heft VIII. 1873.

Die Trift der Hall’schen Nordpolar-Expedition, 16 August bis 15 Oktober, 1872, und die Schollenfahrt der 20 bis zum 30 April, 1873. Von Dr. A. Petermann. Band XIX., Heft X. 1873.

Das offene Polarmeer bestätigt durch das Treibholz an der Nordwestküste von Grönland. Von Dr. A. Petermann. Band XX., Heft V. 1874.

Das arktische Festland und Polarmeer. Von Dr. Joseph Chavanne. Band XX., Heft VII. 1874.

Die Umkehr der Hall’schen Polar-Expedition nach den Aussagen der Offiziere. Von Dr. A. Petermann. Band XX., Heft VII. 1874.

Die zweite österreichisch-ungarische Nordpolar-Expedition unter Weyprecht und Payer, 1872−4. Von Dr. A. Petermann. Band XX., Heft X. 1874.

Beiträge zur Klimatologie und Meteorologie des Ost-polar-Meeres. Von Professor Möhn. Band XX., Heft V. 1874.

Kapitän David Gray’s Reise und Beobachtungen im ost-grönländischen Meere, 1874, und seine Ansichten über den besten Weg zum Nordpol. Original-Mittheilungen an A. Petermann, d.D., Peterhead, Dezember, 1874. Band XXI., Heft III. 1875.

VIII.

LIST OF PAPERS BY THE AUTHOR TO WHICH REFERENCE IS MADE IN THIS VOLUME.

On the Influence of the Tidal Wave on the Earth’s Rotation and on the Acceleration of the Moon’s Mean Motion.—_Phil. Mag._, April, 1864.

On the Nature of Heat-vibrations.—_Phil. Mag._, May, 1864.

On the Cause of the Cooling Effect produced on Solids by Tension.—_Phil. Mag._, May, 1864.

On the Physical Cause of the Change of Climate during Geological Epochs.—_Phil. Mag._, August, 1864.

On the Physical Cause of the Submergence of the Land during the Glacial Epoch.—The _Reader_, September 2nd and October 14th, 1865.

On Glacial Submergence.—The _Reader_, December 2nd and 9th, 1865.

On the Eccentricity of the Earth’s Orbit.—_Phil. Mag._, January, 1866.

Glacial Submergence on the Supposition that the Interior of the Globe is in a Fluid Condition.—The _Reader_, January 13th, 1866.

On the Physical Cause of the Submergence and Emergence of the Land during the Glacial Epoch, with a Note by Professor Sir William Thomson.—_Phil. Mag._, April, 1866.

On the Influence of the Tidal Wave on the Motion of the Moon.—_Phil. Mag._, August and November, 1866.

On the Reason why the Change of Climate in Canada since the Glacial Epoch has been less complete than in Scotland.—_Trans. Geol. Soc. of Glasgow_, 1866.

On the Eccentricity of the Earth’s Orbit, and its Physical Relations to the Glacial Epoch.—_Phil. Mag._, February, 1867.

On the Reason why the Difference of Reading between a Thermometer exposed to direct Sunshine and one shaded diminishes as we ascend in the Atmosphere.—_Phil. Mag._, March, 1867.

On the Change in the Obliquity of the Ecliptic; its Influence on the Climate of the Polar Regions and Level of the Sea.—_Trans. Geol. Soc. of Glasgow_, vol. ii., p. 177. _Phil. Mag._, June, 1867.

Remarks on the Change in the Obliquity of the Ecliptic, and its Influence on Climate.—_Phil. Mag._, August, 1867.

On certain Hypothetical Elements in the Theory of Gravitation and generally received Conceptions regarding the Constitution of Matter.—_Phil. Mag._, December, 1867.

On Geological Time, and the probable Date of the Glacial and the Upper Miocene Period.—_Phil. Mag._, May, August, and November, 1868.

On the Physical Cause of the Motions of Glaciers.—_Phil. Mag._, March, 1869. _Scientific Opinion_, April 14th, 1869.

On the Influence of the Gulf-stream.—_Geol. Mag._, April, 1869. _Scientific Opinion_, April 21st and 28th, 1869.

On Mr. Murphy’s Theory of the Cause of the Glacial Climate.—_Geol. Mag._, August, 1869. _Scientific Opinion_, September 1st, 1869.

On the Opinion that the Southern Hemisphere loses by Radiation more Heat than the Northern, and the supposed Influence that this has on Climate.—_Phil. Mag._, September, 1869. _Scientific Opinion_, September 29th and October 6th, 1869.

On Two River Channels buried under Drift belonging to a Period when the Land stood several hundred feet higher than at present.—_Trans. Geol. Soc. of Edinburgh_, vol. i., p. 330.

On Ocean-currents: Ocean-currents in Relation to the Distribution of Heat over the Globe.—_Phil. Mag._, February, 1870.

On Ocean-currents: Ocean-currents in Relation to the Physical Theory of Secular Changes of Climate.—_Phil. Mag._, March, 1870.

The Boulder Clay of Caithness a Product of Land-ice.—_Geol. Mag._, May and June, 1870.

On the Cause of the Motion of Glaciers.—_Phil. Mag._, September, 1870.

On Ocean-currents: On the Physical Cause of Ocean-currents. Examination of Lieutenant Maury’s Theory.—_Phil. Mag._, October, 1870.

On the Transport of the Wastdale Granite Boulders.—_Geol. Mag._, January, 1871.

On a Method of determining the Mean Thickness of the Sedimentary Rocks of the Globe.—_Geol. Mag._, March, 1871.

Mean Thickness of the Sedimentary Rocks.—_Geol. Mag._, June, 1871.

On the Age of the Earth as determined from Tidal Retardation.—_Nature_, August 24th, 1871.

Ocean-currents: On the Physical Cause of Ocean-currents. Examination of Dr. Carpenter’s Theory.—_Phil. Mag._, October, 1871.

Ocean-currents: Further Examination of the Gravitation Theory.—_Phil. Mag._, February, 1874.

Ocean-currents: The Wind Theory of Oceanic Circulation.—_Phil. Mag._, March, 1874.

Ocean-currents.—_Nature_, May 21st, 1874.

The Physical Cause of Ocean-currents.—_Phil. Mag._, June, 1874. _American Journal of Science and Art_, September, 1874.

On the Physical Cause of the Submergence and Emergence of the Land during the Glacial Epoch.—_Geol. Mag._, July and August, 1874.

INDEX.

Absolute heating-power of ocean-currents, 23 〃 amount of heat received from the sun per day, 26

Adhémar, M., theory founded upon a mistake in regard to radiation, 81, 85 〃 on submergence, 368 〃 on influence of eccentricity on climate, 542

Aërial currents increased in action by formation of snow and ice, 76 〃 function of, stated, 51 〃 heat conveyed by, 27

Africa, South, glacial and inter-glacial periods of, 242 〃 boulder clay of Permian age, 300

Age and origin of the sun, 346

Air, on absorption of rays by, 59 〃 when humid, absorbs rays which agree with it in period, 59 〃 when perfectly dry incapable of absorbing radiant heat, 59

Airy, Professor, earth’s axis of rotation permanent, 7

Aitken’s, Mr., experiment on density of polar water, 129

Aland islands, striation of, 447

Alternate cold and warm periods, 236

Allermuir, striations on summit of, 441

America, low temperature in January, 72 〃 thickness of ice-sheet of North, 381

Anderson, Captain Sir James, never observed a stone on an iceberg, 282

Antarctic regions, mean summer temperature of, below freezing-point, 63

Antarctic ice-cap, probable thickness of, 375 〃 diagram representing thickness of, 377 〃 thickness of, estimated from icebergs, 384

Antarctic snowfall, estimates of, 382

Aphelion, glacial conditions at maximum when winter solstice is at, 77

Arago, M., on influence of eccentricity on climate, 536

Arctic climate, influence of ocean-currents on, during glacial period, 260

Arctic regions, influence of Gulf-stream on climate of, 45 〃 mean summer temperature of, 63

Arctic regions, amount of heat received by, per unit surface, 195 〃 warm periods best marked in, 258 〃 warm inter-glacial periods in, 258−265 〃 state of, during glacial period, 260 〃 evidence of warm periods in, 261 〃 occurrence of recent trees in, 261, 265 〃 evidence of warm inter-glacial periods, 293 〃 warm climate during Old Red Sandstone period in, 295 〃 glacial period during Carboniferous age in, 297 〃 warm climate during Permian period in, 301 〃 list of papers relating to, 556

Arctic Ocean, area of, 195 〃 according to gravitation theory ought to be warmer than Atlantic in torrid zone, 195 〃 heat conveyed into, by currents, compared with that received by it from the sun, 195 〃 blocked up with polar ice, 444

Armagh, boulder beds of, 299

Arran, Island of, glacial conglomerate of Permian age in, 299

Astronomical causes of change of climate, 10

Astronomy and geology, supposed analogy between, 355

Atlantic, atmospheric pressure on middle of, 33 〃 inability of, to heat the south-west winds without the Gulf-stream, 34 〃 mean annual temperature of, 36 〃 mean temperature of, raised by Gulf-stream, 36, 40 〃 isothermal lines of, compared with those of the Pacific, 46 〃 area of, from equator to Tropic of Cancer, 194 〃 inquiry whether the area of, is sufficient to supply heat according to Dr. Carpenter’s theory, 194

Atlantic, North, heat received by, from torrid zone by currents, 194 〃 according to Dr. Carpenter’s theory ought to be warmer in temperate regions than in the torrid zone, 195 〃 great depth of warm water in, 198 〃 North, an immense whirlpool, 216 〃 above the level of equator, 221 〃 probable antiquity of, 367 〃 from Scandinavia to Greenland probably filled with ice, 451

Atmosphere-pressure in Atlantic a cause of south-west winds, 33

Atmosphere, on difference between black-bulbed and shaded thermometer in upper strata of, 547

Australia, evidence of ice-action in conglomerate of, 295

Ayrshire, ice-action during Silurian period in, 293

Bakewell, Mr. R., on influence of eccentricity on climate, 540

Banks’s Land, discovery of ancient forest in, 261 〃 Professor Heer, on fossilized wood of, 309

Ball, Mr., objection to Canon Moseley’s results, 501

Baltic current, 171

Baltic, glaciation of islands in, 448

Baltic glacier, passage of, over Denmark, 449

Bath, grooved rock surfaces of, 464

Bay-ice grinds but does not striate rocks, 277

Belcher, Sir E., tree dug up by, in latitude 75° N., 263 〃 carboniferous fossils found in arctic regions by, 298

Belle-Isle, Strait of, observations on action of icebergs in, 276

Bell, Mr. A., on Mediterranean forms in glacial bed at Greenock, 254

Belt, Mr. Thomas, theory of the cause of glacial epochs, 415

Bennie, Mr. James, on surface geology, 468 〃 on deposits filling buried channel, 486

Blanford, Mr., on ice-action during Carboniferous age in India, 297

Borings, evidence of inter-glacial beds from, 254 〃 examination of drift by, 467 〃 journals of, 483, 484

Boulder clays of former glacial epochs, why so rare, 269 〃 a product of land-ice, 284 〃 if formed from icebergs must be stratified, 284 〃 scarcity of fossils in, 285 〃 formed chiefly from rock on which it lies, 285 〃 of Caithness a product of land-ice, 435 〃 on summit of Allermuir, 441

Boulders, how carried from a lower to a higher level, 527

Boussingault on absorption of carbon by vegetation, 428

Britain, climate of, affected most by south-eastern portion of Gulf-stream, 33

Brown, Dr. R., cited on Greenland ice-sheet, 378, 380 〃 on inland ice of Greenland, 284 〃 on cretaceous formation of Greenland, 305 〃 on Miocene beds of the Disco district, 310

Brown, Mr. Robert, on growth of coal plants, 421

Brown and Dickeson, on sediment of Mississippi, 330

Buchan, Mr., on atmosphere-pressure in the Atlantic, 33 〃 on force of the wind, 220

Buchanan, Mr. J. Y., on vertical distribution of heat of the ocean, 550

Buckland, Dr., observations by, on occurrence of red chalk on Cotteswold hills, 459

Buff, Professor, on oceanic circulation, 145

Buried river channels, 466 〃 channel from Kilsyth to Grangemouth, 468 〃 section at Grangemouth, 474 〃 from Kilsyth to Clyde, 481 〃 not excavated by sea nor by ice, 469 〃 other examples of, 488−494

Caithness, difficulty of accounting for the origin of the boulder clay of, 435

Caithness, boulder clay of, a product of land-ice, 435 〃 boulder clay not formed by icebergs, 437 〃 theories regarding the origin of the boulder clay of, 437 〃 why the ice was forced over it, 444 〃 Professor Geikie and B. N. Peach on path of ice over, 453

Cambrian conglomerate of Islay, 292

Campbell, Mr., observations of, on icebergs, 276 〃 on supposed striation of rocks by large icebergs, 278 〃 evidence that river-ice does not striate rocks, 279

Canada, change of climate less complete than in Scotland, 71

Carboniferous period of arctic regions, 298 〃 evidence of glacial epoch during, 296−298 〃 temperate climate of, 422

Carboniferous limestone, mode of formation, 433

Carpenter’s, Dr., objections examined, 141 〃 theory, mechanics of, 145 〃 idea of a 〃vertical circulation〃 stated, 153

Carpenter’s, Dr., radical error in theory of, 155 〃 on difference of density between waters of Atlantic and Mediterranean, 168 〃 theory, inadequacy of, 191 〃 estimate of thermal work of Gulf-stream, 199

