The Plurality of Worlds

28. It should be recollected that there must be some system which is the most complete of all systems, some planet which is the largest of all planets. And if that largest planet, in the most complete system, be, after being for ages tenanted by irrational creatures, at last, and alone of all, occupied by a rational race, that race must necessarily have the power of asking such questions as these: Why they should be alone rational? Why their planet should be alone thus favored? If the case be ours, we may hope to be then able to answer these questions, when we can explain the most certain fact which they involve; Why the Earth was occupied so long by irrational creatures, before the rational race was placed upon it? The mere power of asking such questions can prove or disprove nothing; for it is a power which must equally subsist, whether the human inhabitants of the earth be or be not the only rational population which the universe contains. If there be a race thus favored by the Creator, they must, at that stage of their knowledge in which man now is, be able to doubt, as man does, of the extent and greatness of the privilege which they enjoy.

29. The argument that the Fixed Stars are like the Sun, and therefore the centres of inhabited systems as the Sun is, is sometimes called an argument from Analogy; and this word Analogy is urged, as giving great force to the reasoning. But it must be recollected, that precisely the point in question is, whether there is an analogy. The stars, it is said, are like the Sun. In what respects? We know of none, except in being self-luminous; and this they have in common with the nebulæ, which, as we have seen, are not centres of inhabited systems. Nor does this quality of being self-luminous at all determine the degree of condensation of a star. Sirius may be less than a hundredth or a thousandth of the density of the Sun. But the Stars, it may be further urged, are like the Sun in turning on their axes. To this we reply, that we know this only of those stars in which, the very phenomenon which proves their revolution, proves also that they are unlike the Sun, in having one side darker than the other. Add to which, their revolution is not connected with the existence of planets, still less of inhabitants of planets, in any intelligible manner. The resemblance, therefore, so far as it bears upon the question, is confined to one single point, in the highest degree ambiguous and inconclusive; and any argument drawn from this one point of resemblance, has little claim to be termed an argument from analogy.[41 - Humboldt, very justly, regards the force of analogy as tending in the opposite direction. "After all," he asks, (Cosmos, iii. 373,) "is the assumption of satellites to the Fixed Stars so absolutely necessary? If we were to begin from the outer planets, Jupiter, &c., analogy might seem to require that all planets have satellites. But yet this is not true for Mars, Venus, Mercury." To which we may further add the twenty-three Planetoids. In this case there is a much greater number of bodies which have not satellites, than which have them.]

30. On a subject on which we know so little, it is difficult to present any view which deserves to be regarded as an analogy. We see, among the stars, nebulæ more or less condensed, which are possibly, in some cases, stages of a connected progress towards a definite star; and it may be, to a star with planets in permanent orbits. We see, in our planet, evidence of successive stages of a connected series of brute animals, preceded perhaps by various stages of lifeless chaos. If the histories of the Sun, and of all the stars, are governed by a common analogy, the nebulous condensation, and the stages of animal life, may be parts of the same continued series of events; and different stars may be at different points of that series. But even on this supposition, but a few of the stars may be the seats of conscious life, and none, of intelligence. For among the stars which have condensed to a permanent form, how many have failed in throwing off a permanent planet! How many may be in some stage of lifeless chaos! We must needs suppose a vast number of stages between a nebular chaos and the lowest forms of conscious life. Perhaps as many as there are fixed stars; and far more than there are of stars which become fertile of life: so that no two systems may be at the same stage of the planetary progress. And if this be so,—our system being so complicated, that we must suppose it peculiarly developed, having the largest Sun that we know of, and our Earth being (as we shall hereafter attempt to prove) the largest solid planet that we know of,—this Earth may be the sole seat of the highest stage of planetary development.

31. The assumption that there is anything of the nature of a regular law or order of progress from nebular matter to conscious life,—a law which extends to all the stars, or to many of them,—is in the highest degree precarious and unsupported; but since it is sometimes employed in such speculations as we are pursuing, we may make a remark or two connected with it. If we suppose, on the planets of other systems, a progress in some degree analogous to that which geology shows to have occurred on the Earth, there may be, in those planets, creatures in some way analogous to our vegetables and animals; but analogy also requires that they should differ far more from the terrestrial vegetables and animals of any epoch, than those of one epoch do from those of another; since they belong to a different stellar system, and probably exist under very different conditions from any that ever prevailed on the Earth. We are forbidden, therefore, by analogy, to suppose that on any other planet there was such an anatomical progression towards the form of man, as we can discern (according to some eminent physiologists) among the tribes which have occupied the Earth. Are we to conceive that the creatures on the planets of other systems are, like the most perfect terrestrial animals, symmetrical as to right and left, vertebrate, with fore limbs and hind limbs, heads, organs of sense in their heads, and the like? Every one can see how rash and fanciful it would be to make such suppositions. Those who have, in the play of their invention, imagined inhabitants of other planets, have tried to avoid this servile imitation of terrestrial forms. Here is Sir Humphry Davy's account of the inhabitants of Saturn. "I saw moving on the surface below me, immense masses, the forms of which I find it impossible to describe. They had systems for locomotion similar to that of the morse or sea-horse, but I saw with great surprise that they moved from place to place by six extremely thin membranes, which they used as wings. I saw numerous convolutions of tubes, more analogous to the trunk of the elephant, than to anything else I can imagine, occupying what I supposed to be the upper parts of the body."[42 - Consolations in Travel. Dial. 1.] The attendant Genius informs the narrator, that though these creatures look like zoophytes, they have a sphere of sensibility and intellectual enjoyment far superior to that of the inhabitants of the Earth. If we were to reason upon a work of fancy like this, we might say, that it was just as easy to ascribe superior sensibility and intelligence to zoophyte-formed creatures upon the Earth, as in Saturn. Even fancy cannot aid us in giving consistent form to the inhabitants of other planets.

32. But even if we could assent to the opinion, as probable, that there may occur, on some other planet, progressions of organized forms analogous in some way to that series of animal forms which has appeared upon the earth, we should still have no ground to assume that this series must terminate in a rational and intelligent creature like man. For the introduction of reason and intelligence upon the Earth is no part nor consequence of the series of animal forms. It is a fact of an entirely new kind. The transition from brute to man does not come within the analogy of the transition from brute to brute. The thread of analogy, even if it could lead us so far, would break here. We may conceive analogues to other animals, but we could have no analogue to man, except man. Man is not merely a higher kind of animal; he is a creature of a superior order, participating in the attributes of a higher nature; as we have already said, and as we hope hereafter further to show. Even, therefore, if we were to assume the general analogy of the Stars and of the Sun, and were to join to that the information which geology gives us of the history of our own planet; though we might, on this precarious path, be led to think of other planets as peopled with unimagined monsters; we should still find a chasm in our reasoning, if we tried, in this way, to find intelligent and rational creatures in planets which may revolve round Sirius or Arcturus.

