Part 3

Another, and the most remote planet in our system, is Neptune, revolving at the immense distance of 2850 millions of miles from the sun, and taking more than 164 years to perform the journey. This planet is only to be seen by the most powerful glasses, and was discovered under very peculiar circumstances, not (like other planets) by chance; its existence was recognised as necessary to account for certain "perturbations" or deviations in the orbit of Uranus, which was found to take a course differing from what it should, according to computation, and which were only to be accounted for by supposing that another planet existed far out in space, which affected the course of Uranus by its attraction. The idea of finding out where this orb should be, occurred to M. Le Verrier and Mr. Adams, independent of each other; they both arrived at nearly similar conclusions, for the positions assigned to the supposed planet so nearly agreed with each other and with its real position, that their calculations have been looked on as the greatest feat of astronomical research. It was discovered by Dr. Galle of Berlin, upon his hearing from Le Verrier the position in which at that time it should be sought for.

THE PLANETOIDS.

Upon observing the relative distances of the planets from each other in passing outwards from Mercury, it will be found that each one is placed about double the distance of the one next before it (not exactly, but sufficiently near to form a coincidence almost amounting to a law), they are as follows:--

Mercury 36,700,000 Venus 68,770,000 The Earth 95,000,000 Mars 144,780,000 Jupiter 494,000,000 Saturn 906,000,000 Uranus 1822,000,000 Neptune 2850,000,000

It will be seen, however, that in this table of distances there is one great exception to the rule, namely the distance between Mars and Jupiter, which is nearly double what it should be according to this rule, giving rise to the idea that a planet ought to be placed between them. Professor Bode was so convinced that something of the kind was necessary to complete the harmony of the series, that he caused to be instituted a search for the supposed planet, and according to his conjecture one was discovered as nearly as possible in the situation indicated, but upon more accurate information being obtained, this planet was found to be so small (scarcely 150 miles diameter) that it could hardly be considered as a planet ranging with the others, and having so large a space of the heavens to itself; shortly afterwards another small planet was discovered, having nearly the same orbit. From time to time others have been discovered, to the number of forty-two, up to the present date. Dr. Olbers upon discovering the second of these small bodies, where one great one was anticipated, put forth the curious hypothesis, that one large planet had really existed there at some former time, that it had been shattered into pieces by some accident, such as an internal explosion, and that more of these pieces would be found; it is curious how accurately this prediction has been verified.

If we suppose the original planet to have been liquid when shattered to pieces, it would fully account for the fragments being spherical, just as globules of mercury assume this form from their own attraction of gravitation, and that the planets, our own earth amongst them, are liquid, is very nearly proved by their forms, which are exactly such as fluid masses rotating on an axis would assume, moreover the specific gravity of some of the planets is but little more than that of water, and with respect to our earth, the evidence of internal heat, increasing as we descend below the surface, &c., show almost beyond doubt that at the present time it is in a liquid state (molten) covered over by a few miles' thickness of hardened crust, which bears such a small proportion to the bulk of the globe, that the whole may be considered as liquid.

THE STELLAR SYSTEM.

The thousands of stars which spangle the heavens are all part of one "system;" but it has been found by the aid of the telescope that this system is but one out of many. Our system of stars occupies a space somewhat in the form of a thick lens or much-flattened sphere, but others are of very different forms, and some have but little regularity of form at all; our sun is one of the stars of this system. It is not known whether there are planets revolving round the other stars, the distance being far too great for any telescope to render them visible. Our sun occupies a somewhat central position in the system. The stars are classed into sizes, as first magnitude, second magnitude, &c., on to the thirteenth or fourteenth magnitude, but all beyond the fifth magnitude (by far the greater number) are only visible by the aid of the telescope. There are about 5000 stars visible to the naked eye. But fourteen stars of the first magnitude are in our hemisphere of the heavens, and about fifty of the second, but the number of stars of each magnitude increases prodigiously in the higher numbers, so that those stars capable only of being seen by the aid of powerful telescopes, amount to many millions; these are chiefly situated in a great belt which encircles the heavens, called the "Milky Way," which is caused by the line of vision passing through the breadth of our starry system, and consequently meeting with a greater number of stars than in other directions, where it only crosses its thickness.