Charpentier’s, M., theory of glacier-motion, 513

Carse clays, date of, 405

Cattegat, ice-markings on shore of, 446

Cave and river deposits, 251

Chalk, erratic blocks found in, 304 〃 _débris_, conclusion of Mr. Searles Wood, 460

_Challenger’s_ temperature-soundings at equator, 119 〃 crucial test of the wind and gravitation theories, 220

Chambers, Dr. Robert, on striated pavements, 255 〃 observations on glaciation of Gothland, 446

Champlain Lake, inter-glacial bed of, 241

Chapelhall, ancient buried channel at, 491 〃 inter-glacial sand-bed, 244

Chart showing the agreement between system of currents and system of winds, 212

Christianstadt, crossed by Baltic glacier, 450

Circulation without difference of level, 176

Climate, Secular changes of, intensified by reaction of physical causes, 75, 76 〃 affected most by temperature of the surface of ground, 88 〃 ocean-currents in relation to, 226 〃 cold conditions of, inferred from absence of fossils, 288 〃 cold condition of, difficulty of determining, from fossil remains, 289 〃 warm, of arctic regions during Old Red Sandstone period, 295 〃 rough sketch of the history of, during the last 60,000 years, 409 〃 of Coal period inter-glacial in character, 420 〃 alternate changes of, during Coal period, 426

Climates, Mr. J. Geikie on difficulty of detecting evidence of ancient glacial conditions, 289 〃 evidence of, from ancient sea-bottoms, 289

Coal an inter-glacial formation, 420

Coal beds, alternate submergence and emergence during formation of, 424 〃 preservation of, by submergence, 426

Coal period, flatness of the land during, 430

Coal plants, conditions necessary for, preservation of, 423

Coal seams, thickness of, indicative of length of inter-glacial periods, 428

Coal seams, time occupied in formation of, 429

Coal strata, on absence of ice-action in, 429

Coal measures, oscillations of sea-level during formation of, 425

Cold periods best marked in temperate regions, 258

Colding, Dr., oceanic circulation, 95

Confusion of ideas in reference to the agency of polar cold, 179

Continental ice, inadequate conceptions of, 385 〃 absence of, during glacial epochs of Coal period, 432

Contorted drift near Musselburgh, 465

Cook, Captain, description of Sandwich Land by, 60 〃 on South Georgia, 60

Cornwall, striated rocks of, 464

Cotteswold hills, red chalk from Yorkshire found on, 459

Couthony, Mr., on action of icebergs, 275

Coutts, Mr. J., on buried channel, 493

Craig, Mr. Robert, on inter-glacial beds at Overton Hillhead and Crofthead, 247

Craiglockhart hill, inter-glacial bed of, 245

“Crawling” theory considered, 507

“Crevasses,” origin of, according to molecular theory, 521

Cretaceous period, evidence of ice-action during, 303−305

Cretaceous age, evidence of warm periods during, 304

Cretaceous formation of Greenland, 305

Crofthead, inter-glacial bed at, 248

Cromer forest bed, 250

Crosskey, Rev. Mr., comparison of Clyde and Canada shell beds, 71 〃 on southern shells in Clyde beds, 253

Croydon, block of granite found in chalk at, 303

Crucial test of the wind and gravitation theories, 220

Crystallization, force of, a cause of glacier-motion, 523

Currents, effects of their stoppage on temperatures of equator and poles, 42 〃 produced by saltness neutralize those produced by temperature, 106

Dalager, excursion in Greenland by, 378

Dana, Professor, on action of icebergs, 275 〃 on striations by icebergs, 275 〃 on thickness of ice-sheet of North America, 381

Darwin, Mr., on alternate cold and warm periods, 231 〃 on migration of plants and animals during glacial epoch, 395 〃 on peat of Falkland Islands, 422

Date of the 40-foot beach, 409

Date when conditions were favourable to formations of the Carse clay, 409

Davis’ Straits, current of, 132

Dawkins, Mr. Boyd, on the animals of cave and river deposits, 251

Dawson, Principal, on esker of Carboniferous age, 296

〃 on habitats of coal plants, 424

Deflection of ocean-currents chief cause of change of climate, 68

De la Beche, Sir H. T., on influence of eccentricity on climate, 539

De Mairan, on influence of eccentricity on climate, 528

Denmark, crossed by Baltic glacier, 449−452

Denudation, method of measuring rate of, 329 〃 as a measure of geological time, 329 〃 measured by sediment of Mississippi, 330 〃 subaërial rate of, 331 〃 law which determines rate of, 333 〃 marine, trifling, 337

Deposition, rates of, generally adopted, quite arbitrary, 360 〃 rate of, determined by rate of denudation, 362 〃 range of, restricted to a narrow fringe surrounding the continents, 364 〃 area of, 365 〃 during glacial epoch probably less than present, 366

Deposits from icebergs cannot be wholly unstratified, 437

Despretz, tables by, of temperature of maximum density of sea-water, 117

Desor, M., on tropical fauna of the Eocene formation in Switzerland, 306

Derbyshire, breaks in limestone of, marks of cold periods, 434

Derbyshire limestone a product of inter-glacial periods, 434

Devonshire, boulder clay discovered in, 463

Diagram illustrating descent of water from equator to poles, 155 〃 showing variations of eccentricity, 313 〃 illustrative of fluidity of interior of the earth, 396 〃 showing formation of coal beds, 426

Dick, Mr., chalk flints in boulder clay, 454

Dick, Mr. R., on buried channel, 491

Difference of level essential to gravitation theory, 176

Dilatation of sea-water by increase of temperature calculated by Sir John Herschel, 116

Disco district, Dr. R. Brown cited on Miocene beds of, 310

Disco Island, Upper Miocene period of, 307−308

Distribution, how effected by ocean-currents, 231

Dove, Professor, method of constructing normal temperature tables by, 40 〃 on mean annual temperature, 401

Dover, mass of coal imbedded in chalk found at, 303

Drayson, Lieutenant-Colonel, on obliquity of ecliptic, 410

Drayson, Lieutenant-Colonel, theory of the cause of the glacial epoch, 410

Drift, examination by borings, 467

Drumry, deep surface deposits at, 482

Dubuat’s, M., experiments, 182 〃 experiments by, on water flowing down an incline, 120

Duncan, Captain, on under current in Davis’ Strait, 134

Dürnten lignite beds, 240

Dürnten beds an example of inter-glacial coal formation, 433

Durham, buried river channel at, 488

Earth’s axis of rotation permanent, 7

Earth, mean temperature of, increased by water at equator, 30 〃 not habitable without ocean-currents, 54 〃 mean temperature of, greatest in aphelion, 77, 78 〃 centre of gravity of, effects of ice-cap on, 370, 371

Eccentricity of the earth’s orbit, Mr. Stockwell’s researches regarding, 54 〃 primary cause of change of climate, 54 〃 primary cause of glacial epochs, 77 〃 how it affects the winds, 228 〃 tables of, 314−321 〃 its influence on temperature, 323 〃 explanation of tables of, 324 〃 De Marian, on influence of, on climate, 528 〃 Sir J. F. Herschel, on influence of, on climate, 529 〃 Œpinus, on influence of, on climate, 529 〃 R. Kirwan, on influence of, on climate, 529 〃 of planetary orbits, superior limits as determined by Lagrange, Leverrier, and Mr. Stockwell, 531 〃 Sir Charles Lyell, on influence of, on climate, 529, 535 〃 M. Arago, on influence of, on climate, 536 〃 Baron Humboldt, on influence of, on climate, 538 〃 Sir H. T. de la Beche, on influence of, on climate, 539 〃 Professor Phillips, on influence of, on climate, 539 〃 Mrs. Somerville, on influence of, on climate, 540 〃 L. W. Meech, on influence of, on climate, 540 〃 Mr. R. Bakewell, on influence of, on climate, 540 〃 M. Jean Reynaud, on influence of, on climate, 541 〃 M. Adhémar, on influence of, on climate, 542

Equator, reduction of level by denudation, 336

Ecliptic, supposed effect of a change of obliquity of, 8 〃 changes of, effects on climate, 398−417 〃 obliquity of, Lieutenant-Colonel Drayson on, 410

Emergence, physical cause of, 368

England, inter-glacial beds of, 249 〃 glacial origin of Old Red Sandstone of, 294 〃 ice-action during Permian period in, 298 〃 North of, ice-sheet of, 456 〃 ice-sheet of South of, 463

Eocene period, total absence of fossils in flysch, 286 〃 glacial epoch of, 305

Eocene and Miocene periods, date of, 357

Equator, heat received per square mile at, 26 〃 temperature of earth increased by water at, 30 〃 and poles, effects of stoppage of currents on temperature of, 42 〃 surface-currents warmer than the under currents, 92 〃 heat transferred by currents from southern hemisphere compared with that received by land at, 93 〃 temperature soundings at, 119 〃 temperature of sea at, decreases most rapidly at the surface, 119 〃 heat received by the three zones compared with that received by the, 194 〃 migration across, 234 〃 glaciation of, 234

Equatorial current, displacement of, 229

Erratic blocks in stratified rocks, evidence of former land-ice, 269 〃 in chalk, 304 〃 why not found in coal strata, 432

Erratics extend further south in America than in Europe, 72

Etheridge, R., jun., on glacial conglomerate in Australia of Old Red Sandstone age, 295

Europe, influence of Gulf-stream on climate of, 31 〃 effect of deflection of Gulf-stream on condition of, 68 〃 glacial condition of, if Gulf-stream was stopped, 71 〃 river systems of, unaltered since glacial period, 393

Faraday, Professor, on cause of regelation, 554

Faroe Islands glaciated by land-ice from Scandinavia, 450

Ferrel, Mr., on Dr. Carpenter’s theory, 126 〃 argument from the tides, 184

Findlay, Mr. A. G., objection by, considered, 31, 203 〃 estimate of heat conveyed by Gulf-stream, 206

Fisher, Rev. O., on the 〃trail〃 of Norwich, 251 〃 on glacial submergence, 387

Fitzroy, Admiral, on temperature of Atlantic, 36

Fluid molecules crystallize in interstices, 523

Fluvio-marine beds of Norwich, 250

“Flysch” of Eocene period, absence of fossils in, 286 〃 of Switzerland of glacial origin, 306

Fogs prevent the sun’s heat from melting ice and snow in arctic regions, 60

Forbes, Professor J. D., method adopted by, of ascertaining temperatures, 48 〃 on temperature of equator and poles, 48 〃 on the conductivity of different kinds of rock, 86 〃 on underground temperature, 86 〃 experiments by, on the power of different rocks to store up heat, 86

Forest bed of Cromer, 250

Former glacial periods, 266−310 〃 why so little known of, 266 〃 geological evidence of, 292

France, evidence of ice-action during Carboniferous period in, 296

Fraserburgh, glaciation of, 450 〃 crossed by North Sea ice, 454

Fundamental problem of geology, 1

Ganges, amount of sediment conveyed by, 331

Gases, radiation of, 38

Gastaldi, M., on the Miocene glacial epoch of Italy, 306

Geikie, Professor, on geological agencies, 1 〃 on inter-glacial beds of Scotland, 243 〃 remarks on inter-glacial beds, 245 〃 on striated pavements, 256 〃 on ice-markings on Scandinavian coast, 281 〃 striated stones found in carboniferous conglomerate by, 296 〃 on sediment of European rivers, 332 〃 on modern denudation, 332 〃 suggestion regarding the loess, 452 〃 on striation of Caithness, 453 〃 on buried channel at Chapelhall, 491 〃 and Mr. James, on glacial conglomerate of Lower Carboniferous age, 296

Geikie, Mr. James, on Crofthead inter-glacial bed, 248 〃 on the gravels of Switzerland, 268 〃 on difficulty of recognising former glacial periods, 289 〃 on Cambrian conglomerate of north-west of Scotland, 293 〃 on ice-action in Ayrshire during Silurian period, 293 〃 on boulder conglomerate of Sutherland, 301 〃 on buried channels, 492

Geogr. Mittheilungen, list of papers in, relating to arctic regions, 556

Geological agencies climatic, 2

Geological principle, nature of, 4

Geological climates, theories of, 6

Geological time, 311−359 〃 measurable from astronomical data, 311 〃 why it has been over-estimated, 325 〃 method of measuring, 328, 329 〃 Professor Ramsay on, 343

Geology, fundamental problem of, 1 〃 a dynamical science, 5 〃 and astronomy, supposed analogy between, 355

German Polar Expedition on density of polar water, 151 〃 list of papers relating to, 556

German Ocean once dry land, 479

Germany, Professor Ramsay on Permian breccia of, 300

Gibraltar current, Dr. Carpenter’s theory of, 167 〃 cause of, 215

Glacial conditions increased by reaction of various physical causes, 75 〃 reach maximum when winter solstice arrives at aphelion, 77

Glacial epoch, date of, 327 〃 circumstances which show recent date of, 341 〃 Mr. Belt’s theory of cause of, 415

Glacial epochs dependent upon deflection of ocean-currents, 68 〃 caused primarily by eccentricity, 77 〃 why so little known of, formerly, 266 〃 boulder clays of former, why so rare, 269 〃 geological evidence of former, 292

Glacial period in America more severe than in Western Europe, 73 〃 mean temperature of the earth greatest at aphelion during, 78 〃 records of, fast disappearing, 270 〃 of the Eocene formation, 305