33. The reasonable view of the matter appears to be this. The assumption that the Fixed Stars are of exactly the same nature as the Sun, was, at the first, when their vast distance and probable great size were newly ascertained, a bold guess; to be confirmed or refuted by subsequent observations and discoveries. Any appearances, tending in any degree to confirm this guess, would have deserved the most considerate attention. But there has not been a vestige of any such confirmatory fact. No planet, nor anything which can fairly be regarded as indicating the existence of a planet, revolving about a star, has anywhere been discerned. The discovery of nebulæ, of binary systems, of clusters of stars, of periodical stars, of varying and accelerated periods of such stars, all seem to point the other way. And if all these facts be held to be but small in amount, as to the information which they convey, about the larger, and perhaps nearer stars; still they leave the original assumption a mere guess, unsupported by all that three centuries of most diligent, and in other respects successful research, has been able to bring to light. That Copernicus, that Galileo, that Kepler, should believe the stars to be Suns, in every sense of the term, was a natural result of the expansion of thought which their great discoveries produced, in them and in their contemporaries. Nor are we yet called upon to withdraw from them our sympathy; or entitled to contradict their conjecture. But all the knowledge that the succeeding times have given us; the extreme tenuity of much of the luminous matter in the skies; the existence of gyratory motion among the stars, quite different from planetary systems; the absence of any observed motions at all resembling such systems; the appearance of changes in stars, quite inconsistent with such permanent systems; the disclosure of the history of our own planet, as one in which changes have constantly been going on; the certainty that by far the greater part of the duration of its existence, it has been tenanted by creatures entirely different from those which give an interest, and thence, a persuasiveness, to the belief of inhabitants in worlds appended to each star; the impossibility, which appears, on the gravest consideration, of transferring to other worlds such interests as belong to our own race in this world; all these considerations should, it would seem, have prevented that old and arbitrary conjecture from growing up, among a generation professing philosophical caution, and scientific discipline, into a settled belief.

34. Some of the moral and theological views which tend to encourage and uphold this belief, may be taken under our more special consideration hereafter: but here, where we are reasoning principally upon astronomical grounds, we may conclude what we have to remark about the Fixed Stars, as the centres of inhabited systems of worlds, by saying; that it will be time enough to speculate about the inhabitants of the planets which belong to such systems, when we have ascertained that there are such planets, or one such planet. When that is done, we can then apply to them any reasons which may exist, for believing that all, or many planets, are the seats of habitation of living things. What reasons of this kind can be adduced, and what is their force with regard to our own solar system, we must now proceed to discuss.[43 - What is said in Art. 15, that in consequence of the time employed in the transmission of visual impressions, our seeing a star is evidence, not that it exists now, but that it existed, it may be, many thousands of years ago; may seem, to some readers, to throw doubts upon reasonings which we have employed. It may be said that a star which was a mere chaos, when the light, by which we see it, set out from it, may, in the thousands of years which have since elapsed, have grown into an orderly world. To which bare possibility, we may oppose another supposition at least equally possible:—that the distant stars were sparks or fragments struck off in the formation of the Solar System, which are really long since extinct; and survive in appearance, only by the light which they at first emitted.]

CHAPTER IX

THE PLANETS

1. When it was discovered, by Copernicus and Galileo, that Mercury, Venus, Mars, Jupiter, Saturn, which had hitherto been regarded only as "wandering fires, that move in mystic dance," were really, in many circumstances, bodies resembling the Earth;—that they and the Earth alike, were opaque globes, revolving about the Sun in orbits nearly circular, revolving also about their own axes, and some of them accompanied by their Satellites, as the Earth is by the Moon;—it was inevitable that the conjecture should arise, that they too had inhabitants, as the Earth has. Each of these bodies were seemingly coherent and solid; furnished with an arrangement for producing day and night, summer and winter; and might therefore, it was naturally conceived, have inhabitants moving upon its solid surface, and reckoning their lives and their employment by days, and months, and years. This was an unavoidable guess. It was far less bold and sweeping than the guess that there are inhabitants in the region of the Fixed Stars, but still, like that, it was, for the time at least, only a guess; and like that, it must depend upon future explorations of these bodies and their conditions, whether the guess was confirmed or discredited. The conjecture could not, by any moderately cautious man, be regarded as so overwhelmingly probable, that it had no need of further proof. Its final acceptance or rejection must depend on the subsequent progress of astronomy, and of science in general.

2. We have to consider then how far subsequent discoveries have given additional value to this conjecture. And, as, in the first place, important among such discoveries, we must note the addition of several new planets to our system. It was found, by the elder Herschel, (in 1781,) that, far beyond Saturn, there was another planet, which, for a time, was called by the name of its sagacious discoverer; but more recently, in order to conform the nomenclature of the planets to the mythology with which they had been so long connected, has been termed Uranus. This was a vast extension of the limits of the solar system. The Earth is, as we have already said, nearly a hundred millions of miles from the Sun. Jupiter is at more than five times, and Saturn nearly at ten times this distance: but Uranus, it was found, describes an orbit of which the radius is about nineteen times as great as that of the Earth. But this did not terminate the extension of the solar system which the progress of astronomy revealed. In 1846, a new planet, still more remote, was discovered: its existence having been divined, before it was seen, by two mathematicians, Mr. Adams, of Cambridge, and M. Leverrier, of Paris, from the effects of its force upon Uranus. This new planet was termed Neptune: its distance from the Sun is about thirty times the Earth's distance. Besides these discoveries of large planets, a great number of small planets were detected in the region of the solar system which lies between the orbits of Mars and Jupiter. This series of discoveries began on the first day of 1801, when Ceres was detected by Piazzi at Palermo; and has gone on up to the present time, when twenty-three of these small bodies have been brought to light; and probably the group is not yet exhausted.

3. Now if we have to discuss the probability that all these bodies are inhabited, we may begin with the outermost of them at present known, namely Neptune. How far is it likely that this globe is occupied by living creatures which enjoy, like the creatures on the Earth, the light and heat of the Sun, about which the planet revolves? It is plain, in the first place, that this light and heat must be very feeble. Since Neptune is thirty times as far from the sun as the earth is, the diameter of the sun as seen from Neptune will only be one-thirtieth as large as it is, seen from the earth. It will, in fact, be reduced to a mere star. It will be about the diameter under which Jupiter appears when he is nearest to us. Of course its brightness will be much greater than that of Jupiter; nearly as much indeed, as the sun is brighter than the moon, both being nearly of the same size: but still, with our full-moonlight reduced to the amount of illumination which we receive from a full Jupiter, and our sun-light reduced in nearly the same proportion, we should have but a dark, and also a cold world. In fact, the light and the heat which reach Neptune, so far as they depend on the distance of the sun, will each be about nine hundred times smaller than they are on the earth. Now are we to conceive animals, with their vital powers unfolded, and their vital enjoyments cherished, by this amount of light and heat? Of course, we cannot say, with certainty, that any feebleness of light and heat are inconsistent with the existence of animal life: and if we had good reason to believe that Neptune is inhabited by animals, we might try to conceive in what manner their vital scheme is accommodated to this scanty supply of heat and light. If it were certain that they were there, we might inquire how they could live there, and what manner of creatures they could be. If there were any general grounds for assuming inhabitants, we might consider what modifications of life their particular conditions would require.

4. But is there any such general ground!? Such a ground we should have, if we could venture to assume that all the bodies of the Solar System are inhabited;—if we could proceed upon such a principle, we might reject or postpone the difficulties of particular cases.