The other systems of stars, called "nebulæ," from their resemblance to little clouds, were supposed to consist of luminous matter of but little density, and which might at some future period be condensed into stars; but the improved power and construction of telescopes have enabled astronomers to resolve many of these nebulæ into clusters of stars, and there is but little doubt that all could be thus resolved, were the telescope of sufficient power; and thus it appears that in the infinity of space collections of systems are placed, each one too distant from the others to be calculated or written in numbers, but each consisting of thousands of suns many hundred times greater than this earth, and many millions of miles from each other. What an idea of space does this afford, and how soon do all our narrow notions of possibility and impossibility vanish before such facts accomplished by the hands of God!

Amongst the stars composing our system there are certain conspicuous groups or constellations, which were named by the earliest astronomers, and compose a list of the most ridiculous imaginary figures, as useless to the casual observer of the stars as to the astronomer, and have not the most distant resemblance to the figures after which they are named, as for instance the "Great Bear" (Ursa Major) fig. 24. The grouping of stars into constellations serves, however, to find any one required (provided the groups on the chart can be identified with those in the heavens), as for instance the three conspicuous stars forming "Orion's belt" (fig. 25), from which a line produced eastward will point to Sirius, the brightest star in the heavens, and another line produced westward will serve to point out "Aldebaran," also a star of the first magnitude, &c.; also a line drawn from _a_ to _b_ in the Great Bear will nearly point to the pole-star--the star situated nearly (although not exactly) at the pole, or that part of the heavens which would be indicated by a line drawn through the earth at its axis of rotation. Amongst the stars scattered over the vault of heaven there are many which to the naked eye appear single, but which when seen through the telescope prove to be two stars closely approaching each other. These "double stars" are of two kinds, "optical" and "physical;" optical double stars are those which appear to be near each other merely from the accident of one being placed behind the other, nearly in a straight line, although in reality at an immense distance and in no way connected. Physical double stars (usually called "binary systems") consist of two suns comparatively near each other and revolving about their common centre of gravity. Sir William Herschell first discovered their physical connection, and thereby proved that the great law of gravitation was not confined to our system, but was the ruling power which controlled and regulated other systems. Astronomers have calculated the orbits of only fourteen such "binary systems" at present with any degree of certainty; about a thousand double stars are known, but only about one hundred "binary systems." In some "binary systems" the two suns are of different colours; white and purple, red and green, or yellow and blue. If any planets revolve about such suns, when situated between the two, what strange phenomena must occur! Imagine a day during which a red sun had tinted everything crimson, being succeeded at sunset (not, as with us, by darkness) by the rising of a green sun, changing the colour of every object from red to green, and how curious and beautiful must be the shades of light and colour during the transition from one day to the other. Can anything be conceived more gorgeous?--no shadow, but every object tinted with a combination of colours of the most brilliant hues! Triple, quadruple, and multiple stars are known in abundance, but all probably optically so, and not in any way connected with each other by attraction. Others of the stars are called "variable," they are those whose light gradually diminishes and after a time regains its former brightness; some of them not only vary in brilliancy but in colour; the cause of these phenomena is totally unknown, and fortunate will he be who discovers it; it will be a great stride in the science of astronomy, for at present everything proposed has been quite inadequate to account for such phenomena. About 60 of these variable stars are at present known, but others are constantly being added to the list. Hind has discovered twenty-one, Pogson seven, and other astronomers have taken their share in discerning them.

But, of all the heavenly bodies, none have excited so much wonder, and in former times so much fear, as the Comets. The orbits of these are for the most part in very eccentric ellipses, some of them comprised within the limits of our solar system, while those of others extend millions of miles beyond it, so that they only reappear to us after hundreds of years. The matter of which these bodies is composed is certainly not solid, as the smallest telescopic stars may be seen through the very middle of it, although many thousand miles in thickness. When comets approach the sun, their substance appears to become more condensed, and therefore to possess a greater power of reflecting light. The cause of the "tails" which appear appended to comets when they approach the sun is not known, but this tail extends always in a direction from the sun, and passes over many degrees of the heavens' space. As comets obey strictly the law of gravitation, they must possess some amount of weight or mass, however small it may be, and that it is inconceivably small is proved by the fact of comets having passed close to Jupiter, right amongst his four moons, without in any way affecting or disturbing their well-known motions; the comets have, however, been terribly tossed about on such occasions, one having been totally lost to us by such an occurrence.