Glacial periods, indirect evidence of, in Eocene and Miocene formations, 287 〃 difficulty of determining, from fossil remains, 289

Glacial submergence resulting from displacement of the earth’s centre of gravity, 389

Glaciation a cause of submergence, 390 〃 remains of, found chiefly on land surfaces, 267 〃 of Scandinavia inexplicable by theory of local glaciers, 448

Glacier des Bois, 497

Glacier-motion, Canon Moseley’s theory of, 507 〃 Professor James Thomson’s theory of, 512 〃 M. Charpentier’s theory of, 513 〃 molecular, 516

Glacier-motion, present state of the question, 514 〃 molecular theory of, 514−527 〃 heat necessary to, 515 〃 due to force of crystallization, 523 〃 due chiefly to internal molecular pressure, 523

Glaciers, pressure exerted by, 274 〃 physical cause of the motion of, 495−527 〃 difficulties in accounting for motion of, 495

Glasgow, actual January temperature of, 28° above normal, 72

Godwin-Austen, Mr., on ice-action during the Carboniferous period in France, 296 〃 on evidence of ice-action during Cretaceous period, 303 〃 on mass of coal found in chalk at Dover, 304 〃 on the flatness of the land during Coal period, 430

Gothland, glaciation of, 446

Grangemouth, buried river channel at, 468 〃 surface-drift of, 484

Gravitation, the whole work of, performed by descent of water down the slope, 154 〃 of sun’s mass, 348 〃 insufficient to account for sun’s heat, 349, 350

Gravitation theory, its relation to the theory of Secular changes of climate, 97 〃 three modes of determining it, 115 〃 mechanics of, 145 〃 of the Gibraltar current, 167 〃 inadequacy of, 191 〃 _crucial_ test of, 220 〃 of the sun’s heat, 346−355

Gravity, force of, impelling water from equator to poles, 119, 120 〃 force of, insensible at a short distance below the surface, 120 〃 work performed by, 150 〃 diagram illustrating the action of, in producing currents, 155 〃 amount of work performed by, due solely to _difference_ of temperature between equatorial and polar waters, 164 〃 specific difference in, between water of Atlantic and Mediterranean insufficient to produce currents, 169 〃 centre of, displacement, by polar ice-cap, 368

Greenland, summer warm if free from ice, 59 〃 receives as much heat in summer as England, 66 〃 continental ice free from clay or mud, 284 〃 North, warm climate during Oolitic period in, 302 〃 Cretaceous formation of, 305

Greenland, evidence of warm conditions during Miocene period in, 307 〃 Professor Heer cited on Miocene flora of, 308, 309 〃 state of, during glacial period, 259 〃 effect of removal of ice from, 260

Greenland ice-sheet, probable thickness of, 378 〃 invaded the American continent, 445

Greenland inland ice, 379

Gulf-stream, estimate of its volume, 24 〃 United States’ coast survey of, 24 〃 absolute amount of heat conveyed by, 25, 26 〃 heat conveyed by, compared with that carried by aërial currents, 27 〃 heat conveyed by, compared with that received by the frigid zone from the sun, 27 〃 influence on climate of Europe, 31 〃 efficiency of, due to the slowness of its motion, 32 〃 climate of Britain influenced by south-eastern portion of, 33 〃 heat conveyed by, compared with that derived by temperate regions from the sun, 34 〃 heat of, expressed in foot-pounds of energy, 35 〃 mean temperature of Atlantic increased one-fourth by, 36 〃 the only current that can heat arctic regions, 45 〃 influence of, on climate of arctic regions, 45 〃 the compensating warm current, 46 〃 palæontological objections to influence of, 53 〃 agencies which deflect the, in glacial periods, 69 〃 result, if stopped, 71 〃 large portion of the heat derived from southern hemisphere, 94 〃 Lieut. Maury on propulsion of, by specific gravity, 102 〃 contradictory nature of, the causes supposed by Lieut. Maury for the, 110 〃 higher temperature of, considered by Lieut. Maury as the real cause of its motion, 111 〃 amount of heat conveyed by, not over-estimated, 197 〃 amount of heat conveyed by, 192 〃 amount of heat conveyed by, compared with that by general oceanic circulation, 194 〃 heat conveyed by, compared with that received by torrid zone from the sun, 194 〃 heat conveyed by, into Arctic Ocean compared with that received by it from the sun, 195 〃 Capt. Nares’s observations of, 198 〃 Dr. Carpenter’s estimate of the thermal work of, 199

Gulf-stream, volume and temperature of, according to Mr. A. G. Findlay, 203, 206 〃 erroneous notion regarding depth of, 207 〃 list of papers relating to, 556

Haughton, Professor, on recent trees in arctic regions, 263 〃 on fragments of granite in carboniferous limestone, 296 〃 on coal beds of arctic regions, 298 〃 on _Ammonites_ of Oolitic period in arctic regions, 303

Hayes, Dr., on Greenland ice-sheet, 379

Heat received from the sun per day, 26 〃 received by temperate regions from the sun, 34 〃 radiant, absorbed by ice remains insensible, 60 〃 sun’s, amount of, stored up in ground, 87 〃 transferred from southern to northern hemisphere, 93 〃 internal, supposed influence of, 176 〃 received by the three zones compared with that received by the equator, 194 〃 amount radiated from the sun, 346 〃 received by polar regions 11,700 years ago, 403 〃 necessary to glacier-motion, 515 〃 how transmitted through ice, 517

Heat-vibrations, nature of, 544

Heath, Mr. D. D., on glacial submergence, 387

Heer, Professor, on Dürnten lignite beds, 241 〃 on Miocene flora of Greenland, 308−310 〃 on Miocene flora of Spitzbergen, 309

Hills, ice-markings on summits of, as evidence of continental ice, 458

Helmholtz’s gravitation theory of sun’s heat, 348

Henderson, Mr. John, on inter-glacial bed at Redhall quarry, 247

Herschel, Sir John, on influence of eccentricity, 11 〃 estimate of the Gulf-stream by, 25 〃 on the amount of the sun’s heat, 26 〃 on inadequacy of specific gravity to produce ocean-currents, 116 〃 his objections to specific gravity not accepted, 117 〃 on influence of eccentricity on climate, 529

Home, Mr. Milne, on buried river channels, 478

Hooker, Sir W., on tree dug up by Capt. Belcher, 264

Hooker, Dr., on preponderance of ferns among coal plants, 421

Horne, Mr. J., on conglomerates of Isle of Man, 295

Hoxne, inter-glacial bed of, 241

Hudson’s Bay, low mean temperature of, in June, 62

Hull, Professor, on ice-action during Permian age in Ireland, 299 〃 on equable temperature of Coal period, 421 〃 on estuarine origin of coal measures, 424

Hull, buried channel at, 489

Humboldt, Baron, on loss of heat from radiation, 82 〃 on rate of growth of coal, 429 〃 on influence of eccentricity on climate, 538

Humphreys and Abbot on sediment of Mississippi, 330

Ice, latent heat of, 60

Ice, effects of removal of, from polar regions, 64 〃 heat absorbed by, employed wholly in mechanical work, 60 〃 slope necessary for motion of continental, 375 〃 does not shear in the solid state, 516 〃 how heat is transmitted through, 517 〃 how it can ascend a slope, 525 〃 how it can excavate a rock basin, 525

Icebergs do not striate sea-bottom, 272 〃 markings made by, are soon effaced, 273 〃 exerting little pressure perform little work, 273 〃 behaviour of, when stranded, 274 〃 action of, on sea-bottoms, 274 〃 rocks ground smooth, but not striated by, 276 〃 stones seldom seen on, 281 〃 evidence of, in Miocene formation of Italy, 307 〃 comparative thickness of arctic and antarctic, 381 〃 great thickness of antarctic, 382

Ice-cap, effects of, on the earth’s centre of gravity, 369 〃 probable thickness of antarctic, 375 〃 evidence from icebergs as to thickness of antarctic, 383−385

Ice-markings, modern, observed by Sir Charles Lyell, 280

Ice-sheet, probable thickness of in Greenland, 380 〃 of north of England, 456

Ice-worn pebbles found on summit of Allermuir, 441

Iceland, lignite of Miocene age in, 308 〃 probably glaciated by land-ice from North Greenland, 451

India, evidences of glacial action of Carboniferous age in, 297

Indian Ocean, low temperature at bottom, 123

Internal heat, no influence on climate, 6 〃 supposed influence of, 176

Inter-tropical regions, greater portion of moisture falls as rain, 29

Inter-glacial bed at Slitrig, 243 〃 at Chapelhall, 244 〃 of Craiglockhart hill, 245 〃 at Kilmaurs, 248

Inter-glacial beds, Professor Geikie on, 243 〃 of Dürnten, 240 〃 of Scotland, 243 〃 of England, 249 〃 at Norwich, 250 〃 evidence of, from borings, 254

Inter-glacial character of cave and river deposits, 251

Inter-glacial climate during Old Red Sandstone period in arctic regions, 295

Inter-glacial periods, 236 〃 reason why overlooked, 237 〃 of Switzerland, 239 〃 evidence of, from shell-beds, 252 〃 evidence from striated pavements of, 255 〃 reasons why so few vestiges remain of, 257 〃 in arctic regions, 258−265 〃 of Silurian age in arctic regions, 293 〃 of Carboniferous age in arctic regions, 297 〃 of Eocene formation in Switzerland, 306 〃 formation of coal during, 420 〃 length of, indicated by thickness of coal-seams, 428

Inglefield, Captain, erect trees found in Greenland by, 309

Ireland, on ice-action during Permian age in, 299

Isbister, Mr., on carboniferous limestone of arctic regions, 297

Islay, Cambrian conglomerate of, 292

Italy, glacial epoch of Miocene period in, 306

Jack, Mr. R. L., on deflection of ice across England, 461

Jamieson, Mr. T. F., on boulder clay of Caithness, 435 〃 opinion that Caithness was glaciated by floating ice, 437 〃 on thickness of ice in the north Highlands, 439 〃 glaciation of headland of Fraserburgh, 450, 455

January temperature of Glasgow and Cumberland, difference between, 72

Jeffreys, Mr. Gwyn, on Swedish glacial shell beds, 253

Johnston, Dr. A. Keith, on coast-line of the globe, 337

Joule’s, Dr., experiments on the thermal effect of tension, 552

Judd, Mr., on boulders of Jurassic age in the Highlands, 302

Jukes, Mr., on warm climate of North Greenland during Oolitic period, 302

July, why hotter than June, 89

Kane, Dr., on mean temperature of Von Rensselaer Harbour, 62

Karoo beds, glacial character of, 301 〃 evidence of subtropical during deposition of, 301

Kelvin, ancient bed of, 481

Kielsen, Mr., excursion upon Greenland ice-sheet, by, 378

Kilmours, inter-glacial bed at, 248

Kirwan, Richard, on influence of eccentricity on climate, 529

Kyles of Bute, southern shell bed in, 253

Labrador, mean temperature of, for January, 72 〃 Mr. Packard on glacial phenomena of, 282

Lagrange, M., on eccentricity of the earth’s orbit, 54 〃 table of superior limits of eccentricity, 531

Land at equator would retain the heat at equator, 30 〃 radiates heat faster than water, 91 〃 elevation of, will not explain glacial epoch, 391 〃 submergence and emergence during glacial epoch, 368−397 〃 successive upheavals and depressions of, 391

Land-ice necessarily exerts enormous pressure, 274 〃 evidence of former, from erratic blocks on stratified deposits, 269

Land-surfaces, remains of glaciation found chiefly on, 267 〃 (ancient) scarcity of, 268

Laplace, M., on obliquity of ecliptic, 398

Laughton, Mr., on cause of Gibraltar current, 215

Leith Walk, inter-glacial bed at, 246

Leverrier, M., on superior limit of eccentricity, 54 〃 on obliquity of ecliptic, 398 〃 table, by, of superior limits of eccentricity, 531 〃 formulæ, of, 312

Lignite beds of Dürnten, 240

Loess, origin of, 452

London, temperature of, raised 40° degrees by Gulf-stream, 43

Lomonds, ice-worn pebbles found on, 439

Lubbock, Sir J., on cave and river deposits, 252

Lucy, Mr. W. C., on glaciation of West Somerset, 463 〃 on northern derivation of drift on Cotteswold hills, 460

Lyell’s, Sir C., theory of the effect of distribution of land and water, 8 〃 on action of river-ice, 280 〃 on tropical character of the fauna of the Cretaceous formation, 305 〃 on warm conditions during Miocene period in Greenland, 307 〃 on influence of eccentricity, 324 〃 on sediment of Mississippi, 331 〃 on comparison of existing rocks with those removed, 362 〃 on submerged areas during Tertiary period, 392 〃 on change of obliquity of ecliptic, 418 〃 on climate best adapted for coal plants, 420 〃 on influence of eccentricity on climate, 529, 535

Mackintosh, Mr., observations on the glaciation of Wastdale Crag, 457

Magellan, Straits of, temperature at midsummer, 61

Mahony, Mr. J. A., on Crofthead inter-glacial bed, 248

Mälar Lake crossed by ice, 447

Man, Isle of, Mr. Cumming on glacial origin of Old Red Sandstone of, 294

Mars, uncertainty as to its climatic condition, 80 〃 objection from present condition of, 79

Marine denudation trifling, 337

Markham, Clements, on density of Gulf-stream water, 129 〃 on motion of icebergs in Davis’ Straits, 133