5. But is such an assumption true? Is such a principle well founded? The best chance which we have of learning whether it is so, is to endeavor to ascertain the fact, in the body which is nearest to us; and thus, the best placed for our closer scrutiny. This is, of course, the Moon; and with regard to the Moon, we have, again, this advantage in beginning the inquiry with her:—that she, at least, is in circumstances, as to light and heat, so far as the Sun's distance affects them, which we know to be quite consistent with animal and vegetable life. For her distance from the Sun is not appreciably different from that of the Earth; her revolutions round the earth do not make nearly so great a difference, in her distance from the sun, as does the earth's different distances from the sun in summer and in winter: the fact also being, that the earth is considerably nearer to the sun in the winter of this our northern hemisphere, than in the summer. The moon's distance from the sun then, adapts her for habitation: is she inhabited?

6. The answer to this question, so far as we can answer it, may involve something more than those mere astronomical conditions, her distance from the sun, and the nature of her motions. But still, if we are compelled to answer it in the negative;—if it appear, by strong evidence, that the Moon is not inhabited; then is there an end of the general principle, that, all the bodies of the solar system are inhabited, and that we must begin our speculations about each, with this assumption. If the Moon be not inhabited, then, it would seem, the belief that each special body in the system is inhabited, must depend upon reasons specially belonging to that body; and cannot be taken for granted without such reasons. Of the two bodies of the solar system which alone we can examine closely, so as to know anything about them, the Earth and the Moon, if the one be inhabited, and the other blank of inhabitants, we have no right to assume at once, that any other body in the solar system belongs to the former of these classes rather than to the latter. If, even under terrestrial conditions of light and heat, we have a total absence of the phenomenon of life, known to us only as a terrestrial phenomenon; we are surely not entitled to assume that when these conditions fail, we have still the phenomenon, life. We are not entitled to assume it; however it may be capable of being afterwards proved, in any special case, by special reasons; a question afterwards to be discussed.

7. Is, then, the Moon inhabited? From the moon's proximity to us, (she is distant only thirty diameters of the earth, less than ten times the earth's circumference; a railroad carriage, at its ordinary rate of travelling, would reach her in a month,) she can be examined by the astronomer with peculiar advantages. The present powers of the telescope enable him to examine her mountains as distinctly as he could the Alps at a few hundred miles distance, with the naked eye; with the additional advantage that her mountains are much more brilliantly illuminated by the Sun, and much more favorably placed for examination, than the Alps are. He can map and model the inequalities of her surface, as faithfully and exactly as he can those of the surface of Switzerland. He can trace the streams that seem to have flowed from eruptive orifices over her plains, as he can the streams of lava from the craters of Etna or Hecla.

8. Now, this minute examination of the Moon's surface being possible, and having been made, by many careful and skilful astronomers, what is the conviction which has been conveyed to their minds, with regard to the fact of her being the seat of vegetable or animal life? Without exception, it would seem, they have all been led to the belief, that the Moon is not inhabited; that she is, so far as life and organization are concerned, waste and barren, like the streams of lava or of volcanic ashes on the earth, before any vestige of vegetation has been impressed upon them: or like the sands of Africa, where no blade of grass finds root. It is held, by such observers, that they can discern and examine portions of the moon's surface as small as a square mile;[44 - More recently, at the meeting of the British Association in September, 1853, Professor Phillips has declared, that astronomers can discern the shape of a spot on the Moon's surface, which is a few hundred feet in breadth.] yet, in their examination, they have never perceived any alteration, such as the cycle of vegetable changes through the revolutions of seasons would produce. Sir William Herschel did not doubt that if a change had taken place on the visible part of the Moon, as great as the growth or the destruction of a great city, as great, for instance, as the destruction of London by the great fire of 1666, it would have been perceptible to his powers of observation. Yet nothing of the kind has ever been observed. If there were lunar astronomers, as well provided as terrestrial ones are, with artificial helps of vision, they would undoubtedly be able to perceive the differences which the progress of generations brings about on the surface of our globe; the clearing of the forests of Germany or North America; the embankment of Holland; the change of the modes of culture which alter the color of the ground in Europe; the establishment of great nests of manufactures which shroud portions of the land in smoke, as those which have their centres at Birmingham or at Manchester. However obscurely they might discern the nature of those changes, they would still see that change was going on. And so should we, if the like changes were going on upon the face of the Moon. Yet no such changes have ever been noticed. Nor even have such changes been remarked, as might occur in a mere brute mass without life;—the formation of new streams of lava, new craters, new crevices, new elevations. The Moon exhibits strong evidences, which strike all telescopic observers, of an action resembling, in many respects, volcanic action, by which its present surface has been formed.[45 - A person visiting the Eifel, a region of extinct volcanoes, west of the Rhine, can hardly fail to be struck with the resemblance of the craters there, to those seen in the moon through a telescope.] But, if it have been produced by such internal fires, the fires seem to be extinguished; the volcanoes to be burned out. It is a mere cinder; a collection of sheets of rigid slag, and inactive craters. And if the Moon and the Earth were both, at first in a condition in which igneous eruptions from their interior produced the ridges and cones which roughen their surfaces; the Earth has had this state succeeded by a series of states of life in innumerable forms, till at last it has become the dwelling-place of man; while the Moon, smaller in dimensions, has at an earlier period completely cooled down, as to its exterior at least, without ever being judged fit or worthy by its Creator of being the seat of life; and remains, hung in the sky, as an object on which man may gaze, and perhaps, from which he may learn something of the constitution of the universe; and among other lessons this; that he must not take for granted, that all the other globes of the solar system are tenanted, like that on which he has his appointed place.

9. It is true, that in coming to this conclusion, the astronomers of whom I speak, have been governed by other reasons, besides those which I have mentioned, the absence of any changes, either rapid or slow, discoverable in the Moon's face. They have seen reason to believe that water and air, elements so essential to terrestrial life, do not exist in the Moon. The dark spaces on her disk, which were called seas by those who first depicted them, have an appearance inconsistent with their being oceans of water. They are not level and smooth, as water would be; nor uniform in their color, but marked with permanent streaks and shades, implying a rigid form. And the absence of an atmosphere of transparent vapor and air, surrounding the moon, as our atmosphere surrounds the earth, is still more clearly proved, by the absence of all the optical effects of such an atmosphere, when stars pass behind the moon's disk, and by the phenomena which are seen in solar eclipses, when her solid mass is masked by the Sun.[46 - Bessel has discussed and refuted (it was hardly necessary) the conjecture of some persons (he describes them as "the feeling hearts who would find sympathy even in the Moon") that there may be in the Moon's valleys air enough to support life, though it does not rise above the hills.—Populäre Vorlesungen, p. 78.] This absence of moisture and air in the Moon, of course, entirely confirms our previous conclusion, of the absence of vegetable and animal life; and leaves us, as we have said, to examine the question for the other bodies, on their special grounds, without any previous presumption that such life exists. Undoubtedly the aspect of the case will be different in one feature, when we see reason to believe that other bodies have an atmosphere; and if there be in any planet sufficient light and heat, and clouds and winds, and a due adjustment of the power of gravity, and the strength of the materials of which organized frames consist, there may be, so far as we can judge, life of some kind or other. But yet, even in those cases, we should be led to judge also, by analogy, that the life which they sustain is more different from the terrestrial life of the present period of the earth, than that is from the terrestrial life of any former geological period, in proportion as the conditions of light and heat, and attraction and density, are more different on any other planet, than they can have been on the earth, at any period of its history.

