Chapter XIII: Transits of Venus 273 (5)

When we inquire what these tints are, we find that they are tints which are _sometimes_ wanting even from direct sunlight. When the sun sinks very low and looks like a great red ball through the moisture-laden air, his spectrum is not the same exactly as that of the sun shining high in the mid heaven. It shows other gaps than those corresponding to the ordinary myriads of missing tints. Its red colour shows indeed that some thing has happened to the sunlight; but, oddly enough (at first sight at least), the gaps are chiefly in the red part of the spectrum, just what one would expect if the sun's light showed a want instead of an excess of ruddy light. The fact is, however, that the violet, indigo, and blue are weakened altogether, not by the mere abstraction of tints here and there. The red suffers under a few abstractions of tint, but remains on the whole little weakened. Now the same gaps which at such times appear in the spectrum of the sun are found (generally, if not always) in the spectrum of the planet Mars, even when he is shining high in the heavens, so that his light is _not_ at the time absorbed by the denser portions of our air. In fact the gaps have been seen in the spectrum of Mars when the planet has been shining higher in the heavens than the moon, whose spectrum was found on trial (at the time) not to show the same gaps,--as of course it must have done, and even more markedly, if the missing tints had been abstracted by our own air.

No doubt can remain, then, that the sun's light, which reaches us after falling on Mars, has suffered _at Mars_ the same absorption which our own air produces on the rays of the sun when he is low down. But we know what it is in our air which causes this absorption. It is the aqueous vapour. We know this from several independent series of researches. It was proved first by an American physicist, Professor Cooke of Harvard, who found that these lines in the red are always darker when the air is moister. Then by Janssen, who observed the spectrum of great bonfires lit at a distance of many miles, on the Swiss mountains, finding these same lines in the spectrum of the fire-light when the air was heavily laden with moisture. Wherefore we know that the air of Mars must also contain the same substance--the vapour of water--which, in our own air, produces these dark lines. We can, indeed, understand that the ruddy colour of Mars is in part due to this moisture, which, precisely as in our own air it makes the sun and moon look red, would, in the air of a planet, make the planet itself look red.

But how much follows from the discovery that there is moisture in the air of Mars! This moisture can only come from water in sufficient quantities. There must, therefore, be seas on Mars. We should be sure of this from the spectroscopic evidence, even without the evidence given by the telescope. We cannot doubt for a moment, however, knowing as we do how the telescope shows greenish markings on Mars, that these really are the seas and oceans of the planet. And again, the white spots at the poles of Mars can no longer be regarded doubtfully. If we could not see them, but knew only, from the spectroscopic evidence, that Mars must have large seas, we should be sure that his polar regions must be covered with everlasting ice and snow, varying with the seasons, but always surrounding, in enormous masses, the poles themselves. Seeing that the telescope presents spots to our view which, long before the spectroscopic evidence had been obtained or hoped for, had been regarded as analogues of our polar snows, we can now entertain no manner of doubt that they really are so.

But again, recognising the presence of enormous masses of snow and ice around the poles of Mars, and knowing that not only are there wide oceans, seas, and lakes, but that there is an atmosphere capable of carrying mist and cloud, how many circumstances, corresponding to those which we associate with the wants of living creatures, present themselves to our consideration! It remains that I should now consider some of these points.