Martins’s, Professor Charles, objections, 79

Mathews, Mr., on Canon Moseley’s experiment, 499

Maury, Lieutenant, his estimate of the Gulf-stream, 25 〃 his theory examined, 95 〃 on temperature as a cause of difference of specific gravity, 102 〃 on difference of saltness as a cause of ocean-currents, 103 〃 discussion of his views of the causes of ocean-currents, 104 〃 his objection to wind theory of ocean-currents, 211

McClure, Captain, discovery of ancient forest in Banks’s Land, 261

Mecham, Lieutenant, discovery of recent trees in Prince Patrick’s Island, 261

Mechanics of gravitation theory, 145

Mediterranean shells in glacial shell bed of Udevalla, 253 〃 shells in glacial beds at Greenock, 254

Meech, Mr., on amount of sun’s rays cut off by the atmosphere, 26 〃 on influence of eccentricity on climate, 540

Melville Island, summer temperature of, 65 〃 discovery of recent trees in, 262 〃 plants found in coal of, 298

Mer de Glace, Professor Tyndall’s observations on, 498

Meteoric theory of sun’s heat, 347

Method of measuring rate of denudation, 329

Miller, Hugh, on absence of hills in the land of the Coal period, 431

Migration of plants and animals, how influenced by ocean-currents, 231 〃 across equator, 234

Millichen, remarkable section of drift at, 483

Miocene glacial period, 286

Miocene period, glacial epoch of, in Italy, 306

Miocene, warm period of, in Greenland, 307

Miocene and Eocene periods, date of, 357

Mississippi, amount of sediment in, 330 〃 volume of, 330

Mitchell, Mr., on cause of Gulf-stream, 131

Molecular theory of origin of 〃Crevasses,” 521 〃 modification of, 523

Moore, Mr. J. Carrick, on ice-action of Silurian age in Wigtownshire, 293

Moore, Mr. Charles, on grooved rocks in Bath district, 464

Morlot, M., on inter-glacial periods of Switzerland, 240

Moseley, Canon, experiment to determine unit of shear, 498 〃 on motion of glaciers, 498 〃 unit of shear uncertain, 504 〃 his theory examined, 507

Motion of the sea, how communicated to a great depth, 136

Motion in space, origin of sun’s heat, 353

Mühry, M., on circumpolar basin, 133, 556

Mundsley, freshwater beds of, 250

Muncke on the expansion of sea-water, 118

Murchison, Sir R., on southern shells at Worcester, 253 〃 on trees in arctic regions, 262 〃 on striation of islands in the Baltic, 448

Murphy’s, Mr., theory, 66

Musselburgh, section of contorted drift near, 465

Nares, Captain, on low temperature of antarctic regions, 64 〃 discovery of great depth of warm water in North Atlantic, 198 〃 estimate of volume and temperature of Gulf-stream, 198 〃 temperature soundings by, 119, 222 〃 thermal condition of Southern Ocean, 225

Natal, boulder clay of, 300

Newberry, Professor, on inter-glacial peat-bed of Ohio, 249 〃 on boulder of quartzite found in seam of coal, 296

Nicholson, Dr., on Wastdale Crag, 457

Nicol, Professor, on inter-glacial buried channel, 244

Nordenskjöld, Professor, on inland ice of Greenland, 379

North Sea rendered shallow by drift deposits, 443

Northern seas probably filled with land-ice during glacial period, 438

Northern hemisphere, condition of, when deprived of heat from ocean-current, 68

Norway, southern species in glacial shell beds, 253

Norwich Crag, its glacial character, 249

Norwich fluvio-marine beds, 250

Norwich inter-glacial beds, 250

Obliquity of ecliptic, its effects on climate, 398−419 〃 change of, influence on sea-level, 403 〃 Lieutenant-Colonel Drayson on, 410 〃 Mr. Belt on change of, 415 〃 Sir Charles Lyell on change of, 418

Ocean, imperfect conception of its area, 135 〃 condition of, inconsistent with the gravitation theory, 136 〃 low temperature at bottom a result of under currents, 142 〃 circulation, pressure as a cause of, 187 〃 antiquity of, 367

Ocean-currents, absolute heating power of, 23 〃 influence of, on normal temperatures overlooked, 40 〃 maximum effects of, reached at equator and poles, 49 〃 compensatory at only one point, 49 〃 heating effects of, greatest at the poles, 50 〃 cooling effects of, greatest at equator, 50 〃 earth not habitable without, 51 〃 result of deflection into Southern Ocean, 68 〃 palæontological objections against influence of, 53 〃 deflection of, the chief cause of changes of climate, 68 〃 how deflected by eccentricity, 69 〃 deflected by trade-winds, 70 〃 temperature of southern hemisphere lowered by transference of heat to northern hemisphere by, 92 〃 take their rise in the Southern Ocean, 92 〃 cause of, never specially examined by physicists, 95 〃 if due to specific gravity, strongest on cold hemisphere, 97 〃 if due to eccentricity, strongest on warm hemisphere, 97 〃 if due to specific gravity, act only by descent, 99 〃 mode by which specific gravity causes, 100, 101 〃 the true method of estimating the amount of heat conveyed by, 207 〃 due to system of winds, 212 〃 system of, agrees with the system of the winds, 213 〃 how they mutually intersect, 219 〃 in relation to climate, 226 〃 direction of, depends on direction of winds, 227 〃 causes which deflect, affect climate, 228 〃 in relation to distribution of plants and animals, 231 〃 effects of, on Greenland during glacial period, 260

Œpinus on influence of eccentricity on climate, 529

Ohio inter-glacial beds, 249

Old Red Sandstone, evidence of ice-action in conglomerate of, 294, 295

Oolite of Sutherlandshire, 454

Oolitic period, evidence of ice-action during, 301−303 〃 warm climate in North Greenland during, 302

Organic remains, absence of, in glacial conglomerate of Upper Miocene period, 286

Organic life, paucity of, a characteristic of glacial periods, 287

Orkney Islands, glaciated by land-ice, 444

Osborne, Captain, remarks on recent forest trees in arctic regions, 262, 263

Oudemans, Dr., on planet Mars, 80

Overton Quarry, inter-glacial bed in, 247

Pacific Ocean, depth of, 147

Packard, Mr., on glacial phenomena of Labrador, 282

Page, Professor, on temperate climate of Coal period, 422 〃 on character of coal plants, 421 〃 on old watercourse at Hailes quarry, 490

Palæontological objections against influence of ocean-currents, 53

Palæontological evidence of last glacial period, 285

Parry, Captain, discovery of recent trees in Melville Island by, 262

Peach, Mr. C. W., on inter-glacial bed at Leith Walk, 246 〃 on boulder clay of Caithness, 436 〃 on striated rock surfaces in Cornwall, 464

Peach, Mr. B. N., on striation of Caithness, 453

Pengelly, Mr. W., on raised beaches, 407

Perigee, nearness of sun in, cause of snow and ice, 74

Perihelion, warm conditions at maximum when winter solstice is at, 77

Permian period, evidence of ice-action in, 298−303

Perthshire hills, ice-worn surfaces at elevations of 2,200 feet on the, 440

Petermann, Dr. A., on Dr. Carpenter’s theory, 138 〃 on thermal condition of the sea, 138 〃 chart of Gulf-stream and Polar current, 219 〃 _Geogr. Mittheilungen_ of, list of papers in relation to arctic regions, 556

Phillips, Professor, on influence of eccentricity on climate, 539

Poisson’s theory of hot and cold parts of space, 7

Polar regions, effect of removal of ice from, 64 〃 influence of ice on climate, 64 〃 low summer temperature of, 66

Polar cold considered by Dr. Carpenter the _primum mobile_ of ocean-currents, 173 〃 confusion of ideas regarding its influence, 180 〃 influence of, according to Dr. Carpenter, 180

Polar ice-cap, displacement of the earth’s centre of gravity by, 368

Port Bowen, mean temperature of, 63

Portobello, striated pavements near, 255, 256

Post-tertiary formations, hypothetical thickness of, 366

Pouillet, M., on the amount of the sun’s heat, 26 〃 on amount of sun’s rays cut off by the atmosphere, 26

Pratt, Archdeacon, on glacial submergence, 387

Prestwich, Professor, on Hoxne inter-glacial bed, 241

Pressure as a cause of circulation, 187

Principles of geology, nature of, 4

Prince Patrick’s Island, discovery of recent tree in, 261

Radiation, rate of, increases with increase of temperature, 37 〃 of gases, 38 〃 the way by which the earth loses heat, 39 〃 how affected by snow covering the ground, 58 〃 how affected by humid air, 59 〃 accelerated by increased formation of snow and ice, 75

Raised beaches, date of, 407 〃 Mr. Pengelly on, 407

Ramsay, Professor, on glacial origin of Old Red Sandstone of North of England, 294 〃 on Old Red Sandstone, 367 〃 on geological time, 343 〃 on ice-action during Permian period, 298 〃 on boulders of Permian age in Natal, 301 〃 on thickness of stratified rocks of Britain, 267, 361

Redhall Quarry, inter-glacial bed in, 247

Red Sea, why almost rainless, 30

Regelation, _rationale_ of, 520, 554 〃 Professor James Thomson on cause of, 554 〃 Professor Faraday on cause of, 554

Regnault, M., on specific heat of sandstone, 86

Reynaud, Jean, on influence of eccentricity on climate, 541

Rhine, ancient, bed in German Ocean, 480

Ridge between Capes Trafalgar and Spartel, influence of, 167

Rink, Dr., on inland ice of Greenland, 380

River-ice, effect of, 279

River-ice does not produce striations, 279

River systems, carrying-power measure of denudation, 336

River valleys, how striated across, 525

Robertson, Mr. David, on Crofthead and Hillhead inter-glacial beds, 247, 248 〃 on foraminifera in red clay, 485

Rock-basins, how excavated by ice, 525

Rocks removed by denudation, 361

Ross, Capt. Sir James, on South Shetland, 61 〃 on temperature of antarctic regions in summer, 63

Sandwich Land, description by Capt. Cook, 60 〃 cold summers of, not due to latitude, 64

Salter, Mr., on carboniferous fossils of arctic regions, 298 〃 on warm climate of North Greenland during Oolitic period, 302

Saltness of the ocean, difference of, as a cause of motion, 103 〃 in direct opposition to temperature in producing ocean-currents, 104

Scandinavian ice, track of, 447

Scandinavian ice-sheet in the North Sea, 444

Scoresby, Dr., on condition of arctic regions in summer, 58, 62 〃 on density of Gulf-stream water, 129

Scotland, inter-glacial beds of, 243−249 〃 evidence of ice-action in carboniferous conglomerate of, 296 〃 buried under ice, 439 〃 ice-sheet of, in North Sea, 442 〃 why ice-sheet was so thick, 452

Sea, height of, at equator above poles, 119 〃 rise of, due to combined effect of eccentricity and obliquity, 403 〃 bottoms not striated by icebergs, 272

Sea and land, present arrangement indispensable to life, 52

Sea-level, oscillations of, in relation to distribution, 394 〃 oscillations of, during formation of coal measures, 424 〃 raised, by melting of antarctic ice-cap, 388 〃 influence of obliquity of ecliptic on, 403

Section of Mid-Atlantic, 222

Section across antarctic ice-cap, 377

Sedimentary rocks existing fragmentary, 361 〃 of the globe, mean thickness of, hitherto unknown, 361 〃 how mean thickness might be determined, 362 〃 mean thickness of, over-estimated, 364

Shearing-force of ice, 496 〃 momentary loss of, 518

Shetland islands glaciated by land-ice from Scandinavia, 450

Shetland, South, glacial condition of, 61

Shell-beds, evidence of warm inter-glacial periods from, 252

Shells of the boulder clay of Caithness, 450

Shore-ice, striations produced by, in Bay of Fundy, 280

Silurian period, ice-action in Ayrshire during, 293 〃 evidence in Wigtownshire of ice-action during, 293

Slitrig, inter-glacial bed of, 243

Slope of surface of maximum density has no power to produce motion, 120 〃 from equator to pole, erroneous view regarding, 120

Smith, Mr. Leigh, temperature soundings, 129

Smith, Mr., of Jordanhill, on striated pavements, 256

Snow, how radiation is affected by, 58 〃 common in summer in arctic regions, 62 〃 rate of accumulation of, increased by sun’s rays being cut off by fogs, 75 〃 formation increased by radiation, 75

Somerset, West, glaciation of, 463

Somerville, Mrs., on influence of eccentricity on climate, 540

South Africa, glaciation of, 242 〃 boulder clay of Permian age in, 300

South of England ice-sheet, 463

South Shetland, glacial condition of, at mid summer, 61

South-west winds, heat conveyed by, not derived from equatorial regions, 28 〃 heat conveyed by, derived from Gulf-stream, 28

Southern hemisphere, present extension of ice on, due partly to eccentricity, 78 〃 why colder than northern, 81−92 〃 absorbs more heat than the northern, 90 〃 lower temperature of, due to ocean-currents, 92 〃 surface currents from, warmer than under currents to, 92 〃 glacial and inter-glacial periods of, 242

Southern Ocean, thermal condition of, 225

Specific gravity can act only by causing water to descend a slope, 99 〃 mode of action in causing ocean-currents, 100 〃 inadequacy of, to produce ocean-currents demonstrated by Sir John Herschel, 116

Spitzbergen, Gulf-stream and under current at, 134 〃 Miocene flora of, 309

Stellar space, temperature of, 35 〃 received temperature of, probably too high, 39