10. Let us then consider the state of these elements of being in the other planets. I have mentioned, among them, the force of gravity, and the density of materials; because these are important elements in the question. It may seem strange, that we are able, not only to measure the planets, but to weigh them; yet so it is. The wonderful discovery of universal gravitation, so firmly established, as the law which embraces every particle of matter in the solar system, enables us to do this, with the most perfect confidence. The revolutions of the satellites round their primary planets, give us a measure of the force by which the planets retain them in their orbits; and in this way, a measure of the quantity of matter of which each planet consists. And other effects of the same universal law, enable us to measure, though less easily and less exactly, the masses, even of those planets which have no satellites. And thus we can, as it were, put the Earth, and Jupiter or Saturn, in the balance against each other; and tell the proportionate number of pounds which they would weigh, if so poised. And again, by another kind of experiment, we can, as we have said, weigh the earth against a known mountain; or even against a small sphere of lead duly adjusted for the purpose. And this has been done; and the results are extremely curious; and very important in our speculations relative to the constitution of the universe.

11. And in the first place, we may remark that the Earth is really much less heavy than we should expect, from what we know of the materials of which it consists. For, measuring the density, or specific gravity, of materials, (that is their comparative weight in the same bulk,) by their proportion to water, which is the usual way, the density of iron is 8, that of lead 11, that of gold 19: the ordinary rocks at the Earth's surface have a density of 3 or 4. Moreover, all the substances with which we are acquainted, contract into a smaller space, and have their density increased, by being subjected to pressure. Air does this, in an obvious manner; and hence it is, that the lower parts of our atmosphere are denser than the upper parts; being pressed by a greater superincumbent weight, the weight of the superior parts of the atmosphere itself. Air is thus obviously and eminently elastic. But all substances, though less obviously and eminently, are still, really, and in some degree, elastic. They all contract by compression. Water for instance, if pressed by a column of water 100000 feet high, would be reduced to a bulk one-tenth less than before. In the same manner iron, compressed by a column of iron 90000 feet high, loses one-tenth of its bulk, and of course gains so much in density. And the like takes place, in different amounts, with all material whatever. This is the rate at which compression produces its effect of increasing the density, in bodies which are in the condition of those which lie around us. But if this law were to go on at the same rate, when the compression is greatly increased, the density of bodies deep down towards the centre of the Earth must be immense. The Earth's radius is above 20 million feet. At a million feet depth we should have matter subjected to the pressure of a column of a million feet of superincumbent matter, heavier than water; and hence we should have a compression of water 10 times as great as we have mentioned; and, therefore, the bulk of the water would be reduced almost to nothing, its density increased almost indefinitely: and the same would be the case with other materials, as metals and stones. If, therefore, this law of compression were to hold for these great pressures, all materials whatever, contained in the depths of the Earth's mass, must be immensely denser, and immensely specifically heavier, than they are at the surface. And thus, the Earth consisting of these far denser materials towards the centre, but, nearer the surface, of lighter materials, such as rock, and metals, in their ordinary state, must, we should expect, be, on the whole, much heavier than if it consisted of the heaviest ordinary materials; heavier than iron, or than lead; hundreds of times perhaps heavier than stone.

12. This, however, is not found to be so. The expectation of the great density of the Earth, which we might have derived from the known laws of condensation of terrestrial substances, is not confirmed. The mass of the Earth being weighed, by means of such processes as we have already referred to, is found to be only five times heavier than so much water: less heavy than if it were made of iron: less than twice as heavy as if it were made of ordinary rock. This, of course, shows us that the condensation of the interior parts of the Earth's mass, is by no means so great as we should have expected it to be, from what we know of the laws of condensation here; and from considering the enormous pressure of superincumbent materials to which those interior parts are subjected. The laws of condensation, it would seem, do not go on operating for these enormous pressures, by the same progression as for smaller pressure. If a mass of a material is compressed into nine-tenths its bulk by the weight of a column of 100000 feet high, it does not follow that it will be again compressed into nine-tenths of its condensed bulk, by another column of 100000 feet high. The compression and condensation reach, or tend to, a limit; and probably, before they have gone very far. It may be possible to compress a piece of iron by one-thousandth part, even by such forces as we can use; and yet it may not be possible to compress the same piece of iron into one half its bulk, even by the weight of the whole Earth, if made to bear upon it. This appears to be probable: and this will explain, how it is, that the materials of the Earth are not so violently condensed as we should have supposed; and thus, why, the Earth is so light.

13. We must avoid drawing inferences too boldly, on a subject where our means of knowledge are so obscure as they are with regard to the interior of the Earth; but yet, perhaps, we may be allowed to say, that the result which we have just stated, that the Earth is so light, suggests to us the belief that the interior consists of the same materials as the exterior, slightly condensed by pressure.[47 - The doctrine that the interior nucleus of the Earth is fluid, whether accepted or rejected, does not materially affect this argument. It appears, that in some cases, at least, the melting of substances is prevented, by their being subjected to extreme pressure; but the density, the element from which we reason, is measured by methods quite independent of such questions.] We find no encouragement to believe that there is a nucleus within, of some material, different from what we have on the outside; some metal, for instance, heavier than lead. If the earth were of granite, or of lava, to the centre, it would, so far as we can judge, have much the same weight which it now has. Such a central mass, covered with the various layers of stone, which form the upper crust of the Earth, would naturally make this globe of at least the weight which it really has. And therefore, if we were to learn that a planet was much lighter than this, as to its materials,—much less dense, taking the whole mass together,—we should be compelled to infer that it was, throughout, or nearly so, formed of less compact matter than metal and stone; or else, that it had internal cavities, or some other complex structure, which it would be absurd to assume, without positive reasons.

14. Now having decided these views from an examination of the Earth, let us apply them to other planets, as bearing upon the question of their being inhabited; and in the first place, to Jupiter. We can, as we have said, easily compare the mass of Jupiter and of the Earth; for both of them have Satellites. It is ascertained, by this means, that the mass of weight of Jupiter is about 333 times the weight of the earth; but as his diameter is also 11 times that of the earth, his bulk is 1331 times that of the earth: (the cube of 11 is 1331); and, therefore, the density of Jupiter is to that of the earth, only as 333 to 1331, or about 1 to 4. Thus the density of Jupiter, taken as a whole, is about a quarter of the earth's density; less than that of any of the stones which form the crust of the earth; and not much greater than the density of water. Indeed, it is tolerably certain, that the density of Jupiter is not greater than it would be, if his entire globe were composed of water; making allowance for the compression which the interior parts would suffer by the pressure of those parts superincumbent. We might, therefore, offer it as a conjecture not quite arbitrary, that Jupiter is a mere sphere of water.

15. But is there anything further in the appearance of Jupiter, which may serve to contradict, or to confirm, this conjecture? There is one circumstance in Jupiter's form, which is, to say the least, perfectly consistent with the supposition, that he is a fluid mass; namely, that he is not an exact sphere, but oblate, like an orange. Such a form is produced, in a fluid sphere, by a rotation upon its axis. It is produced, even in a sphere which is (at present at least,) partly solid and partly fluid; and the oblateness of the earth is accounted for in this way. But Jupiter, who, while he is much larger than the earth, revolves much more rapidly, is much more oblate than the earth. His polar and equatorial diameters are in the proportion of 13 to 14. Now it is a remarkable circumstance, that this is the amount of oblateness, which, on mechanical principles, would result from his time of revolution, if he were entirely fluid, and of the same density throughout.[48 - Herschel, 512. Bessel, however, holds that the oblateness of Jupiter proves that his interior is somewhat denser than his exterior. Pop. Vorles. p. 91.] So far, then, we have some confirmation at least, of his being composed entirely of some fluid which in its density agrees with water.