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We have seen that Mars has water in all its forms, solid, liquid, and vaporous. We perceive also that his polar regions do not extend very much farther towards his equator than do the polar ice and snows of our own earth. (Of course the former do not extend so far in actual distance; I refer to their extent compared with the globe they belong to.) It would appear then, at a first view, that the climate of Mars cannot be very unlike that of our earth. Yet this is scarcely possible. For Mars is so much farther than we are from the sun that he receives less than half as much light and heat from that luminary. And it is not easy to conceive that the deficiency can be compensated by any effects due to the nature of the Martian air. It is more likely by far that this air is much rarer than that it is much denser than ours. For not only can it be shown that with the same relative quantity of air a smaller planet would have a smaller quantity above each square mile of its surface than would a larger one,[12] but the gravity at the surface of the smaller planet being less, the air there is much less compressed by its own weight (having in fact much less weight), and is therefore rarer. Thus the probability is that the air of Mars is like that at (or even above) the summits of our highest mountains, where we know that an intense cold prevails. It is not that the sun's rays do not fall there with as much heating power as at the sea-level, for experiment shows that they fall with even greater power. But there is less air to be warmed and to retain the heat. The difference may be compared in fact to that between a well-watered country near the sea and an arid desert. The sun's rays fall as fiercely on one as on the other, but because there is no moisture in the desert to receive (after the fashion characteristic of water) the solar heat and retain it, the heat passes away so soon as the sun has set, and intense cold prevails, while over the well-watered region the temperature is much more uniform, and warm nights prevail. So is it at the summits of lofty mountains. The sun's rays are poured on them as hotly as elsewhere, but there is little air to retain the moisture, so that the heat passes away almost as quickly as it is received, and during the night as much fresh snow is formed as had been melted during the day. And so it would certainly be with Mars, if, other things being the same, the air were as rare as it is at the summits of our loftiest mountains. If, as seems probable, the air is still rarer than this, the cold would be still more intense.

It would seem, then, that either some important difference exists, by which the Martian air is enabled to retain the sun's heat even more effectively than our air does (for the climate as indicated by the limits of the polar snows seems the same, though the distance from the sun is greater); or else there is some mistake in the supposition that the same general state of things prevails on Mars as on our own earth.

I confess that though Professor Tyndall has shown clearly how the atmosphere of a more distant planet _might_ make up for the deficient supply of solar heat, by more effectively retaining the heat, I know of nothing in either the telescopic or the spectroscopic evidence respecting any of the planets which tends to show, or even renders it likely, that any such arrangement exists,--excepting always the peculiarity in Mars's case which we are now endeavouring to explain. Insomuch that should any other explanation of the difficulty be suggested, and appear to have weight in its favour, I apprehend that the mere possibility of an atmospheric arrangement, such as has been suggested, should not prevent our admitting this other explanation.

I am inclined to think that there is such an explanation. It seems to me that there are good reasons for regarding Mars as a planet which has passed to a much later stage of planetary life than that through which our earth is now passing, and that in this circumstance some of the peculiarities of his appearance find their explanation. As a planet outside the earth, Mars must probably be regarded as one formed somewhat before the earth. As a much smaller planet, he would be not only less heated when first found (whatever theory of planetary formation we adopt), but would also have parted much more rapidly (relatively) with his heat, according to the same law which makes a small mass of metal cool more quickly than a large one. If he has a rarer atmosphere he would be a colder planet on that account also. Being also remoter from the sun, he receives less heat from that orb, and we thus have a fourth reason for regarding Mars as a much colder planet than our earth, both as to inherent heat and as to heat received from without. It seems to me that we may in this consideration find the real meaning of the comparatively limited extension of the Martian snows. It has been well pointed out by Professor Tyndall that for the formation of great glacial masses, not great cold only, but great heat also is required. The snows which fall on mountain slopes, to be compacted into ice and afterwards to form great glaciers, were raised into the air by the sun's heat. Every ice particle represents the action of that heat upon the particles of water at the surface of ocean, sea, or lake, or of wet soil. If the sun's heat suddenly died out, there would prevail an intense cold, and the snows and ice now existing would assuredly remain. The waters also of the earth would congeal. But no new snows would fall. The congealed seas viewed from some remote planet would appear unchanged. For they would not be covered with snow and broken ice, nor therefore white; but would consist of pure ice throughout, retaining the partial transparency and greenish colour of deep-sea water. No winds would disturb the surface of the frozen seas, for winds have their origin in heat, and with the death of the solar heat the winds would utterly die out also.

If we are to choose between these two explanations,--one that the snows and ice have not the great range we should expect, because the temperature is somehow raised despite Mars's greater distance to the same temperature which we experience, and the other that it is not heat but cold which diminishes the quantity of Martian snow, I conceive that there is every reason the case admits of for accepting the latter instead of the former explanation. As extreme cold would certainly prevent glacial masses from being very large and deep, simply because the stores whence the ice was gathered would be less, the snow caps of a very cold planet would vary as readily with varying seasons as those of a planet like our earth. For though less heat would be poured upon them with the returning summer, less heat would be required to melt away their outskirts.