Stewart, Professor Balfour, experiment on radiation, 37 〃 on cause of glacial cold, 79

Stirling, Mr., on old watercourse near Grangemouth, 481

St. John’s River, action of ice on banks of, 279

St. Lawrence, action of ice on bank of river, 279

Stockwell, Mr., on eccentricity of earth’s orbit, 54 〃 on obliquity of ecliptic, 399 〃 table of superior limits of eccentricity, 531

Stone, Mr., on eccentricity of the earth’s orbit, 322

Stow, G. W., on glacial beds of South Africa, 242 〃 on Karoo beds, 301

Striæ, direction of, show the clay of Caithness came from the sea, 436

Striations obliterated rather than produced by icebergs, 274

Striated pavements why so seldom observed, 256 〃 evidence of inter-glacial periods from, 255

Striated stones found in conglomerate of Lower Carboniferous age by Professor Geikie, 296 〃 in Permian breccias, 299 〃 in the glacial conglomerate of the Superga, Turin, 306

Stratified rocks may be formed at all possible rates, 360 〃 rate of formation of, as estimated by Professor Huxley, 363

Struve, M., formula of obliquity of ecliptic, 404

Subaërial denudation, rate of, 331

Submarine forests, 409 〃 (ancient), coal seams the remains of, 428

Submergence, physical causes of, 368 〃 coincident with glaciation, 389 〃 of land resulting from melting of antarctic ice-cap, 389 〃 how affected by fluidity of interior of the earth, 395 〃 necessary for preservation of coal plants, 423 〃 frequent during formation of coal beds, 426

Subsidence insufficient to account for general submergence, 390 〃 necessary to accumulation of coal seams, 427

Sun supposed by some to be a variable star, 8 〃 maximum and minimum distance of, 55 〃 rays of, cut off by fogs in ice-covered regions, 60 〃 nearness in perigee a cause of snow and ice, 74 〃 total amount of heat radiated from, 346 〃 age and origin of, 346 〃 source of its energy, 347 〃 heat of, origin and chief source of, 349 〃 originally an incandescent mass, 350 〃 energy of, may have originally been derived from motion in space, 355

Surface currents which cross the equator warmer than the compensatory under currents, 92

Surface currents from poles to equator, according to Maury, produced by saltness, 108

Sutherland, Dr., observations by, on stranding of icebergs, 275 〃 testimony, that icebergs do not striate rocks, 278 〃 on the boulder clay of Natal, 300

Sutherland, boulder conglomerate of Oolitic period of, 302

Sweden, Southern, shells in glacial shell beds of, 253

Switzerland, inter-glacial period of, 239 〃 M. Morlat on inter-glacial periods of, 240 〃 gravels of, by Mr. James Geikie, 268 〃 Eocene glacial epoch in, 305

Table of June temperatures in different latitudes, 65 〃 soundings in temperate regions, 222

Tables of eccentricity, 314−321 〃 of eccentricity, explanation of, 322

Tay, valley of, striated across, 526 〃 ancient buried channel of, 490

Temperate regions, cold periods best marked in, 258

Temperature of space, 532 〃 reasons why it should be reconsidered, 39

Temperature (mean) of equator and poles compared, 41 〃 why so low in polar regions during summer, 66 〃 how difference of specific gravity is caused by, 102 〃 higher, of the waters of Gulf-stream considered by Lieutenant Maury as the real causes of its motion, 111 〃 of sea at equator decreases most rapidly at the surface, 119 〃 of Greenland in Miocene period, 310 〃 of poles when obliquity was at its superior limit, 402

Tension, effect of, on ice, 522 〃 the cause of the cooling effect produced by, 552

Tertiary period, climate of, error in regard to, 288

Thermal condition of Southern Ocean, 225

Thibet, table-land of, 418

Thomson, Professor James, on cause of regelation, 554 〃 theory of glacier-motion, 512

Thomson, Mr. James, on glacial conglomerate in Arran, 299 〃 on ice-action in Cambrian conglomerate of Islay, 292

Thomson, Professor Wyville, on Dr. Carpenter’s theory, 129 〃 cited, 130 〃 thermal condition of the sea, 138

Thomson, Sir W., amount of internal heat passing through earth’s crust, 142 〃 on limit to age of the globe, 343 〃 on influence of ice-cap on sea-level, 372 〃 climate not affected by internal heat, 6 〃 earth’s axis of rotation permanent, 7 〃 on volume and mass of the sun, 347

Tidal wave, effect of friction, 336

Tides, supposed argument from, 184

Time, geological, 311−359 〃 as represented by geological phenomena, 326 〃 represented by existing rocks, 361

Torrid zone, annual quantity of heat received by, per unit of surface, 194

Towncroft farm, section of channel at, 474

Towson, Mr., on icebergs of Southern Ocean, 383

Trade-winds (anti), heat conveyed by, over-estimated, 28 〃 (anti) derive their heat from the Gulf-stream, 32 〃 of warm hemisphere overborne by those of cold hemisphere, 70 〃 causes which determine the strength of, 70 〃 strongest on glaciated hemisphere, 70 〃 reaction upon trade-winds by formation of snow and ice, 76 〃 influence of, in turning ocean-currents on warm hemisphere, 97 〃 do not explain the antarctic current, 211

Tiddeman on North of England ice-sheet, 458 〃 displacement of, 230

Transport of boulders and rubbish the proper function of icebergs, 281

Trafalgar, effect of ridge between Capes Spartel and on Gibraltar current, 167

Turner, Professor, on arctic seal found at Grangemouth, 485

Tylor, Alfred, on denudation of Mississippi basin, 333

Tyndall, Professor, on heat in aqueous vapour, 29 〃 on sifted rays, 47 〃 on diathermancy of air, 59 〃 on glacial epoch, 78

Udevalla, Mediterranean shell in glacial shells, bed of, 253

Under currents to southern hemisphere colder than surface currents from, 92 〃 produced by saltness, flow from equator to poles, 106 〃 account for cold water at equator, 124, 142 〃 in Davis’ Strait, 134 〃 take path of least resistance, 130 〃 why considered improbable, 135 〃 difficulty regarding, obviated, 217 〃 theory of, 217

Underground temperature, Professor J. D. Forbes on, 86

Underground temperature exerts no influence on the climate, 88 〃 absolute amount of heat derived from, 142 〃 supposed influence of, 176

Uniformity, modern doctrine of, 325

United States’ coast survey of Gulf-stream, 24 〃 hydrographic department, papers published by, 556

Unstratified boulder clay must be the product of land-ice, 437

Upsala and Stockholm striated by Baltic glacier, 447

Vertical circulation, Lieutenant Maury’s theory of, 108 〃 according to Dr. Carpenter, 153

Vertical descent of polar column caused by extra pressure of water upon it, 154 〃 effects of, and slope, the same, whether performed simultaneously or alternately, 159 〃 of polar column illustrated by diagram, 160

Vertical distribution of heat in the ocean, Mr. Buchanan’s theory, 550

Vogt, Professor, on Dürnten lignite bed, 241

Warm hemisphere made warmer by increased reaction of physical causes, 76

Warm periods best marked in arctic regions, 258 〃 in arctic regions, evidence of, 261 〃 better represented by fossils than cold periods, 288 〃 evidence of, during Cretaceous age, 304

Warm inter-glacial periods in arctic regions, 258−265

Water at equator the best means of distributing heat derived from the sun, 30

Water, a worse radiator than land, 91

Wastdale granite boulders, difficulty of accounting for transport of, 456

Wastdale Crag glaciated by continental ice, 457

Weibye, M., striation observed by, 280

Wilkes, Lieutenant, on cold experienced in antarctic regions in summer, 63

Wellington Sound, ancient trees found at, 265

Winter-drift of ice on coast of Labrador, 276

West winds, moisture of, derived from Gulf-stream, 29

Wind, work in impelling currents, 219

Winds, ocean-currents produced by, 212 〃 system of, agrees with the system of ocean-currents, 213

Wind theory of oceanic circulation, 210 〃 crucial test of, 220

Wigtownshire, ice-action during Silurian age, 293

Work performed by descent of polar column, 157

Wood, Mr. Nicholas, on buried channel, 488

Wood, Jun., Mr. Searles, middle drift, 250 〃 on occurrence of chalk _débris_ in south-west of England, 460

Woodward, Mr. H. B., on boulder clay in Devonshire, 463

Wunsch, Mr. E. A., on glacial conglomerate in Arran, 299

Yare, ancient buried channel of, 489

Young, Mr. J., objection considered, 482

Yorkshire drift common in south of England, 460

Zenger, Professor, on the moon’s influence on climate, 324

THE END.