16. But there are other circumstances in the appearances of Jupiter, which still further confirm this conjecture of his watery constitution. His belts,—certain bands of darker and lighter color, which run parallel to his equator, and which, in some degree, change their form, and breadth, and place, from time to time,—have been conjectured, by almost all astronomers, to arise from lines of cloud, alternating with tracts comparatively clear, and having their direction determined by currents analogous to our trade-winds, but of a much more steady and decided character, in consequence of the great rotatory velocity.[49 - Herschel, 513.] Now vapors, supplying the materials of such masses of cloud, would naturally be raised from such a watery sphere as we have supposed, by the action of the Sun; would form such lines; and would change their form from slight causes of irregularity, as the belts are seen to do. The existence of these lines of cloud does of itself show that there is much water on Jupiter's surface, and is quite consistent with our conjecture, that his whole mass is water.[50 - A difficulty may be raised, founded on what we may suppose to be the fact, as to the extreme cold of those regions of the Solar System. It may be supposed that water under such a temperature could exist in no other form than ice. And that the cold must there be intense, according to our notion, there is strong reason to believe. Even in the outer regions of our atmosphere, the cold is probably very many degrees below freezing, and in the blank and airless void beyond, it may be colder still. It has been calculated by physical philosophers, on grounds which seem to be solid, that the cold of the space beyond our atmosphere is 100° below zero. The space near to Jupiter, if an absolute vacuum, in which there is no matter to receive and retain heat emitted from the Sun, may, perhaps, be no colder than it is nearer the Sun. And as to the effect the great cold would produce on Jupiter's watery material, we may remark, that if there be a free surface, there will be vapor produced by the Sun's heat; and if there be air, there will be clouds. We may add, that so far as we have reason to believe, below the freezing point, no accession of cold produces any material change in ice. Even in the expeditions of our Arctic navigators, a cold of 40° below zero was experienced, and ice was still but ice, and there were vapors and clouds as in our climate. It is quite an arbitrary assumption, to suppose that any cold which may exist in Jupiter would prevent the state of things which we suppose.]

17. Perhaps some persons may be disposed to doubt whether, if Jupiter be, as we suppose, merely or principally a mass of water and of vapor, we are entitled to extend to him the law of universal gravitation, which is the basis of our speculations. But this doubt may be easily dismissed. We know that the waters of the earth are affected by gravitation; not only towards the earth, as shown by their weight, but towards those distant bodies, the Sun and the Moon; for this gravitation produces the tides of the ocean. And our atmosphere also has weight, as we know; and probably has also solar and lunar tides, though these are marked by many other causes of diurnal change. We have, then, the same reason for supposing that air and water, in other parts of the system, are governed by universal gravitation, and exercise themselves the attractive force of gravitation, which we have for making the like suppositions with regard to the most solid bodies. Whatever argument proves universal gravitation, proves it for all matter alike; and Newton, in the course of his magnificent generalization of the law, took care to demonstrate, by experiment, as well as by reasoning, that it might be so generalized.

18. As bearing upon the question of life in Jupiter, there is another point which requires to be considered; the force of gravity at his surface. Though, equal bulk for equal bulk, he is lighter than the earth, yet his bulk is so great that, as we have seen, he is altogether much heavier than the earth. This, his greater mass, makes bodies, at equal distances from the centres, ponderate proportionally more to him than they would do to the earth. And though his surface is 11 times further from his centre than the earth's is, and therefore the gravity at the surface is thereby diminished, yet, even after this deduction, gravity at the surface of Jupiter is nearly two and a half times that on the earth.[51 - Herschel, 508.] And thus a man transferred to the surface of Jupiter would feel a stone, carried in his hands, and would feel his own limbs also, (for his muscular power would not be altered by the transfer,) become 2

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times as heavy, as difficult to raise, as they were before. Under such circumstances animals of large dimensions would be oppressed with their own weight. In the smaller creatures on the earth, as in insects, the muscular power bears a great proportion to the weight, and they might continue to run and to leap, even if gravity were tripled or quadrupled. But an elephant could not trot with two or three elephants placed upon his back. A lion or tiger could not spring, with twice or thrice his own weight hung about his neck. Such an increase of gravity would be inconsistent then, with the present constitution and life of the largest terrestrial animals; and if we are to suppose planets inhabited, in which gravity is much more energetic than it is upon the earth, we must suppose classes of animals which are adapted to such a different mechanical condition.

19. Taking into account then, these circumstances in Jupiter's state; his (probably) bottomless waters; his light, if any, solid materials; the strong hand with which gravity presses down such materials as there are; the small amount of light and heat which reaches him, at 5 times the earth's distance from the sun; what kind of inhabitants shall we be led to assign to him? Can they have skeletons where no substance so dense as bone is found, at least in large masses? It would seem not probable.[52 - It may be thought fanciful to suppose that because there is little or no solid matter (of any kind known to us) in Jupiter, his animals are not likely to have solid skeletons. The analogy is not very strong; but also, the weight assigned to it in the argument is small. Valeat quantum valere debet.] And it would seem they must be dwellers in the waters, for against the existence there of solid land, we have much evidence. They must, with so little of light and heat, have a low degree of vitality. They must then, it would seem, be cartilaginous and glutinous masses; peopling the waters with minute forms: perhaps also with larger monsters; for the weight of a bulky creature, floating in the fluid, would be much more easily sustained than on solid ground. If we are resolved to have such a population, and that they shall live by food, we must suppose that the waters contain at least so much solid matter as is requisite for the sustenance of the lowest classes; for the higher classes of animals will probably find their food in consuming the lower. I do not know whether the advocates of peopled worlds will think such a population as this worth contending for: but I think the only doubt can be, between such a population, and none. If Jupiter be a mere mass of water, with perhaps a few cinders at the centre, and an envelope of clouds around it, it seems very possible that he may not be the seat of life at all. But if life be there, it does not seem in any way likely, that the living things can be anything higher in the scale of being, than such boneless, watery, pulpy creatures as I have imagined.

20. Perhaps it may occur to some one to ask, if this planet, which presents so glorious an aspect to our eyes, be thus the abode only of such imperfect and embryotic lumps of vitality as I have described; to what purpose was all that gorgeous array of satellites appended to him, which would present, to intelligent spectators on his surface, a spectacle far more splendid than any that our skies offer to us: four moons, some as great, and others hardly less, than our moon, performing their regular revolutions in the vault of heaven. To which it will suffice, at present, to reply, that the use of those moons, under such a supposition, would be precisely the same, as the use of our moon, during the myriads of years which elapsed while the earth was tenanted by corals and madrepores, shell-fish and belemnites, the cartilaginous fishes of the Old Red Sandstone, or the Saurian monsters of the Lias; and in short, through all the countless ages which elapsed, before the last few thousand years: before man was placed upon the earth "to eye the blue vault and bless the useful light:" to reckon by it his months and years: to discover by means of it, the structure of the universe, and perhaps, the special care of his Creator for him alone of all his creatures. The moons of Jupiter, may in this way be of use, as our own moon is. Indeed we know that they have been turned to most important purposes, in astronomy and navigation. And knowing this, we may be content not to know how, either the satellites of Jupiter, or the satellite of the Earth, tend to the advantage of the brute inhabitants of the waters.