I think we may fairly regard Mars as in all probability a somewhat old and decrepit planet. He is not absolutely dead, like our own moon, where we see neither seas nor clouds, neither snow nor ice, no effects, in fine, of either heat or cold. But I think he has passed far on the road towards planetary death,--that is, towards that stage of a planet's existence when at least the higher forms of life can no longer exist upon the planet's surface.

There is one peculiarity of the planet's appearance which seems strikingly to accord with this view that Mars holds a position intermediate between that of our earth and the moon,--as indeed we might fairly expect from his intermediate proportions. The seas of our earth cover nearly three-quarters of her entire globe. The moon has no visible water on her surface. If we examine the chart of Mars at page 167, we see that the seas and oceans of the planet are much smaller (relatively as well as actually) than are the seas of our own earth. I have carefully estimated their relative extent in the following simple but effective way. I drew a chart such as the above-mentioned, but on a projection of my own invention, in which equal surfaces on a globe are represented by equal surfaces on the planisphere. Then I cut out with a pair of scissors the parts representing land and the parts representing water (leaving the polar parts as doubtful), and carefully weighed these in a delicate balance. I found that they were almost exactly equal: whatever preponderance there was seemed to be in favour of the land. Thus, if we assume that, when in the same stage of planetary existence, Mars had as great a relative extent of water surface as our earth, or that about 72/100 of the surface of Mars were originally water, we should have to admit that the water had so far been withdrawn into the planet's interior as to diminish the water-surface by 22/100 (for there are now barely 50/100). At a very fair assumption as to the slopes of the Martian sea-bottoms, it would follow that more than half the Martian water originally existing above the surface had been withdrawn into the interior, as the planet's mass gradually cooled.

I am aware the assumption above mentioned is in itself somewhat daring, and is not supported by direct evidence. But, since we have very strong reasons for considering that the moon once had seas, which have been withdrawn in the way suggested, and since Mars unquestionably holds a position midway between the earth and moon as to size and presumably as to age,[13] it seems not unreasonable to find in the character of her seas,--less extended relatively than the earth's, but, unlike the moon's, still existing,--the evidence that she has gone partially through the process through which the moon has long since passed completely.

I think it very likely that the recent discovery of two Martian satellites will lead many to look with more disfavour than ever on the idea that Mars may not at present be the abode of life. For moons seem so manifestly convenient additions to a planet's surroundings, as light-givers, time-measurers, and tide-rulers, that many will regard the mere fact that these conveniences exist as proof positive that they are at this present time subserving the purposes which they are capable of subserving. I would point out, however, that our own moon must have existed for ages before any living creatures, far less any reasoning beings, could profit by her light, or by the regularity of her motions, or by her action in swaying the waters of ocean. And doubtless she will continue to exist for ages after all life shall have passed away from the earth. Again, there can be no question that our earth would present a most attractive scene if she were viewed from the moon, and would be a most useful ornament of the lunar skies. Yet we have every reason to believe that there is not a living creature on the moon at present to profit by her light. The case may well be the same (apart from the actual evidence that it _is_ the same) with Mars. His satellites may long since have served most useful purposes to his inhabitants; but it by no means follows that because if there were inhabitants on Mars now the same purposes would still be subserved, therefore there are inhabitants there.

Let us, however, without considering the question whether the satellites of Mars serve such special purposes for creatures living on the planet, consider briefly the history of their discovery, their nature, and the laws of their motion around the planet.

Astronomers had long examined the neighbourhood of Mars with very powerful telescopes, in the hope of discovering Martian moons. But the hope had so thoroughly been abandoned for many years that the planet had come to be known as "moonless Mars." The construction, however, of the fine telescope which has been mounted at Washington, with an object-glass twenty-six inches in diameter, caused at least American astronomers to hope that after all a Martian moon or two might be discovered. Taking advantage of the exceptionally favourable opportunity presented during the planet's close approach to our earth in the autumn of 1877, Prof. Asaph Hall, of the Washington Observatory, paid special attention to the search for Martian moons. At last, on August 16, 1877, he detected close by the planet a faint point of light, which he was unable to examine further at the time (to see if it behaved as a satellite, or as one of the fixed stars). But on the 18th he saw it again, and determined its nature. He also saw another still fainter point of light closer to the planet; and subsequent observations shewed that this object also was a satellite. During the next few weeks both the moons were observed as closely as possible, in fact, whenever weather permitted, and the result is that we now know the true nature of their paths.