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+----------------------------------------------------------------------+ | FOOTNOTES: | | | | [1] Trans. of Edin. Geol. Soc., vol. ii. p. 252. | | | | [2] Phil. Mag., January, 1863. | | | | [3] _Athenæum_, September 22, 1860. | | | | [4] Trans. Glasgow Geol. Soc., vol. iv., p. 313. | | | | [5] See Mr. Hopkin’s remarks on this theory, Quart. Journ. Geol. | | Soc., vol. viii. | | | | [6] See Chap. xxv. | | | | [7] See Chap. iv. | | | | [8] “Treatise on Astronomy,” § 315; “Outlines,” § 368. | | | | [9] _Annuaire_ for 1834, p. 199. Edin. New Phil. Journ., April, | | 1834, p. 224. | | | | [10] “Cosmos,” vol. iv. p. 459 (Bohn’s Edition). “Physical | | Description of the Heavens,” p. 336. | | | | [11] Phil. Mag. for February, 1867, p. 127. | | | | [12] The Gulf-stream at the narrowest place examined by the Coast | | Survey, and where also its velocity was greatest, was found to be | | over 30 statute miles broad and 1,950 feet deep. But we must not | | suppose that this represents all the warm water which is received | | by the Atlantic from the equator; a great mass flows into the | | Atlantic without passing through the Straits of Florida. | | | | [13] It is probable that a large proportion of the water | | constituting the south-eastern branch of the Gulf-stream is never | | cooled down to 40°; but, on the other hand, the north-eastern | | branch, which passes into the arctic regions, will be cooled far | | below 40°, probably below 30°. Hence I cannot be over-estimating | | the extent to which the water of the Gulf-stream is cooled down in | | fixing upon 40° as the average minimum temperature. | | | | [14] “Physical Geography of the Sea,” § 24, 6th edition. | | | | [15] “Physical Geography,” § 54. | | | | [16] Trans. of Roy. Soc. of Edin., vol. xxi., p. 57. Phil. Mag., § | | 4, vol. ix., p. 36. | | | | [17] “Smithsonian Contributions to Knowledge,” vol. ix. | | | | [18] “Heat as a Mode of Motion,” art. 240. | | | | [19] Trans. Roy. Soc. of Edin., vol. xxv., part 2. | | | | [20] See “Smithsonian Contributions to Knowledge,” vol. ix. | | | | [21] “Meteorology,” section 36. | | | | [22] _Comptes-Rendus_, July 9, 1838. Taylor’s “Scientific Memoirs,” | | vol. iv., p. 44 (1846). | | | | [23] The mean temperature of the Atlantic between the tropics and | | the arctic circle, according to Admiral Fitzroy’s chart, is about | | 60°. But he assigns far too high a temperature for latitudes above | | 50°. It is probable that 56° is not far from the truth. | | | | [24] The probable physical cause of this will be considered in the | | Appendix. | | | | [25] The mean temperature of the equator, according to Dove, is | | 79°·7, and that of the north pole 2°·3. But as there is, of course, | | some uncertainty regarding the actual mean temperature of the | | poles, we may take the difference in round numbers at 80°. | | | | [26] Trans. of Roy. Soc. Edin., vol. xxii., p. 75. | | | | [27] _Connaissance des Temps_ for 1863 (Additions). Lagrange’s | | determination makes the superior limit 0·07641 (Memoirs of the | | Berlin Academy for 1782, p. 273). Recently the laborious task of | | re-investigating the whole subject of the secular variations of the | | elements of the planetary orbits was undertaken by Mr. Stockwell, | | of the United States. He has taken into account the disturbing | | influence of the planet Neptune, the existence of which was not | | known when Leverrier’s computations were made; and he finds that | | the eccentricity of the earth’s orbit will always be included | | within the limits of 0 and 0·0693888. Mr. Stockwell’s elaborate | | Memoir, extending over no fewer than two hundred pages, will be | | found in the eighteenth volume of the “Smithsonian Contributions to | | Knowledge.” | | | | [28] When the eccentricity is at its superior limit, the absolute | | quantity of heat received by the earth during the year is, however, | | about one three-hundredth part greater than at present. But this | | does not affect the question at issue. | | | | [29] Scoresby’s “Arctic Regions,” vol. ii., p. 379. Daniell’s | | “Meteorology,” vol. ii., p. 123. | | | | [30] Tyndall, “On Heat,” article 364. | | | | [31] Tyndall, “On Heat,” article 364. | | | | [32] See Phil. Mag., March, 1870, p. | | | | [33] Captain Cook’s “Second Voyage,” vol. ii., pp. 232, 235. | | | | [34] “Antarctic Regions,” vol. ii., pp. 345−349. | | | | [35] Ibid., vol. i., p. 167. | | | | [36] Ibid., vol. ii., p. 362. | | | | [37] Edinburgh Philosophical Journal, vol. iv., p. 266. | | | | [38] Scoresby’s “Arctic Regions,” vol. i., p. 378. | | | | [39] Ibid., p. 425. | | | | [40] See Meech’s memoir “On the Intensity of the Sun’s Heat and | | Light,” “Smithsonian Contributions,” vol. ix. | | | | [41] “Antarctic Regions,” vol. i., p. 240. | | | | [42] _Challenger_ Reports, No. 2, p. 10. | | | | [43] See “Smithsonian Contributions,” vol. ix. | | | | [44] Quart. Journ. Geol. Soc., vol. xxv., p. 350. | | | | [45] Trans. of Glasgow Geol. Soc. for 1866. | | | | [46] _Revue des Deux Mondes_ for 1867. | | | | [47] Letter to the author, February, 1870. | | | | [48] “Révolutions de la Mer,” p. 37 (second edition). | | | | [49] Edin. Phil. Journ., vol. iv., p. 262 (1821). | | | | [50] Phil. Mag., § 4, vol. xxviii., p. 131. _Reader_, December 2nd, | | 1865. | | | | [51] This point will be found discussed at considerable length in | | the Phil. Mag. for September, 1869. | | | | [52] See Phil. Mag. for October, 1870, p. 259. | | | | [53] Proceedings of the Royal Society, No. 138, p. 596, foot-note. | | | | [54] The edition from which I quote, unless the contrary is stated, | | is the one published by Messrs. T. Nelson and Sons, 1870, which is | | a reprint of the new edition published in 1859 by Messrs. Sampson | | Low and Co. | | | | [55] “Physical Geography,” article 57. | | | | [56] Philosophical Magazine, vol. xii. p. 1 (1838). | | | | [57] “Mémoires par divers Savans,” tom. i., p. 318, St. | | Petersburgh, 1831. See also twelfth number of Meteorological | | Papers, published by the Board of Trade, 1865, p. 16. | | | | [58] Dubuat’s “Hydraulique,” tom. i., p. 64 (1816). See also | | British Association Report for 1834, pp. 422, 451. | | | | [59] See Proceedings of the Royal Society for December, 1868, | | November, 1869. Lecture delivered at the Royal Institute, _Nature_, | | vol. i., p. 490. Proceedings of the Royal Geographical Society, | | vol. xv. | | | | [60] Trans. of Glasgow Geol. Soc. for April, 1867. Phil. Mag. for | | February, 1867, and June, 1867 (Supplement). | | | | [61] Phil. Mag. for February, 1870. | | | | [62] “The Depths of the Sea,” pp. 376 and 377. | | | | [63] “The Threshold of the Unknown Region,” p. 95. | | | | [64] See “Physical Geography of the Sea,” chap. ix., new edition, | | and Dr. A. Mühry “On Ocean-currents in the Circumpolar Basin of the | | North Hemisphere.” | | | | [65] “Depths of the Sea,” _Nature_ for July 28, 1870. | | | | [66] “Memoir on the Gulf-stream,” _Geographische Mittheilungen_, | | vol. xvi. (1870). | | | | [67] Dr. Carpenter “On the Gulf-stream,” Proceedings of Royal | | Geographical Society for January 9, 1871, § 29. | | | | [68] Dr. Petermann’s _Mittheilungen_ for 1872, p. 315. | | | | [69] Proceedings of the Royal Society, vol. xvii., p. 187, xviii., | | p. 463. | | | | [70] The average depth of the Pacific Ocean, as found by the | | soundings of Captain Belknap, of the U.S. steamer _Tuscarora_, made | | during January and February, 1874, is about 2,400 fathoms. The | | depth of the Atlantic is somewhat less. | | | | [71] Proceedings of Royal Geographical Society, vol. xv., § 22. | | | | [72] It is a well-established fact that in polar regions the | | temperature of the sea decreases from the surface downwards; | | and the German Polar Expedition found that the water in very | | high latitudes is actually less dense at the surface than at | | considerable depths, thus proving that the surface-water could not | | sink in consequence of its greater density. | | | | [73] Proceedings of the Royal Society, vol. xix., p. 215. | | | | [74] _Nature_ for July 6, 1871. | | | | [75] Since the above objection to the Gravitation Theory of the | | Gibraltar Current was advanced three years ago, Dr. Carpenter | | appears to have abandoned the theory to a great extent. He now | | admits (Proceedings of Royal Geographical Society, vol. xviii., | | pp. 319−334, 1874) that the current is almost wholly due not to | | difference of specific gravity, but to an excess of evaporation in | | the Mediterranean over the return by rain and rivers. | | | | [76] Proceedings of Royal Society, No. 138, § 26. | | | | [77] Proceedings of Royal Geographical Society, January 9, 1871. | | | | [78] Ibid. | | | | [79] See §§ 20, 34; also Brit. Assoc. Report for 1872, p. 49, and | | other places. | | | | [80] See also to the same effect Brit. Assoc. Report, 1872, p. 50. | | | | [81] Phil. Mag. for Oct. 1871. | | | | [82] The actual slope, however, does not amount to more than 1 in | | 7,000,000. | | | | [83] Proc. of Roy. Geog. Soc., January 9, 1871, § 29. | | | | [84] Trans. of Geol. Soc. of Glasgow for April, 1867; Phil. Mag. | | for June, 1867. | | | | [85] _Nature_, vol. i., p. 541. Proc. Roy. Soc., vol. xviii., p. | | 473. | | | | [86] Chapter II. | | | | [87] Chapter II. | | | | [88] Chapter II. | | | | [89] Mr. Findlay considers that the daily discharge does not exceed | | 333 cubic miles (Brit. Assoc. Rep., 1869, p. 160). My estimate | | makes it 378 cubic miles. Mr. Laughton’s estimate is 630 cubic | | miles (Paper “On Ocean-currents,” Journal of Royal United-Service | | Institution, vol. xv.). | | | | [90] Proceedings of the Royal Geographical Society, vol. xviii., p. | | 393. | | | | [91] Phil. Mag. for October, 1871, p. 274. | | | | [92] Proceedings of the Royal Geographical Society, vol. xv. | | | | [93] Phil. Mag., February, 1870. | | | | [94] Brit. Assoc. Report, 1869, Sections, p. 160. | | | | [95] Journal of Royal United-Service Institute, vol. xv. | | | | [96] Dr. Carpenter (Proc. of Roy. Geog. Soc., vol. xviii., p. | | 334) misapprehends me in supposing that I attribute the Gibraltar | | current wholly to the Gulf-stream. In the very page from which he | | derives or could derive his opinion as to my views on the subject | | (Phil. Mag. for March, 1874, p. 182), I distinctly state that | | “the excess of evaporation over that of precipitation within the | | Mediterranean area would of itself produce a considerable current | | through the Strait.” That the Gibraltar current is due to two | | causes, (1) the pressure of the Gulf-stream, and (2) excess of | | evaporation over precipitation in the Mediterranean, has always | | appeared to me so perfectly obvious, that I never held nor could | | have held any other opinion on the subject. | | | | [97] Paper read to the Edinburgh Botanical Society on January 8, | | 1874. | | | | [98] Proc. Roy. Geog. Soc., vol. xviii., p. 362. A more | | advantageous section might have been chosen, but this will suffice. | | The section referred to is shown in Plate III. The peculiarity of | | this section, as will be observed, is the thinness of the warm | | strata at the equator, as compared with that of the heated water in | | the North Atlantic. | | | | [99] The temperature of column C in Dr. Carpenter’s section is | | somewhat less than that given in the foregoing table; so that, | | according to that section, the difference of level between column C | | and columns A and B would be greater than my estimate. | | | | [100] Captain Nares’s Report, July 30, 1874. | | | | [101] See Chapter IV. | | | | [102] Phil. Mag. for August, 1864, February, 1867, March, 1870; see | | Chap. IV. | | | | [103] Quarterly Journal of Science for October, 1874. | | | | [104] See a paper by M. Morlot, on “The Post-Tertiary and | | Quaternary Formations of Switzerland.” Edin. New Phil. Journal, New | | Series, vol. ii., 1855. | | | | [105] Edin. New Phil. Journ., New Series, vol. ii., p. 28. | | | | [106] Vogt’s “Lectures on Man,” pp. 318−321. | | | | [107] See Mr. Prestwich on Flint Implements, Phil. Trans. for 1860 | | and 1864. Lyell’s “Antiquity of Man,” Second Edition, p. 168. | | | | [108] Edin. New Phil. Journ., New Series, vol. ii., p. 28. | | Silliman’s Journ., vol. xlvii., p. 259 (1844). | | | | [109] Quart. Journ. Geol. Soc., vol. xxvii., p. 534. | | | | [110] Ibid., vol. xxviii., p. 17. | | | | [111] “Glacial Drift of Scotland,” p. 54. | | | | [112] “Glacial Drift of Scotland,” p. 58. | | | | [113] Quart. Journ. Geol. Soc., vol. v., p. 22. | | | | [114] “Glacial Drift of Scotland,” p. 64. | | | | [115] Trans. Edin. Geol. Soc., vol. ii., p. 391. | | | | [116] Trans. of Geol. Soc. of Glasgow, vol. iv., p. 146. | | | | [117] Geol. Mag., vi., p. 391. | | | | [118] See “Memoirs of Geological Survey of Scotland,” Explanation | | of sheet 22, p. 29. See also Trans. Glasgow Geol. Soc., iv., p. 150. | | | | [119] “Great Ice Age,” p. 374. | | | | [120] “Great Ice Age,” p. 384. | | | | [121] “Geological Survey of Ohio, 1869,” p. 165. See also “Great | | Ice Age,” chap. xxviii. | | | | [122] Quart. Journ. Geol. Soc., xxviii., p. 435. | | | | [123] Brit. Assoc. Report, 1863. | | | | [124] Trans. Glasgow Nat. Hist. Soc., vol. i., p. 115. | | | | [125] Trans. of the Geol. Soc. of Glasgow, vol. iii., p. 133. See | | also “Great Ice Age,” chaps. xii. and xiii. | | | | [126] Chap. XXIX. | | | | [127] Edin. New Phil. Journ., vol. liv., p. 272. | | | | [128] “Newer Pliocene Geology,” p. 129. John Gray & Co., Glasgow. | | | | [129] “Glacial Drift of Scotland,” p. 67. | | | | [130] “Glacial Drift of Scotland,” p. 12. | | | | [131] See Chapter IV. | | | | [132] “Discovery of the North-West Passage,” p. 213. | | | | [133] “Voyage of the _Resolute_,” p. 294. | | | | [134] Quart. Journ. Geol. Soc., vol. xi., p. 540. | | | | [135] “McClure’s North-West Passage,” p. 214. Second Edition. | | | | [136] “British