21. There is another point, connected with this doctrine of the watery nature of Jupiter, which I may notice, though we have little means of knowledge on the subject. Jupiter being thus covered with water, is the water ever converted into ice? The planet is more than 5 times as far from the sun as the earth is: the heat which he receives is, on that account, 25 times less than ours. The veil of clouds which covers a large part of his surface, must diminish the heat still further. What effect the absence of land produces, on the freezing of the ocean, it is not easy to say. We cannot, therefore, pronounce with any confidence whether his waters are ever frozen or not. In the next considerable planet, Mars, astronomers conceive that they do trace the effects of frost; but in Mars we have also appearances of land. In Jupiter, we are left to mere conjecture; whether continents and floating islands of ice still further chill the fluids of the slimy tribes whom we have been led to regard as the only possible inhabitants; or whether the watery globe is converted into a globe of ice; retaining on its surface, of course, as much fluid as is requisite, under the evaporating power of the sun, to supply the currents of vapor which form the belts. In this case, perhaps, we may think it most likely that there are no inhabitants of these shallow pools in a planet of ice: at any rate, it is not worth while to provide any new speculations for such a hypothesis.

22. We may turn our consideration from Jupiter to Saturn; for in many respects the two planets are very similar. But in almost every point, which is of force against the hypothesis of inhabitants, the case is much stronger in Saturn than it is in Jupiter. Light and heat, at his distance, are only one ninetieth of those at the Earth. None but a very low degree of vitality can be sustained under such sluggish influences. The density of his mass is hardly greater than that of cork; much less than that of water: so that, it does not appear what supposition is left for us, except that a large portion of the globe, which we see as his, is vapor. That the outer part of the globe is vapor, is proved, in Saturn as in Jupiter, by the existence of several cloudy streaks or belts running round him parallel to his equator. Yet his mass, taken altogether, is considerable, on account of his great size; and gravity would be greater, at his outer surface, than it is at the earth's. For such reasons, then, as were urged in the case of Jupiter, we must either suppose that he has no inhabitants; or that they are aqueous, gelatinous creatures; too sluggish, almost to be deemed alive, floating on their ice-cold waters, shrouded forever by their humid skies.

23. Whether they have eyes or no, we cannot tell; but probably if they had, they would never see the Sun; and therefore we need not commiserate their lot in not seeing the host of Saturnian satellites; and the Ring, which to an intelligent Saturnian spectator, would be so splendid a celestial object. The Ring is a glorious object for man's view, and his contemplation; and therefore is not altogether without its use. Still less need we (as some appear to do) regard as a serious misfortune to the inhabitants of certain regions of the planet, a solar eclipse of fifteen years' duration, to which they are liable by the interposition of the Ring between them and the Sun.[53 - Herschel, 522.]

24. The cases of Uranus and Neptune are similar to that of Saturn, but of course stronger, in proportion to their smaller light and heat. For Uranus, this is only 1-360th, for Neptune, as we have already said, 1-900th of the light and heat at the earth. Moreover, these two new planets agree with Jupiter and with Saturn, in being of very large size and of very small density; and also we may remark, one of them, probably both, in revolving with great rapidity, and in nearly the same period, namely, about 10 hours: at least, this has been the opinion of astronomers with regard to Uranus. The arguments against the hypothesis of these two planets being inhabited, are of course of the same kind as in the case of Jupiter and Saturn, but much increased in strength; and the supposition of the probably watery nature and low vitality of their inhabitants must be commended to the consideration of those who contend for inhabitants in those remote regions of the solar system.

25. We may now return towards the Sun, and direct our attention to the planet Mars. Here we have some approximation to the condition of the Earth, in circumstances, as in position. It is true, his light and heat, so far as distance from the Sun affects them, are less than half those at the Earth. His density appears to be nearly equal to that of the Earth, but his mass is so much smaller, that gravity at his surface is only one-half of what it is here. Then, as to his physical condition, so far as we can determine it, astronomers discern in his face[54 - Herschel, 510.] the outlines of continents and seas. The ruddy color by which he is distinguished, the red and fiery aspect which he presents, arise, they think, from the color of the land, while the seas appear greenish. Clouds often seem to intercept the astronomer's view of the globe, which with its continents and oceans thus revolves under his eye; and that there is an atmosphere on which such clouds may float, appears to be further proved, by brilliant white spots at the poles of the planet, which are conjectured to be snow; for they disappear when they have been long exposed to the sun, and are greatest when just emerging from the long night of their polar winter; the snow-line then extending to about six degrees (reckoned upon the meridian of the planet) from the pole. Moreover, Mars agrees with the earth, in the period of his rotation; which is about 24 hours; and in having his axis inclined to his orbit, so as to produce a cycle of long and short days and nights, a return of summer and winter, in every revolution of the planet.

26. We have here a number of circumstances which speak far more persuasively for a similarity of condition, in this planet and the Earth, than in any of the cases previously discussed. It is true, Mars is much smaller than the earth, and has not been judged worthy of the attendance of a satellite, although further from the Sun; but still, he may have been judged worthy of inhabitants by his Creator. Perhaps we are not quite certain about the existence of an atmosphere; and without such an appendage, we can hardly accord him tenants. But if he have inhabitants, let us consider of what kind they must be conceived to be, according to any judgment which we can form. The force of his gravity is so small, that we may allow his animals to be large, without fearing that they will break down by their own weight. In a planet so dense, they may very likely have solid skeletons. The ice about his poles will cumber the seas, cold even for the want of solar heat, as it does in our arctic and antarctic oceans; and we may easily imagine that these seas are tenanted, like those, by huge creatures of the nature of whales and seals, and by other creatures which the existence of these requires and implies. Or rather, since, as we have said, we must suppose the population of other planets to be more different from our existing population, than the population of other ages of our own planet, we may suppose the population of the seas and of the land of Mars, (if there be any, and if we are not carrying it too high in the scale of vital activity,) to differ from any terrestrial animals, in something of the same way in which the great land and sea saurians, or the iguanodon and dinotherium, differed from the animals which now live on the earth.

27. That we need not discuss the question, whether there are intelligent beings living on the surface of Mars, perhaps the reader will allow, till we have some better evidence that there are living things there at all; if he calls to mind the immense proportion which, on the earth, far better fitted for the habitation of the only intelligent creature which we know or can conceive, the duration of unintelligent life has borne to that of intelligent. Here, on this Earth, a few thousand years ago, began the life of a creature who can speculate about the past and the future, the near and the absent, the Universe and its Maker, duty and immortality. This began a few thousand years ago, after ages and myriads of ages, after immense varieties of lives and generations, of corals and mollusks, saurians, iguanodons, and dinotheriums. No doubt the Creator might place an intelligent creature upon a planet, without all this preparation, all this preliminary life. He has not chosen to do so on the earth, as we know; and that is by much the best evidence attainable by us, of what His purposes are. It is also possible that He should, on another planet, have established creatures of the nature of corals and mollusks, saurians and iguanodons, without having yet arrived at the period of intelligent creatures: especially if that other planet have longer years, a colder climate, a smaller mass, and perhaps no atmosphere. It is also possible that He should have put that smaller planet near the Earth, resembling it in some respects, as the Moon does, but without any inhabitants, as she has none; and that Mars may be such a planet. The probability against such a belief can hardly be considered as strong, if the arguments already offered be regarded as effective against the opinion of inhabitants in the other planets, and in the Moon.