In fig. 23 these paths are shown as they appeared in 1877. Of course the paths themselves are not seen; but if the satellites left behind them a fine train or wake of light, the shape of this train would be as shown in fig. 23. The satellites themselves could not be shown at all in a picture on so small a scale--the diameter of either would certainly be less than the cross-breadth of the fine elliptical line representing its track. The size of the planet is correctly indicated, and the true pose of the planet in 1877 is shown in the figure, his southern pole being somewhat bowed towards the earth. This is the uppermost pole; for the figure represents the planet and his satellites' orbits as they would appear in an astronomical telescope, which inverts objects.

The outer satellite is probably not more than ten miles or so in diameter, the inner one, perhaps, the same; but neither can be measured. In the most powerful telescopes they appear as mere points of light. Nor is it easy to determine, from their lustre, or rather from their faintness, their true dimensions; for we cannot compare them directly in this respect with objects of known size, because their visibility is partly affected by the proximity of the planet, whose overpowering light dims their feeble rays. This remark applies with special force to the inner satellite.

The distance of the outer satellite from Mars's centre is about 14,300 miles, from Mars's surface about 12,000 miles. The inner travels at a distance of about 5,750 miles from the centre, and about 3,450 miles from the surface of Mars.

The motions of the satellites as seen from Mars must be very different from those of our own moon. Thus, our moon moves so slowly among the stars that she requires nearly an hour to traverse a distance equal to her own apparent diameter. The outer moon of Mars traverses a similar distance--that is, not her own apparent diameter, but an arc on the stellar heavens equal to our moon's apparent diameter--in about two and a half minutes, while the inner moon moves so rapidly as to traverse the same distance in about forty seconds. To both moons, therefore, but to the inner in particular, Job's description of our moon as "walking in brightness" would seem singularly applicable, so far at least as the rapidity of their motions is concerned. Their brightness, however, cannot be comparable to our moon's. For notwithstanding their much greater proximity, it is easily shown that they must present much smaller discs, and being illuminated by a more distant sun, their discs cannot shine so brightly as our moon's. That is, not only are the discs smaller, but their intrinsic brightness is less. Assuming the outer moon to be ten miles, the inner fifteen miles in diameter, it is easily shown that the two together, if full at the same time, can hardly give one-twelfth as much light to Martians as our moon gives to us.

Yet there can be no doubt that the Martian moons must be (or _have been_) most useful additions to the Martian skies. They do not give a useful measure of time intermediate in length between the day and the year, as our moon does; for the outer travels round the planet in about thirty and a quarter hours, the inner in about seven and a half hours. Nor can they exert an influence upon the Martian seas corresponding to that exerted by our own moon in generating the lunar tidal wave. But their motions must serve usefully to indicate the progress of time, both by night and by day, for they must be visible by day unless very close to the sun. They must be even more useful than our moon in indicating the longitude of ships at sea, seeing that the accuracy with which a moon indicates longitude is directly proportional to her velocity of motion among the stars.

I have said that there does not seem to be any valid reason for considering that now is the accepted time with these moons; their services may have been of immense value in long past ages, and now be valueless for want of any creatures to be benefited by them. But those who not only believe that no object in nature was made without some special purpose, but that we are able to assign to each object its original purpose, should be well satisfied if they find reason for believing that, during millions of years long, long ago, the moons lately discovered by our astronomers were measuring time for past races of Martians, swaying tides in wider seas than those which now lave the shores of Martian continents, and enabling Martian travellers to guide their course over the trackless ocean and arid desert with far greater safety than can our voyagers by sea and land despite all the advances of modern science.

FOOTNOTES:

[11] Brown is not the right word for the tint of red where the visible spectrum begins. I know, however, of no word properly expressing the colour.