Association Report for 1855,” p. 381. “The Last of | | the Arctic Voyages,” vol. i., p. 381. | | | | [137] Mr. James Geikie informs me that the great accumulations of | | gravel which occur so abundantly in the low grounds of Switzerland, | | and which are, undoubtedly, merely the re-arranged materials | | originally brought down from the Alps as till and as moraines | | by the glaciers during the glacial epoch, rarely or never yield | | a single scratched or glaciated stone. The action of the rivers | | escaping from the melting ice has succeeded in obliterating all | | trace of striæ. It is the same, he says, with the heaps of gravel | | and sand in the lower grounds of Sweden and Norway, Scotland and | | Ireland. These deposits are evidently in the first place merely the | | materials carried down by the swollen rivers that issued from the | | gradually melting ice-fields and glaciers. The stones of the gravel | | derived from the demolition of moraines and till, have lost all | | their striæ and become in most cases well water-worn and rounded. | | | | [138] Report on Icebergs, read before the Association of American | | Geologists, _Silliman’s Journal_, vol. xliii., p. 163 (1842). | | | | [139] “Manual of Geology,” p. 677. | | | | [140] Quart. Journ. Geol. Soc., vol. ix., p. 306. | | | | [141] Dana’s “Manual of Geology,” p. 677. | | | | [142] Quart. Journ. Geol. Soc., vol. ix., p. 306. | | | | [143] “Journal,” vol. i., p. 38. | | | | [144] “Short American Tramp,” pp. 168, 174. | | | | [145] “Short American Tramp,” pp. 239−241. | | | | [146] “Travels in North America,” vol. ii., p. 137. | | | | [147] Ibid., vol. ii., p. 174. | | | | [148] Proceedings of the Royal Society of Edinburgh, Session | | 1865−66, p. 537. | | | | [149] “Short American Tramp,” pp. 77, 81, 111. | | | | [150] “Second Visit,” vol. ii., p. 367. | | | | [151] “Memoirs of Boston Society of Natural History,” vol. i. | | (1867), p. 228. | | | | [152] “Antiquity of Man,” p. 268. Third Edition. | | | | [153] “Great Ice Age,” p. 512. | | | | [154] Brit. Assoc., 1870, p. 88. | | | | [155] Quart. Journ. Geol. Soc., vol. v., p. 10. Phil. Mag. for | | April, 1865, p. 289. | | | | [156] “Great Ice Age,” p. 512. | | | | [157] Jukes’ “Manual of Geology,” p. 421. | | | | [158] See also Quarterly Journal Geological Society, vol. xi., p. | | 510. | | | | [159] The _Reader_ for August 12, 1865. | | | | [160] “History of the Isle of Man,” p. 86. My colleague, Mr. John | | Horne, in his “Sketch of the Geology of the Isle of Man,” Trans. of | | Edin. Geol. Soc., vol. ii., part iii., considers this conglomerate | | to be of Lower Carboniferous age. | | | | [161] See Selwyn, “Phys. Geography and Geology of Victoria.” 1866. | | pp. 15−16; Taylor and Etheridge, _Geol. Survey Vict., Quarter Sheet | | 13, N.E._ | | | | [162] Report on the Geology of the District of Ballan, Victoria. | | 1866. p. 11. | | | | [163] _Atrypa reticularis._ | | | | [164] Quart. Journ. Geol. Soc., vol. xii., p. 58. | | | | [165] “Great Ice Age,” p. 513. | | | | [166] “Great Ice Age,” p. 513. | | | | [167] Brit. Assoc. Report for 1873. | | | | [168] Quart. Journ. Geol. Soc., vol. xi., p. 519. | | | | [169] _Orthis resupinata._ | | | | [170] _Prod. semireticulatus_ var. _Martini_. Sow. | | | | [171] “Belcher’s Voyage,” vol. ii., p. 377. | | | | [172] “Journal of a Boat Voyage through Rupert-Land,” vol. ii., p. | | 208. | | | | [173] Quart. Journ. Geol. Soc., vol. xi., p. 197. | | | | [174] Explanation Memoir to Sheet 47, “Geological Survey of | | Ireland.” | | | | [175] Phil. Mag., vol. xxix., p. 290. | | | | [176] “Memoirs of the Geological Survey of India,” vol. i., part i. | | | | [177] Quart. Journ. Geol. Soc., vol. xxvi., p. 514. | | | | [178] Ibid., vol. xxvii., p. 544. | | | | [179] Phil. Mag., vol. xxix., p. 290. | | | | [180] Journal of the Royal Dublin Society for February, 1857. | | | | [181] Quart. Journ. Geol. Soc., vol. xi., p. 519. | | | | [182] “The Last of the Arctic Voyages,” by Captain Sir E. Belcher, | | vol. ii., p. 389. Appendix Brit. Assoc. Report for 1855, p. 79. | | | | [183] Ibid., vol. ii., p. 379. Appendix. | | | | [184] “Manual of Geology,” pp. 395, 493. | | | | [185] Appendix to McClintock’s “Arctic Discoveries.” | | | | [186] Quart. Journ. Geol. Soc., vol. xiv., p. 262. Brit. Assoc. | | Report for 1857, p. 62. | | | | [187] Quart. Journ. Geol. Soc., vol. xvi., p. 327. _Geologist_, | | 1860, p. 38. | | | | [188] Phil. Mag., vol. xxix., p. 290. | | | | [189] Trans. Geol. Soc. of Glasgow, vol. v., p. 64. | | | | [190] “Principles,” vol. i., p. 209. Eleventh Edition. | | | | [191] “Memoirs of the Royal Academy of Science of Turin,” Second | | Series, vol. xx. I am indebted for the above particulars to | | Professor Ramsay, who visited the spot along with M. Gastaldi. | | | | [192] “Antiquity of Man,” Second Edition, p. 237. | | | | [193] Dr. Robert Brown, in a recent Memoir on the Miocene Beds of | | the Disco District (Trans. Geol. Soc. Glasg., vol. v., p. 55), | | has added considerably to our knowledge of these deposits. He | | describes the strata in detail, and gives lists of the plant and | | animal remains discovered by himself and others, and described by | | Professor Heer. Professor Nordenskjöld has likewise increased the | | data at our command (Transactions of the Swedish Academy, 1873); | | and still further evidence in favour of a warm climate having | | prevailed in Greenland during Miocene times has been obtained by | | the recent second German polar expedition. | | | | [194] The following are M. Leverrier’s formulæ for computing the | | eccentricity of the earth’s orbit, given in his “Memoir” in the | | _Connaissance des Temps_ for 1843:— | | | | Eccentricity in (_t_) years after January 1, 1800 | | _____________ | | = √_h_^2 + _l_^2 where | | | | _h_ = 0·000526 Sin (_gt_ + _ß_) + 0·016611 Sin (_g_{1}t_ + _ß_{1}_) | | + 0·002366 Sin (_g_{2}t_ + _ß_{2}_) | | + 0·010622 Sin (_g_{3}t_ + _ß_{3}_) | | - 0·018925 Sin (_g_{4}t_ + _ß_{4}_) | | + 0·011782 Sin (_g_{5}t_ + _ß_{5}_) | | - 0·016913 Sin (_g_{6}t_ + _ß_{6}_) | | and | | | | _l_ = 0·000526 Cos (_gt_ + _ß_) + 0·016611 Cos (_g_{1}t_ + _ß_{1}_) | | + 0·002366 Cos (_g_{2}t_ + _ß_{2}_) | | + 0·010622 Cos (_g_{3}t_ + _ß_{3}_) | | - 0·018925 Cos (_g_{4}t_ + _ß_{4}_) | | + 0·011782 Cos (_g_{5}t_ + _ß_{5}_) | | - 0·016913 Cos (_g_{6}t_ + _ß_{6}_) | | | | _g_ = 2″·25842 _ß_ = 126° 43′ 15″ | | _g_{1}_ = 3″·71364 _ß_{1}_ = 27 21 26 | | _g_{2}_ = 22″·4273 _ß_{2}_ = 126 44 8 | | _g_{3}_ = 5″·2989 _ß_{3}_ = 85 47 45 | | _g_{4}_ = 7″·5747 _ß_{4}_ = 35 38 43 | | _g_{5}_ = 17″·1527 _ß_{5}_ = −25 11 33 | | _g_{6}_ = 17″·8633 _ß_{6}_ = −45 28 59 | | | | [195] See Professor C. V. Zenger’s paper “On the Periodic Change cf | | Climate caused by the Moon,” Phil. Mag. for June, 1868. | | | | [196] Phil. Mag. for February, 1867. | | | | [197] Phil. Mag. for May, 1868. | | | | [198] Student’s “Elements of Geology,” p. 91. Second Edition. | | | | [199] In an interesting memoir, published in the Phil. Mag. for | | 1850, Mr. Alfred Tylor estimated that the basin of the Mississippi | | is being lowered at the rate of one foot in 10,000 years by the | | removal of the sediment; and he proceeds further, and reasons that | | one foot removed off the general surface of the land during that | | period would raise the sea-level three inches. Had it not been that | | Mr. Tylor’s attention was directed to the effects produced by the | | removal of sediment in raising the level of the ocean rather than | | in lowering the level of the land, he could not have failed to | | perceive that he was in possession of a key to unfold the mystery | | of geological time. | | | | [200] Proc. Roy. Soc., No. 152, 1874. | | | | [201] I have taken for the volume and mass of the sun the values | | given in Professor Sir William Thomson’s memoir, Phil. Mag., vol. | | viii. (1854). | | | | [202] Phil. Mag., § 4, vol. xi., p. 516 (1856). | | | | [203] Phil. Mag. for July, 1872, p. 1. | | | | [204] “Principles,” p. 210. Eleventh Edition. | | | | [205] “Principles,” vol. i., p. 107. Tenth Edition. | | | | [206] The conception of submergence resulting from displacement of | | the earth’s centre of gravity, caused by a heaping up of ice at | | one of the poles, was first advanced by M. Adhémar, in his work | | “_Révolutions de la Mer_,” 1842. When the views stated in this | | chapter appeared in the _Reader_, I was not aware that M. Adhémar | | had written on the subject. An account of his mode of viewing the | | question is given in the Appendix. | | | | [207] Petermann’s _Geog. Mittheilungen_, 1871, Heft. x., p. 377. | | | | [208] Geol. Mag., 1872, vol. ix., p. 360. | | | | [209] “Open Polar Sea,” p. 134. | | | | [210] Journal of the Royal Geographical Society, 1853, vol. xxiii. | | | | [211] “Physics of Arctic Ice,” Quart. Journ. Geol. Soc. for | | February, 1871. | | | | [212] Some writers have objected to the conclusion that the | | antarctic ice-cap is thickest at the pole, on the ground that the | | snowfall there is probably less than at lower latitudes. The fact | | is, however, overlooked, that the greater thickness of an ice-cap | | at its centre is a physical necessity not depending on the rate of | | snowfall. Supposing the snowfall to be greater at, say, lat. 70° | | than at 80°, and greater at 80° than at the pole; nevertheless, the | | ice will continue to accumulate till it is thicker at 80° than at | | 70°, and at the pole than it is at 80°. | | | | [213] It is a pity that at present no record is kept, either by | | the Board of Trade or by the Admiralty, of remarkable icebergs | | which may from time to time be met with. Such a record might be of | | little importance to navigation, but it would certainly be of great | | service to science. | | | | [214] See Chapter XXVII., and also Geol. Mag. for May and June, | | 1870, and January, 1871. | | | | [215] Phil. Mag. for April, 1866, p. 323. | | | | [216] Ibid., for March, 1866, p. 172. | | | | [217] _Reader_, February 10, 1866. | | | | [218] In a former paper I considered the effects of another cause, | | viz., the melting of polar ice resulting from an increase of the | | Obliquity of the Earth’s Orbit.—Trans. Glasgow Geol. Soc., vol. | | ii., p. 177. Phil. Mag., June, 1867. See also Chapter XXV. | | | | [219] Phil. Mag. for November, 1868, p. 376. | | | | [220] Phil. Mag., November, 1868. | | | | [221] “Origin of Species,” chap. xi. Fifth Edition. | | | | [222] Lieutenant-Colonel Drayson (“Last Glacial Epoch of Geology”) | | and also Mr. Belt (Quart. Journ. of Science, October, 1874) state | | that Leverrier has lately investigated the question as to the | | extent of the variation of the plane of the ecliptic, and has | | arrived at results differing considerably from those of Laplace; | | viz., that the variation may amount to 4° 52′, whereas, according | | to Laplace, it amounts to only 1° 21′. I fear they are comparing | | things that are totally different; viz., the variation of the | | plane of the ecliptic in relation to its mean position with its | | variation in relation to the equator. Laplace estimated that the | | plane of the ecliptic would oscillate to the extent of 4° 53′ 33″ | | on each side of its mean position, a result almost identical with | | that of Leverrier, who makes it 4° 51′ 42″. But neither of these | | geometricians ever imagined that the ecliptic could change in | | relation to the equator to even one-third of that amount. | | | | Laplace demonstrated that the change in the plane of the ecliptic | | affected the position of the equator, causing it to vary along with | | it, so that the equator could never possibly recede further than | | 1° 22′ 34″ from its mean position in relation to the ecliptic | | (“_Mécanique Céleste_,” vol. ii., p. 856, Bowditch’s Translation; | | see also Laplace’s memoir, “Sur les Variations de l’Obliquité de | | l’Écliptique,” _Connaissance des Temps_ for 1827, p. 234), and I am | | not aware that Leverrier has arrived at a different conclusion. | | | | [223] Memoir on the Secular Variations of the Elements of the | | Orbits of the Planets, “Smithsonian Contributions to Knowledge,” | | vol. xvii. | | | | [224] “Smithsonian Contributions to Knowledge,” vol. ix. | | | | [225] “Distribution of Heat on the Surface of the Globe,” p. 14. | | | | [226] Chapter IV. | | | | [227] Quart. Journ. Geol. Soc., June, 1866, p. 564. | | | | [228] Quart. Journ. Geol. Soc., vol. xxi., p. 186. | | | | [229] “Geological Observer,” p. 446. See also Mr. James Geikie’s | | valuable Memoir, “On the Buried Forests and Peat Mosses of | | Scotland.” Trans. of the Royal Society of Edinburgh, vol. xxiv., | | and Chambers’ “Ancient Sea-Margins.” | | | | [230] See Lyell’s “Antiquity of Man,” Second Edition, p. 282; | | “Elements,” Sixth Edition, p. 162. | | | | [231] In order to determine the position of the solstice-point | | in relation to the aphelion, it will not do to assume, as is | | commonly done, that the point makes a revolution from aphelion to | | aphelion in any regular given period, such as 21,000 years; for | | it is perfectly evident that owing to the great irregularity in | | the motion of the aphelion, no two revolutions will probably be | | performed in the same length of period. For example, the winter | | solstice was in the aphelion about the following dates: 11,700, | | 33,300, and 61,300 years ago. Here are two consecutive revolutions, | | the one performed in 21,600 years and the other in 28,000 years; | | the difference in the length of the two periods amounting to no | | fewer than 6,400 years. | | | | [232] Quart. Journ. Geol. Soc., vol. xxvii., p. 232. See also “The | | Last Glacial Epoch of Geology,” by the same author. | | | | [233] Quart. Journ. of Science, October, 1874. | | | | [234] The longer diameter passes from long. 14° 23′ E. to long. | | 165° 37′ W. | | | | [235] “Principles,” vol. i., p. 294. Eleventh Edition. | | | | [236] Phil. Mag. for