28. The numerous tribe of small bodies, which revolve between Jupiter and Mars, do not admit of much of the kind of reasoning, which we have applied to the larger planets. They have, with perhaps one exception (Vesta) no disk of visible magnitude; they are mere dots, and we do not even know that their form is spherical. The near coincidence of their orbits has suggested, to astronomers, the conjecture that they have resulted from the explosion of a larger body, and from its fracture into fragments. Perhaps the general phenomena of the universe suggest rather the notion of a collapse of portions of sidereal matter, than of a sudden disruption and dispersion of any portion of it; and these small bodies may be the results of some imperfectly effected concentration of the elements of our system; which, if it had gone on more completely and regularly, might have produced another planet, like Mars or Venus. Perhaps they are only the larger masses, among a great number of smaller ones, resulting from such a process: and it is very conceivable, that the meteoric stones which, from time to time, have fallen upon the earth's surface, are other results of the like process:—bits of planets which have failed in the making, and lost their way, till arrested by the resistance of the earth's atmosphere. A remarkable circumstance in these bodies is, that though thus coming apparently from some remote part of the system, they contain no elements but such as had already been found to exist in the mass of the earth; although some substances, as nickel and chrome, which are somewhat rare in the earth's materials, are common parts of the composition of meteoric stones. Also they are of crystalline structure, and exhibit some peculiarities in their crystallization. Such as these strange visitors are, they seem to show that the other parts of the solar system contain the same elementary substances, and are subject to the same laws of chemical synthesis and crystalline force, which obtain in the terrestrial region. The smallness of these specimens is a necessary condition of their reaching us; for if they had been more massive, they would have followed out the path of their orbits round the sun, however eccentric these might be. The great eccentricity of the smaller planets, their great deviation from the zodiacal path, which is the highway of the large planets, their great number, probably by no means yet exhausted by the discoveries of astronomers; all fall in with the supposition that there are, in the solar system, a vast multitude of such abnormal planetoidal lumps. As I have said, we do not even know that they are approximately spherical; and if they are of the nature of meteoric stones, they are mere crude and irregularly crystallized masses of metal and earth. It will therefore, probably, be deemed unnecessary to give other reasons why these planetoids are not inhabited. But if it be granted that they are not, we have here, in addition to the moon, a large array of examples, to prove how baseless is the assumption, that all the bodies of the solar system are the seats of life.

29. We have thus performed our journey from the extremest verge of the Universe, so far as we have any knowledge of it, to the orbit of our own planet; and have found, till we came into our own most immediate vicinity, strong reasons for rejecting the assumption of inhabited worlds like our own; and indeed, of the habitation of worlds in any sense. And even if Mars, in his present condition, may be some image of the Earth, in some of its remote geological periods, it is at least equally possible that he may be an image of the Earth, in the still remoter geological period before life began. Of peculiar fitnesses which make the earth suited to the sustentation of life, as we know that it is, we shall speak hereafter; and at present pass on to the other planets, Venus and Mercury. But of these, there is, in our point of view, very little to say. Venus, which, when nearest to us, fills a larger angle than any other celestial body, except the Sun and the Moon, might be expected to be the one of which we know most. Yet she is really one of the most difficult to scrutinize with our telescopes. Astronomers cannot discover in her, as in Mars, any traces of continents and seas, mountains and valleys; at least with any certainty.[55 - According to Bessel, Schroeter once saw one bright point on the dark ground, near the boundary of light in Venus. This was taken as proving a mountain, estimated at 60,000 feet high. Pop. Vorles. p. 86.] Her illuminated part shines with an intense lustre which dazzles the sight;[56 - Herschel, 509.] yet she is of herself perfectly dark; and it was the discovery, that she presented the phases of the Moon, made by the telescope of Galileo, which gave the first impulse to planetary research. She is almost as large as the earth; almost as heavy. The light and heat which she receives from the Sun must be about double those which come to the earth. We discern no traces of a gaseous or watery atmosphere surrounding her. Perhaps if we could see her better, we might find that she had a surface like the moon; or perhaps, in the nearer neighborhood of the sun, she may have cooled more slowly and quietly, like a glass which is annealed in the fire; and hence, may have a smooth surface, instead of the furrowed and pimpled visage which the Moon presents to us. With this ignorance of her conditions, it is hard to say what kind of animals we could place in her, if we were disposed to people her surface; except perhaps the microscopic creatures, with siliceous coverings, which, as modern explorers assert, are almost indestructible by heat. To believe that she has a surface like the earth, and tribes of animals, like terrestrial animals, and like man, is an exercise of imagination, which not only is quite gratuitous, but contrary to all the information which the telescope gives us; and with this remark, we may dismiss the hypothesis.

30. Of Mercury we know still less. He receives seven times as much light and heat as the Earth; is much smaller than the earth, but perhaps more dense; and has not, so far as we can tell, any of the conditions which make animal existence conceivable. If it is so difficult to find suitable inhabitants for Venus, the difficulty for Mercury is immensely greater.

31. So far then, we have traversed the Solar System, and have found even here, the strongest grounds that there can be no animal existence, like that which alone we can conceive as animal existence, except in the planet next beyond the earth, Mars; and there, not without great modifications. But we may make some further remarks on the condition of the several planets, with regard to what appears to us to be the necessary elements of animal life.

CHAPTER X

THEORY OF THE SOLAR SYSTEM

1. We have given our views respecting the various planets which constitute the Solar System;—views established, it would seem, by all that we know, of the laws of heat and moisture, density and attraction, organization and life. We have examined and reasoned upon the cases of the different planets separately. But it may serve to confirm this view, and to establish it in the reader's mind, if we give a description of the system which shall combine and connect the views which we have presented, of the constitution and peculiarities, as to physical circumstances, of each of the planets. It will help us in our speculations, if we can regard the planets not only as a collection, but as a scheme;—if we can give, not an enumeration only, but a theory. Now such a scheme, such a theory, appears to offer itself to us.