[12] Suppose there are two planets A and B of equal density, of which A has a diameter twice as great as that of B. Then the volume of A is eight times greater than B's volume. So that if the volume of its atmosphere exceed the volume of B's air in the same degree, the planet A has eight times as much air as the planet B. But the surface of A is only four times as great as the surface of B; so that if A had only four times as much air as B, there would be the same quantity of air above each square mile of A's surface as above each of B's surface. Since then A has eight times--not merely four times--as much air as B, it follows that A has twice as much air over each square mile of surface as B has. And similarly in all such cases, the general law being that the larger planet has more air over each square mile of surface in the same degree that its diameter exceeds that of the other.

[13] By age here I do not mean absolute age, but relative age. I speak of Mars and the Moon as older than the earth in the same sense that I should speak of a fly in autumn as older than a five-year-old raven.

X.

_THE PLANET JUPITER._

Two or three years ago I had occasion to consider in the _Day of Rest_ the giant planet Jupiter, the largest and most massive of all the bodies circling around the sun. I then presented a new theory respecting Jupiter's condition, to which I had been led in 1869, when I was visiting other worlds than ours. Since then, in fact within the last few months, observations have been made which place the new theory on a somewhat firm basis; and I propose now briefly to reconsider the subject in the light of these latest observations.

In the first place I would call the reader's attention to the way in which modern science has altered our ideas respecting time as well as space, though the change has only been noticed specially as it affects space. In former ages men regarded the region of space over which they in some sense had rule as very extensive indeed. This earth was the most important body in the universe, all others being not only made for the service of the earth, but being in all respects, in size, in range, and so forth, altogether subordinate to it. Step by step men passed from this to an entirely different conception of our earth's position in space. Shown first to be a globe within the domain of the heavenly bodies, then to be a globe subordinate to the sun, then to be a member of one family among thousands each with its ruling sun, then to belong to a galaxy of suns which is but one among myriads of millions of such galaxies, and lastly shown to the eye of reason, though not to direct observation, as belonging to a galaxy of galaxies itself but one among millions of the same order, which in turn belong to higher and higher orders endlessly, the earth has come to be regarded, despite its importance to ourselves, as but a point in space. The minutest particle by which a mathematician might attempt to picture the conception of a mathematical point, comparing that particle with any near object however large, a house, a mountain, the earth itself, would be but the grossest representation of a point, by comparison with the massive earth, when she is considered with reference to the universe of the fixed stars or rather to that portion of the universe, itself but a point in space, over which the survey of the astronomer extends.

All this has been admitted. Men have fully learned to recognise, though they are quite unable to conceive, the utter minuteness, one may say the evanescence, of their abode in space.

But along with the extension of our ideas respecting space, a corresponding extension has been made, or should have been made, in our conceptions respecting time. We have learned to recognise the time during which our earth has been and will be a fit abode for living creatures as exceedingly short compared with the time during which she was being fashioned into fitness for that purpose, and with the æons of æons to follow, after life has disappeared from her surface. This, however, is but one step towards the eternities to which modern science points. The earth is but one of many bodies of a system; and though it has been the custom to regard the birth of that system as if it had been effected, if one may so speak, in a single continuous effort (lasting millions of millions of years, mayhap, but bringing all the planets and their central sun simultaneously into fitness for their purpose), there is no reason whatever for supposing this to have been really the case, while there are many reasons for regarding it as utterly unlikely. It seems as though men could not divest themselves of the idea that our earth's history is the history of the solar system and of the universe. Precisely as children can hardly be brought to understand, for a long time, what history really means, how generation after generation of their own race has passed away, and how their own race has succeeded countless others, so science, still young, seems scarcely to appreciate the real meaning of its own discoveries. It follows directly from these that world after world like our earth, in this our own system or among the millions peopling space, has had its day, and that the systems themselves, on which such worlds attend, are but the existent representatives of their order, and succeed countless other systems which have long since served their purpose.