Thus it is seen how the same laws of gravity exist throughout all the infinity of space. The comets traverse our system of suns, and passing onwards with incredible rapidity, for years upon years, through that great space beyond it, at last visit some other system--perhaps to be as much an object of regard and wonderment there as they were here. The same ray of light which emanated from one of the distant stars of some one of the clusters situated far out in space, and which has travelled for thousands of years at the rate of 200,000 miles for every second of time, serves us here on this globe to distinguish it--it enters the tube of our telescope and affects our optic nerve; and the Creator who caused it _there_ to be given off, has _here_ made our senses capable of perceiving it. Is not this a clear demonstration of the fact, that one hand has designed the whole; and one Creator provided for all?

THE AIR.

The earth is surrounded in all directions by a covering of air about forty miles in thickness, which bears about the proportion to the earth itself that a coating one inch thick would to a ball seventeen feet through, being about 1/200th of its diameter. This air, though invisible, may at all times be felt while in motion (such is the wind), and by rapidly passing the hand backwards and forwards it may be perceived to press against and slightly obstruct it; this air is composed of about twenty parts oxygen and eighty parts nitrogen, together with about one part in every thousand of carbonic acid (consisting of carbon and oxygen in union) and a small proportion of vapour of carbonate of ammonia (ammonia in union with carbonic acid). It is the oxygen which enables animals to live in the air, as it is taken into their systems at every breath, it is thus constantly being consumed and would ultimately be so far diminished, that they could live no longer for want of it; but Providence has so arranged, that the necessary quantity is always being supplied by vegetables (which give out oxygen) to meet the deficiency. The air, although a gas, is yet capable of being weighed, of great compressibility, and of expansion to an unlimited extent, this causes the lower part (near the surface of the earth) to be much more dense than the upper regions, and those who have ascended to the tops of high mountains, have described the difficulty of respiration as being very great, owing to the rarefaction or lightness of the air. At the surface of the earth it presses with a weight of about fifteen pounds upon every square inch, but as this pressure is equal in every direction, it is not felt by us, nor does it crush the most fragile flower or insect; but, remove the pressure from one side of anything, and it will be found to press with violence upon the other. If the air were removed from the inside of a drum, the weight of the surrounding air would burst in the parchment and fill it. By way of experiment, fill a large basin with water, take a tumbler in the left hand and a piece of lighted paper in the right, hold the lighted paper for a moment under the inverted tumbler and immediately apply its open mouth to the surface of the water, letting it dip in about half an inch, the heat expands and consequently gets rid of some of the air in the tumbler, and as this cools again it resumes its original bulk; the pressure of the air on the surface of the water in the basin will force it up into the tumbler and nearly fill it (fig. 1). This is the principle of the barometer, which is a tube entirely exhausted of air and the weight of the atmosphere forces a column of mercury (which is many times heavier than water) up to the height of about twenty-nine inches, according to the weight of the air at any particular time; this weight varies according to the state of the weather, whether wet or dry; but the average weight of air is found to be perfectly stationary, and during the twenty years from 1816 to 1836, it was found at Paris not to have varied 1/1200th of an inch. Winds or currents of air proceed from several causes; when the sun shines on a large surface of the earth, it becomes heated and a column of hot air is pressed upwards, for hot air is lighter than cold, and the cold air all around, by its weight, forces it up and rushes in to fill the space; this space may perhaps be many hundred square miles in extent, so that a current of wind is caused to blow towards this spot from all the regions round. This is the general state of things at the equatorial parts of the earth and causes the "trade winds," which uniformly blow (the greater part of the year) from both north and south towards the equator. Another cause of local winds, is the condensation of vapours into water; when the air over a large region is saturated with moisture it is greatly expanded by it, and when the vapour is condensed and falls as rain, the air from the surrounding parts forces itself in to fill the space occupied by the rain while in a state of vapour, which is nearly one hundred thousand times greater than it occupies when in the form of water. There are many causes to determine the condensation of the vapour, which the atmosphere always holds in larger or smaller quantities, but cold is the chief agent, for when a current of air passes over seas or rivers, or the damp surface of the earth, it becomes loaded with moisture, and being capable of holding only a certain quantity (less when cold than hot), it follows that warm air when saturated with moisture must let some of it fall when it becomes cooled. This may occur from entering a colder region, or uniting with another current of air colder than itself. In tropical regions the air is so warm that it takes up a very large quantity of watery vapour and upon the coming of colder weather, the rainy season begins and the air empties itself of its superfluous moisture, causing the most tremendous falls of rain in consequence (fig. 2). When the clouds are overcharged with moisture in cold climates or in cold weather, the vapour freezes as it condenses and forms snow, which under the microscope presents a series of the most beautiful star-like crystals (fig. 3); the same result is produced by the freezing of the vapour or fog near to the earth's surface, this is called "hoar-frost," which is (like dew) deposited on those parts of the surface most cooled by radiation. Hail is caused in all probability by the drops of rain passing through a cold stratum of air and becoming frozen as they fall. There are many wonderful accounts on record of great masses of ice falling from the clouds, some of them several feet thick; the cause of these phenomena (if they ever did occur) is not known. The curious occurrence of red or green snow, which has sometimes been known to fall, is owing to the snow being mixed with myriads of minute cellular plants called the _Protococcus pluvialis_, which in one state of its existence is green and in another red, and they colour the snow accordingly. The cause of their sudden accumulation in such vast quantities is not known, but may be sought for in some peculiar state of the atmosphere favourable to their growth, which in many places is prodigiously rapid.