August, 1864. | | | | [237] “Elementary Geology,” p. 399. | | | | [238] “The Past and Present Life of the Globe,” p. 102. | | | | [239] “Memoirs of the Geological Survey,” vol. ii., Part 2, p. 404. | | | | [240] “Coal Fields of Great Britain,” p. 45. Third Edition. | | | | [241] “Journal of Researches,” chap. xiii. | | | | [242] “Coal Fields of Great Britain,” p. 67. | | | | [243] See “Smithsonian Report for 1857,” p. 138. | | | | [244] Quart. Journ. Geol. Soc., May, 1865, p. civ. | | | | [245] “Geology of Fife and the Lothians,” p. 116. | | | | [246] “Life on the Earth,” p. 133. | | | | [247] Quart. Journ. Geol. Soc., vol. xi., p. 535. | | | | [248] Ibid., vol. xii., p. 39. | | | | [249] Miller’s “Sketch Book of Practical Geology,” p. 192. | | | | [250] From Geological Magazine, May and June, 1870; with a few | | verbal corrections, and a slight re-arrangement of the paragraphs. | | | | [251] See Phil. Mag. for November, 1868, p. 374. | | | | [252] See Phil. Mag. for November, 1868, pp. 366−374. | | | | [253] Journ. Geol. Soc., vol. xxi., p. 165. | | | | [254] Specimens of the striated summit and boulder clay stones are | | to be seen in the Edinburgh Museum of Science and Art. | | | | [255] Phil. Mag. for April, 1866. | | | | [256] “Tracings of the North of Europe,” 1850, pp. 48−51. | | | | [257] Quart. Journ. Geol. Soc., vol. ii., p. 364. | | | | [258] “Tracings of the North of Europe,” by Robert Chambers, pp. | | 259, 285. “Observations sur les Phénomènes d’Erosion en Norvège,” | | by M. Hörbye, 1857. See also Professor Erdmann’s “Formations | | Quaternaires de la Suède.” | | | | [259] “Glacial Drift of Scotland,” p. 29. | | | | [260] Geological Magazine, vol. ii., p. 343. Brit. Assoc. Rep., | | 1864 (sections), p. 59. | | | | [261] Trans. Roy. Soc. Edin., vol. vii., p. 265. | | | | [262] “Tracings of Iceland and the Faroe Islands,” p. 49. | | | | [263] See Chap. XXIII. | | | | [264] Mr. Thomas Belt has subsequently advanced (Quart. Jour. Geol. | | Soc., vol. xxx., p. 490), a similar explanation of the steppes of | | Siberia. He supposes that an overflow of ice from the polar basin | | dammed back all the rivers flowing northward, and formed an immense | | lake which extended over the lowlands of Siberia, and deposited the | | great beds of sand and silt with occasional freshwater shells and | | elephant remains, of which the steppes consist. | | | | [265] Proc. Roy. Phys. Soc., Edin., vols. ii. and iii. | | | | [266] From Geol. Mag. for January, 1871. | | | | [267] Quart. Journ. Geol. Soc., xxvi., p. 517. | | | | [268] British Assoc. Report for 1864 (sections), p. 65. | | | | [269] Quart. Journ. Geol. Soc., xxvi., p. 90. | | | | [270] Geol. Mag., vii., p. 349. | | | | [271] Trans. Edin. Geol. Soc., vol. i., p. 136. | | | | [272] Geol. Mag. for June, 1870. See Chap. XXVII. | | | | [273] This was done by Mr. R. H. Tiddeman of the Geological Survey | | of England (Quart. Journ. Geol. Soc. for November, 1872), and the | | result established the correctness of the above opinion as to the | | existence of a North of England ice-sheet. Additional confirmation | | has been derived from the important observations of Mr. D. | | Mackintosh, and also of Mr. Goodchild, of the Geological Survey of | | England. | | | | [274] Trans. Geol. Soc., vol. v., p. 516 (first series). | | | | [275] Quart. Journ. Geol. Soc., vol. xi., p. 492. “Memoir of the | | Country around Cheltenham,” 1857. “Geology of the Country around | | Woodstock,” 1859. | | | | [276] Geol. Mag., vol. vii., p. 497. | | | | [277] Quart. Journ. Geol. Soc., vol. xxvi., p. 90. | | | | [278] My colleague, Mr. R. L. Jack. | | | | [279] The greater portion of this chapter is from the Trans. of | | Geol. Soc. of Edinburgh, for 1869. | | | | [280] Chapter XV., p. 253. | | | | [281] Trans. of the Geol. Soc. of Glasgow, vol. iii., part i., page | | 133. | | | | [282] Mr. Milne Home has advanced, in his “Estuary of the Firth of | | Forth,” p. 91, the theory that this trough had been scooped out | | during the glacial epoch by icebergs floating through the Midland | | valley from west to east when it was submerged. The bottom of the | | trough, be it observed, at the watershed at Kilsyth, is 300 feet | | above the level of its bottom at Grangemouth; and this Mr. Milne | | Home freely admits. But he has not explained how an iceberg, which | | could float across the shallow water at Kilsyth, say, 100 feet | | deep, could manage to grind the rocky bottom at Grangemouth, where | | it was not less than 400 feet deep. “The impetus acquired in the | | Kyle at Kilsyth,” says Mr. Milne Home, “would keep them moving on, | | and the prevailing westerly winds would also aid, so that when | | _grating_ on the subjacent carboniferous rocks they would not have | | much difficulty in scooping out a channel both wider and deeper | | than at Kilsyth.” But how could they “_grate_ on the subjacent | | carboniferous rocks” at Grangemouth, if they managed to _float_ | | at Kilsyth? Surely an iceberg that could “_grate_” at Grangemouth | | would “_ground_” at Kilsyth. | | | | [283] Trans. of the Geol. Soc. of Glasgow, vol. iii., p. 141. | | | | [284] Mr. John Young and Mr. Milne Home advanced the objection, | | that several trap dykes cross the valley of the Clyde near Bowling, | | and come to so near the present surface of the land, that the | | Clyde at present flows across them with a depth not exceeding | | 20 feet. I fear that Mr. Young and Mr. Milne Home have been | | misinformed in regard to the existence of these dykes. About a mile | | _above_ Bowling there are one or two dykes which approach to the | | river-bank, and may probably cross, but these could not possibly | | cut off a channel entering the Clyde at Bowling. In none of the | | borings or excavations which have been made by the Clyde Trustees | | has the rock been reached from Bowling downwards. I may also state | | that the whole Midland valley, from the Forth of Clyde to the Firth | | of Forth, has been surveyed by the officers of the Geological | | Survey, and only a single dyke has been found to cross the buried | | channels, viz., one (Basalt rock) running eastward from Kilsyth to | | the canal bridge near Dullatur. But as this is not far from the | | watershed between the two channels it cannot affect the question at | | issue. See sheet 31 of Geological Survey Map of Scotland. | | | | [285] Trans. Geol. Soc. Glasgow, vol. iv., p. 166. | | | | [286] “Great Ice Age,” chap. xiii. | | | | [287] See further particulars in Mr. Bennie’s paper on the Surface | | Geology of the district around Glasgow, Trans. Geol. Soc. of | | Glasgow, vol. iii. | | | | [288] See also Smith’s “Newer Pliocene Geology,” p. 139. | | | | [289] British Association Report for 1863, p. 89. _Geologist_ for | | 1863, p. 384. | | | | [290] See Geological Magazine, vol. ii., p. 38. | | | | [291] Proc. Geol. Soc., vol. iii., 1840, p. 342. | | | | [292] “Antiquity of Man” (Third Edition), p. 249. | | | | [293] “Glacial Drift of Scotland,” p. 65. Trans. Geol. Soc. Glas., | | vol. i., part 2. | | | | [294] “Memoir, Geological Survey of Scotland,” Sheet 23, p. 42. | | | | [295] Mr. Robert Dick had previously described, in the Trans. Geol. | | Soc. Edinburgh, vol. i., p. 345, portions of these buried channels. | | He seems, however, to have thought that they formed part of one and | | the same channel. | | | | [296] A description of this channel was read to the Natural History | | Society of Glasgow by Mr. James Coutts, the particulars of which | | will appear in the Transactions of the Society. | | | | [297] “Occasional Papers,” pp. 166, 223. | | | | [298] Memoir read before the Royal Society, January 7, 1869. | | | | [299] “Alpine Journal,” February, 1870. | | | | [300] Phil. Mag., January, 1872. | | | | [301] Phil. Mag., July, 1870; February, 1871. | | | | [302] Philosophical Magazine for January, 1870, p. 8; Proceedings | | of the Royal Society for January, 1869. | | | | [303] Philosophical Magazine for March, 1869. | | | | [304] Proceedings of Bristol Naturalists’ Society, p. 37 (1869). | | | | [305] Ibid., vol. iv., p. 37 (new series). | | | | [306] Phil. Mag., S. 4, vol. x., p. 303. | | | | [307] Proceedings of the Bristol Naturalists’ Society, vol. iv., p. | | 39 (new series). | | | | [308] See Philosophical Transactions, December, 1857. | | | | [309] There is one circumstance tending slightly to prevent the | | rupture of the glacier, when under tension, which I do not remember | | to have seen noticed; that is, the cooling effect which is produced | | in solids, such as ice, when subjected to tension. Tension would | | tend to lower the temperature of the ice-molecules, and this | | lowering of temperature would have the tendency of freezing them | | more firmly together. The cause of this cooling effect will be | | explained in the Appendix. | | | | [310] Phil. Mag., March, 1869; September, 1870. | | | | [311] “Forms of Water,” p. 127. | | | | [312] See text, p. 10. | | | | [313] Mathematical and Physical Series, vol. xxxvi. (1765). | | | | [314] “Memoirs of St. Petersburg Academy,” 1761. | | | | [315] The calculations here referred to were made by Lagrange | | nearly half a century previous to the appearance of this paper, and | | published in the “Mémoires de l’Académie de Berlin,” for 1782, p. | | 273. Lagrange’s results differ but slightly from those afterwards | | obtained by Leverrier, as will be seen from the following table; | | but as he had assigned erroneous values to the masses of the | | smaller planets, particularly that of Venus, the mass of which he | | estimated at one-half more than its true value, full confidence | | could not be placed in his results. | | | | Superior limits of eccentricity as determined by Lagrange, | | Leverrier, and Mr. Stockwell:— | | | | By Lagrange. By Leverrier. By Mr. Stockwell. | | | | Mercury 0·22208 0·225646 0·2317185 | | Venus 0·08271 0·086716 0·0706329 | | Earth 0·07641 0·077747 0·0693888 | | Mars 0·14726 0·142243 0·139655 | | Jupiter 0·06036 0·061548 0·0608274 | | Saturn 0·08408 0·084919 0·0843289 | | Uranus — 0·064666 0·0779652 | | Neptune — — 0·0145066 | | | | [J. C.] | | | | [316] “Mém. de l’Acad. royale des Sciences.” 1827. Tom. vii., p. | | 598. | | | | [317] Absolute zero is now considered to be only 493° Fah. below | | the freezing-point, and Herschel himself has lately determined | | 271° below the freezing-point to be the temperature of space. | | Consequently, a decrease, or an increase of one per cent. in the | | mean annual amount of radiation would not produce anything like the | | effect which is here supposed. But the mean annual amount of heat | | received cannot vary much more than one-tenth part of one per cent. | | In short, the effect of eccentricity on the mean annual supply of | | heat received from the sun, in so far as geological climate is | | concerned, may be practically disregarded.—[J. C.] | | | | [318] “Principles of Geology,” p. 110. “Mr. Lyell, however, in | | stating the actual excess of eight days in the duration of the | | sun’s presence in the northern hemisphere over that in the southern | | as productive of an excess of light and heat annually received by | | the one over the other hemisphere, appears to have misconceived the | | effect of elliptic motion in the passage here cited, since it is | | demonstrable that whatever be the ellipticity of the earth’s orbit | | the two hemispheres must receive equal absolute quantities of light | | and heat per annum, the proximity of the sun in perigee exactly | | compensating the effect of its swifter motion. This follows from a | | very simple theorem, which may be thus stated: ‘The amount of heat | | received by the earth from the sun while describing any part of | | its orbit is proportional to the angle described round the sun’s | | centre,’ so that if the orbit be divided into two portions by a | | line drawn _in any direction_ through the sun’s centre, the heats | | received in describing the two unequal segments of the ellipse so | | produced will be equal.” | | | | [319] When the eccentricity of the earth’s orbit is at its superior | | limit, the absolute quantity of heat received by the globe during | | one year will be increased by only 1/300th part; an amount which | | could produce no sensible influence on climate.—[J. C.] | | | | [320] Sir Charles has recently, to a certain extent, adopted the | | views advocated in the present volume, viz., that the cold of the | | glacial epoch was brought about not by a _decrease_, but by an | | _increase_ of eccentricity. (See vol. i. of “Principles,” tenth | | and eleventh editions.) The decrease in the mean annual quantity | | of heat received from the sun, resulting from the decrease in | | the eccentricity of the earth’s orbit—the astronomical cause to | | which he here refers—could have produced no sensible effect on | | climate.—[J. C.] | | | | [321] It is singular that both Arago and Humboldt should appear to | | have been unaware of the researches of Lagrange on this subject. | | | | [322] “Révolutions de la Mer,” p. 37. Second Edition. | | | | [323] See text, p. 37. | | | | [324] See _Philosophical Magazine_ for December, 1867, p. 457. | | | | [325] _Silliman’s American Journal_ for July, 1864. _Philosophical | | Magazine_ for September, 1864, pp. 193, 196. | | | | [326] _Philosophical Magazine_ for August, 1865, p. 95. | | | | [327] See text, p. 80. | | | | [328] See text, p. 222. | | | | [329] Proc. Roy. Soc., No. 157, 1875. | | | | [330] See text, p. 522. | | | | [331] Phil. Trans. for 1859, p. 91. | | | | [332] See text, p. 527. | | | +----------------------------------------------------------------------+

Transcriber’s Notes:

- Text enclosed by underscores is in italics (_italics_). - Blank pages have been removed. - Obvious typographical errors have been silently corrected. - Some spelling and hyphenation variations have been made consistent.