Yet, strangely enough, students of science continue for the most part to speak of other worlds, and other suns, and other systems, as though this present era, this "bank and shoal of time," were the sole period to which to refer in considering the condition of those worlds and suns and systems. It does not seem to occur to them that,--not possibly or probably, but most certainly,--myriads among the celestial bodies must be passing through stages preceding those which are compatible with the existence or support of life, while myriads of others must long since have passed that stage. And thus ideas appear strange and fanciful to them which, rightly apprehended, are alone in strict accordance with analogy. To consider Jupiter or Saturn as in the extreme youth of planetary existence, still glowing with such heat as pervaded the whole frame of our earth before she became a habitable world, still enveloped in cloud masses containing within them the very oceans of those future worlds, all this is regarded as fanciful and sensational. Yet those who so regard such theories do not hesitate to admit that every planet must once in its life pass through the fiery stage of planetary existence, nor are they prepared to show any reason why the stage must be regarded as past in the case of every planet or even of most of the planets. Seeing that, on the other hand, there are abundant reasons for believing that planets differ very widely as regards the duration of the various stages of their life, and that our earth is by no means one of the longest lived, we may very fairly expect to find among the planets some which are very much younger than our earth,--not younger, it will be understood, in years, but younger in the sense of being less advanced in development. When we further find that all the evidence accords with this view, we may regard it as the one to which true science points.

All that we know about the processes through which our earth has passed suggests the probability, I will even say the certainty, that planets so much larger than she is as are Jupiter and Saturn must require much longer periods for every one of those processes. A vast mass like Jupiter would not cool down from the temperature which our earth possessed when her surface was molten to that which she at present possesses in the same time as the earth, but in a period many times longer.

Supposing Bischoff to be right in assigning 340,000,000 years to that era of our earth's past, I have calculated that Jupiter would require about seven times and Saturn nearly five times as long, or about 2,380,000,000 and 1,500,000,000 years respectively, and by these respective periods would they be behind the earth as respects this stage of development. Suppose, however, on the other hand, that Bischoff has greatly overrated the length of that era--and I must confess that experiments on the cooling of small masses of rock, such as he dealt with, seem to afford very unsatisfactory evidence respecting the cooling of a great globe like our earth. Say that instead of 340,000,000 years we must assign but a tenth part of that time to the era in question. Even then we find for the corresponding era of Jupiter's existence about 238,000,000 years, and for that of Saturn's 150,000,000 years, or in one case more than 200,000,000 years longer, in the other more than 110,000,000 years longer than in our earth's case.

This relates to but one era only of our earth's past. That era was preceded by others which are usually considered to have lasted much longer. The earth, according to the nebular theory of Laplace, was once a mighty ring surrounding the sun, and had to contract into globe form, a process requiring many millions of years. When first formed into a globe she was vaporous, and had to contract--forming the moon in so doing--until she became a mass, first of liquid, then of plastic half solid matter, glowing with fire and covered with tracts of fluent heat. Here was another stage of her past existence, requiring probably many hundreds of millions of years. Jupiter and Saturn had to pass through similar stages of development, and required many times as many years for each of them. Is it then reasonable to suppose that they have arrived at the same stage of development as our earth, or indeed as each other.

Supposing for a moment that we were fully assured that Jupiter and Saturn had separate existence, hundreds of millions of years before our earth had been separated from the great glowing mass of vapour formerly constituting the solar system, and that having this enormous start, so to speak, they need not necessarily be regarded as now very greatly in arrear as respects development, or might even be in advance of the earth, it is altogether improbable that either of them, and far more improbable that both of them, are passing through precisely the same stage of development. If we knew only of two ships, that one had to travel from New York to London, and another from Canton to Liverpool, some time during the year, and that the one which had to make the longer journey was likely to start several weeks before the other, would it not be rather unsafe to conclude, when the former had entered the mouth of the Thames, that in all probability the other was sailing up the Mersey? Yet something like this, or in reality much wilder than this, is the reasoning which permits the student of science to believe, independently of the evidence, or altogether against all evidence, that Jupiter and Saturn are necessarily passing through the very stage of planetary existence through which the one planet we know much about is passing.

It seems to me that the student of science should be prepared to widen his conceptions of time even as he has been compelled to widen his conceptions of space. As he knows that the planets are not, as was once supposed, mere attendants upon our earth and belonging to her special domain in space, so should he understand that neither do the other planets appertain of necessity to the domain of time in which our earth's existence has been cast, or only do so in the same sense that like her they occupy a certain domain in space, not her domain, but the sun's. Their history in time, like hers, doubtless belongs to the history of the solar system, but the duration of that system enormously surpasses the duration of the earth as a planet, and immeasurably surpasses the duration of that particular stage of life through which she is now passing.