2. The planets exterior to Mars, Jupiter, and Saturn especially, as the best known of them, appear, by the best judgment which we can form, to be spheres of water, and of aqueous vapor, combined, it may be, with atmospheric air, in which their cloudy belts float over their deep oceans. Mars seems to have some portion at least of aqueous atmosphere; the earth, we know, has a considerable atmosphere of air, and of vapor; but the Moon, so near to her mistress, has none. On Venus and Mercury, we see nothing of a gaseous or aqueous atmosphere; and they, and Mars, do not differ much in their density from the Earth. Now, does not this look as if the water and the vapor, which belong to the solar system, were driven off into the outer regions of its vast circuit; while the solid masses which are nearest to the focus of heat, are all approximately of the same nature? And if this be so, what is the peculiar physical condition which we are led to ascribe to the Earth? Plainly this: that she is situated just in that region of the system, where the existence of matter, both in a solid, a fluid, and a gaseous condition, is possible. Outside the Earth's orbit, or at least outside Mars and the small Planetoids, there is, in the planets, apparently, no solid matter; or rather, if there be, there is a vast preponderance of watery and vaporous matter. Inside the Earth's orbit, we see, in the planets, no traces of water or vapor, or gas; but solid matter, about the density of terrestrial matter. The Earth, alone, is placed at the border where the conditions of life are combined; ground to stand upon; air to breathe; water to nourish vegetables, and thus, animals; and solid matter to supply the materials for their more solid parts; and with this, a due supply of light and heat, a due energy of the force of weight. All these conditions are, in our conception, requisite for life: that all these conditions meet, elsewhere than in the neighborhood of the Earth's orbit, we see strong reasons to disbelieve. The Earth, then, it would seem, is the abode of life, not because all the globes which revolve round the Sun may be assumed to be the abodes of life; but because the Earth is fitted to be so, by a curious and complex combination of properties and relations, which do not at all apply to the others. That the Earth is inhabited, is not a reason for believing that the other Planets are so, but for believing that they are not so.

3. Can we see any physical reason, for the fact which appears to us so probable, that all the water and vapor of the system is gathered in its outward parts? It would seem that we can. Water and aqueous vapor are driven from the Sun to the outer parts of the solar system, or are allowed to be permanent there only, as they are driven off and retained at a distance by any other source of heat;—to use a homely illustration, as they are driven from wet objects placed near the kitchen-fire: as they are driven from the hot sands of Egypt into the upper air: as they are driven from the tropics to the poles. In this latter case, and generally, in all cases, in which vapor is thus driven from a hotter region, when it comes into a colder, it may again be condensed in water, and fall in rain. So the cold of the air in the temperate zone condenses the aqueous vapors which flow from the tropics; and so, we have our clouds and our showers. And as there is this rainy region, indistinctly defined, between the torrid and the frigid zones on the earth; so is there a region of clouds and rain, of air and water, much more precisely defined, in the solar system, between the central torrid zone and the external frigid zone which surrounds the Sun at a greater distance.

4. The Earth's Orbit is the Temperate Zone of the Solar System. In that Zone only is the play of Hot and Cold, of Moist and Dry, possible. The Torrid Zone of the Earth is not free from moisture; it has its rains, for it has its upper colder atmosphere. But how much hotter are Venus and Mercury than the Torrid Zone? There, no vapors can linger; they are expelled by the fierce solar energy; and there is no cool stratum to catch them and return them. If they were there, they must fly to the outer regions; to the cold abodes of Jupiter and Saturn, if on their way, the Earth did not with cold and airy finger outstretched afar, catch a few drops of their treasures, for the use of plant, and beast, and man. The solid stone only, and the metallic ore which can be fused and solidified with little loss of substance, can bear the continual force of the near solar fire, and be the material of permanent solid planets in that region. But the lava pavement of the Inner Planets bears no superstructure of life; for all life would be scorched away along with water, its first element. On the Earth first, can this superstructure be raised; and there, through we know not what graduation of forms, the waters were made to bring forth abundantly things that had life; plants, and animals nourished by plants, and conspiring with them, to feed on their respective appointed elements, in the air which surrounded them. And so, nourished by the influences of air and water, plants and animals lived and died, and were entombed in the scourings of the land, which the descending streams carried to the bottom of the waters. And then, these beds of dead generations were raised into mountain ranges; perhaps by the yet unextinguished forces of subterraneous fires. And then a new creation of plants and animals succeeded; still living under the fostering influence of the united pair, Air and Water, which never ceased to brood over the World of Life, their Nurseling; and then, perhaps, a new change of the limits of land and water, and a new creation again: till at last, Man was placed upon the Earth; with far higher powers, and far different purposes, from any of the preceding tribes of creatures: and with this, for one of his offices;—that there might be an intelligent being to learn how wonderfully the scheme of creation had been carried on, and to admire, and to worship the Creator.

5. But we have a few more remarks to make on the structure of the Solar System, in this point of view. When we say that the water and vapor of the System were driven to the outer parts, or retained there, by the central heat of the Sun, perhaps it might be supposed to be most simple and natural, that the aqueous vapor, and the water, should assume its place in a distinct circle, or rather a spherical shell, of which the Sun was the centre; thus making an elemental sphere about the centre, such as the ancients imagined in their schemes of the Universe. Nor will we venture to say that such an arrangement of elements might not be; though perhaps it might be shown that no stable equilibrium of the system would be, in this way, mechanically possible. But this at least we may say; that a rotatory motion of all the parts of the universe appears to be a universal law prevalent in it, so far as our observation can reach: and that, by such rotation of the separate masses, the whole is put in a condition which is everywhere one of stable equilibrium. It was, then, agreeable to the general scheme, that the excess of water and vapor, which must necessarily be carried away, or stored up, in the outer regions of the System, should be put into shapes in which it should have a permanent place and form. And thus, it is suitable to the general economy of creation, that this water and vapor should be packed into rotating masses, such as are Jupiter and Saturn, Uranus and Neptune. When once collected in such rotating masses, the attraction of its parts would gather it into spheroidal forms; oblate by the effect of rotation, as Jupiter, or perhaps into annular forms, like the Ring of Saturn;[57 - Other speculators also have regarded Saturn's Ring as a ring of cloud or water. See Cosmos, iii. 527 and 553.] for such also is a mechanically possible form of equilibrium, for a fluid mass. And these spheroids once formed, the water would form a central nucleus, over which would hang a cover of vapor, raised by the evaporating power of the Sun, and forming clouds, where the rarity of the upper strata of vapor allowed the cold of the external space to act; and these clouds, spun into belts by the rotation of the sphere. And thus, the vapor, which would otherwise have wandered loose about the atmosphere, was neatly wound into balls; which, again, were kept in their due place, by being made to revolve in nearly circular orbits about the Sun.

6. And thus, according to our view, water and gases, clouds and vapors, form mainly the planets in the outer part of the solar system; while masses such as result from the fusion of the most solid materials, lie nearer the sun, and are found principally within the orbit of Jupiter.[58 - Humboldt has already remarked (Cosmos, i. 95, and iii. 427), that the inner planets as far as Mars, and the outer ones beginning with Jupiter, form two groups having different properties. Also Encke. (See Humboldt's Note.)] To conceive planetary systems as formed by the gradual contraction of a nebular mass, and by the solidification of some of its parts, is a favorite notion of several speculators. If we adopt this notion, we shall, I think, find additional proofs in favor of our view of the system. For, in the first place, we have the zodiacal light, a nebulous appendage to the Sun, as Herschel conceives, extending beyond the orbits of Mercury and Venus. These planets, then, have not yet fully emerged from the atmosphere in which they had their origin:—the mother-light and mother-fire, in which they began to crystallize, as crystals do in their mother-water. Though they are already opaque, they are still immersed in luminous vapor: and bearing such traces of their chaotic state being not yet ended, we need not wonder, if we find no evidence of their having inhabitants, and some evidence to the contrary. They are within a nebular region, which may easily be conceived to be uninhabitable. And where this nebular region, marked by the zodiacal light, terminates, the world of life begins, namely at the Earth.