The temperature of the air differs very greatly in different situations at the surface of the earth, the extreme difference being about 180 degrees; thus in winter, at the poles, the thermometer frequently stands at 60 degrees below zero, and at the equator, in summer, 120 degrees above, but, in the higher regions of the air, it is even colder than at the poles, for the sun shining on the earth heats it, and the air is heated by contact with it; this is its only source of heat, of which it receives less and less the further it is from the earth, so that at a distance (depending upon the situation and climate) ranging from 3,500 to 17,000 feet from the surface, the air is so cold that it is called "the region of perpetual snow," and all mountains whose tops reach above this altitude are covered with snow (formed by the freezing of the rain and vapour) from this point to their summits (fig. 4), and the sun (although it nearly always shines there, these regions being above most of the clouds) is not able to melt it, for the radiation of heat from its surface is greater than its absorption from the sun's rays, white substances absorbing but very slowly, and rough surfaces (as the snow) radiating very rapidly.

Dew is caused by condensation of the watery vapour held in the air; when the sun goes down the radiation continues from the surface of the earth, and those surfaces which radiate most rapidly, such as fields of grass, get cooled down below the temperature of the surrounding air, and therefore cause a condensation of its watery vapour. This may be imitated by filling a large glass goblet with very cold water, and bringing it into a very warm room, the outside of the glass vessel will become covered with dew, although previously quite dry; the same may be shown by filling a similar glass with water at the ordinary temperature, taking care that the outside is dry, then stirring in an ounce or so of nitre or Rochelle salt, which will cause the water to become cold, and the outside of the glass to be covered, as before, with dew. Dew falls fastest when there are no clouds in the sky, as radiation then takes place from the earth into space, and is not compensated for by a return of radiation from the clouds to the earth.

Clouds are formed by the partial condensation of vapour, and are borne along by the wind, instead of falling as rain; the reason of their not falling is this, when the air below the clouds is saturated with moisture it will absorb no more and the rain falls, but when it is warm and dry, and passed in a constantly renewed current, then the vapour is absorbed before it reaches the earth, and is carried off by the wind. Clouds, therefore, although they often appear stationary, are constantly altering their form and size, portions being absorbed while others are being formed. Fogs are the same as clouds, the vapour arising from wet grass, rivers, &c., being condensed as it ascends, by a current of cold air passing over them. The fogs in London have a brown colour, from admixture of smoke, dust, &c., with which the air is contaminated (fig. 5). Clouds are of various forms and sizes, and indeed of almost every variety, but certain kinds which are frequently seen, have received different names. The "cirrhus" comprises all the feathery white clouds which float high up in the air in fine weather; the "cumulus" consists of the large mountain-like clouds which are generally seen in summer; the "stratus," those horizontal layers of clouds low down in the horizon, so often seen at sunset; and the "nimbus" is the rain-cloud, of a dark grey or leaden hue, with sharp well-marked edges (fig. 6).