Prepared thus to view the other planets independently of preconceived ideas as to their resemblance to our own earth, we shall not find much occasion to hesitate, I think, in accepting the conclusion that Jupiter is a very much younger planet.

We have seen already that the enormous mass of Jupiter, surpassing that of our earth 340 times, is suggestive of the enormous duration of every stage of his existence, and therefore of his present extreme youth. His bulk yet more enormously exceeds that of our earth, as, according to the best measurements, no less than 1230 globes, as large as our earth, might be formed out of the mighty volume of the prince of planets. In this superiority of bulk, nearly four times greater than his vast superiority of mass, we find the first direct evidence from observation in favour of the theory that Jupiter is still intensely hot. How can a mass so vast, possessing an attractive power in its own substance so great that, under similar conditions, it should be compressed to a far greater degree than our earth, and be, therefore, considerably more dense, come to be considerably rarer? We no longer believe that there is any great diversity of material throughout the solar system. We cannot suppose, as Whewell once invited us to do, that Jupiter consists wholly or almost wholly of water. Nor can we imagine that any material much lighter than ordinary rocks constitutes the chief portion of his bulk. We are, to all intents and purposes, forced to believe that the contractive effect due to his mighty attractive energy is counteracted by some other force. Nor can we hesitate, since this is admitted, to look for the resisting force in the expansive effects due to heat. We know that in the case of the sun, where a much mightier contractive power is at work, a much more intense heat so resists it that the sun has a mean density no greater than Jupiter's. We have every reason, then, which bulk and mass can supply, to believe that Jupiter is far hotter than the earth--that in fact, as the sun, exceeding Jupiter more than 1000 times in volume, is many times hotter than he is, so Jupiter, exceeding our earth 1200 times in volume, is very much hotter than the earth.

But when we consider the aspect of Jupiter we find that similar reasoning applies to his atmosphere. The telescope shows Jupiter as an orb continually varying in aspect, so as to assure us that we do not see his real surface. The variable envelope we do see presents, further, all the appearance of being laden with enormously deep clouds. The figure (24) shows the planet as seen by Herr Lohse on February 5, 1872, and serves to illustrate the rounded clouds often seen in Jupiter's equatorial zone, as though floating in the deep atmosphere there. Although rounded clouds such as these are not constantly present, they are very often seen; their appearance, even on a few occasions only, would suffice for the argument I now propose to draw from them. It is impossible to regard them as _flat_ round clouds. Manifestly they are globular. Now they may not be quite as deep as they are long, or even broad, but supposing them only half as deep as they are broad, that would correspond to much more than a third of the diameter of our earth, shown in the same picture. The atmosphere in which they float would necessarily be deeper still, but that depth alone would be about 3,000 miles. Now an atmosphere 3,000 miles deep under the tremendous attraction of Jupiter's mass would be compressed near its base to a density many times exceeding that of the densest solids _if_ (which of course is impossible) it could remain in the gaseous form with such density. The fact, then, that an atmosphere, certainly gaseous, exists around Jupiter to this enormous depth at least, proves to demonstration that there must be some power resisting its attractive energy; and again, we have little choice but to admit that that power is no other than the planet's intense heat.

As we extend our scrutiny into the evidence from direct observation, we find still other proofs independent of those just considered. One proof alone, be it remembered, is all that is required, but it will be found that there are many.

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We have found reasons for believing that the planet Jupiter is expanded by heat in such sort that the contractive or condensing power of his own mighty attractive energy is overcome. We know certainly that, regarding the planet we see as a whole, its globe is of very small density. We have every reason to believe that it is made of the same materials, speaking generally, as our earth. We know that its mass as a whole possesses many times the gravitating power of our earth's mass. It is highly probable, therefore, that the condition of its substance is very different from that of our earth's substance. And as we know of no cause save heat which could keep the planet in this state, it is altogether probable that the planet is extremely hot. The argument, be it noticed, is independent of that based on the probability that Jupiter, owing to his enormous mass, has not cooled nearly so much as our earth has.

We then noticed another very powerful argument, similar in kind, but also quite independent, derived from the aspect of the planet. Jupiter's appearance indicates clearly that he has a deep cloud-laden atmosphere, and we know that such an atmosphere, if of the same temperature as our earth's, would be compressed enormously, whereas the observed mobility of Jupiter's cloud-envelope, and other circumstances, indicate that this enormous compression does not exist. We infer, then, that some cause is at work expanding the atmosphere; and we know of no other cause but heat which could do so effectively.

But now let us consider certain details which the telescope has brought to our knowledge.

In the first place, a number of circumstances indicate a tremendous activity in that deep cloud-laden air.

The cloud-belts sometimes change remarkably in appearance and shape in a very short time. Mr. Webb, in his excellent little treatise, "Celestial Objects for Common Telescopes," gives instances from the observations of South, which I here translate into non-technical terms:--On June 3, 1839, at about nine in the evening, South saw with his large telescope, just below the principal belt of Jupiter, a spot of enormous size. It was dark, and therefore probably represented an opening in a great cloud-layer by which a lower or inner layer was brought into view. (For though the planet's real globe may be so intensely hot as to emit a great deal of light, it is probable that most of the light so emitted is concealed by the enwrapping cloud masses, and that the greater part of the light we receive from the planet is reflected sunlight; so that the inner cloud-layers would be the darker.) South estimated this spot as about 20,000 miles in diameter. "I showed it," he says, "to some gentlemen who were present; its enormous extent was such that on my wishing to have a portrait of it, one of the gentlemen, who was a good draughtsman, kindly undertook to draw me one; whilst I, on the other hand, extremely desirous that its actual magnitude should not rest on estimation, proposed, on account of the scandalous unsteadiness of the large instrument, to measure it with my five-feet telescope. Having obtained for my companion the necessary drawing instruments, I went to work, he preparing himself to commence his." But on looking into the telescope, South was astonished to find that the large dark spot, except at its eastern and western edges, had become much whiter than any of the other parts of the planet; and thirty-four minutes after these observations had commenced, "these" [query three?] "miserable scraps were the only remains of a spot which but a few minutes before had extended over at least 20,000 miles,--or two and a half times the diameter of the earth."

The cloud envelope, then, of Jupiter is certainly not in a state of quiescence. Of course we need not suppose that winds had carried cloud masses athwart the tremendous opening seen by South. That would imply a motion of 10,000 miles in the half-hour or so of observation,--supposing contrary winds to have rushed towards the centre,--or double that velocity if the entire breadth of the spot had been traversed in that time. A velocity of 20,000 miles, and still more of 40,000 miles per hour, may fairly be regarded as incredible. It would exceed more than a hundred-fold (taking the least number) the velocity of our most tremendous hurricanes. And although the solar hurricanes would seem to have a velocity, at times, of 300,000 or 400,000 miles per hour, we have no reason for supposing that winds of tens of thousands of miles per hour could be raised in the atmosphere of Jupiter. As I have said, however, it is not necessary to suppose this. We may conceive that clouds had formed very rapidly at the higher elevation where before they had been wanting. Clouds may form as readily and quickly over an area a thousand miles across as over an area two or three miles across. Indeed Webb, referring to such changes as South witnessed, says that Sir J. Herschel once observed a cloud-bank in our own air, which formed so rapidly that it crossed the sky at the rate of 300 miles an hour, not moving, of course, at that rate, but being formed along different parts of its apparent progress almost simultaneously, so as to appear to travel with this enormous velocity.

But now I wish the reader specially to notice how this observation of South's may serve to explain another, equally remarkable and at first sight far more perplexing; and how, when the two observations are brought together, a very singular piece of information is obtained respecting Jupiter's cloud-envelope.

Let _a b_, fig. 25, represent the great dark space seen by South, just below the principal belt, and let us suppose the planet turned round until this dark space, or rather this opening in the planet's outer cloud-envelope, is brought to the edge as at _a c d_, fig. 26. Then this opening would really cause a depression in the planet's outline at _d c_, the shaded part being depressed. The depression might not be observable in any ordinary telescope. For at the edge of Jupiter the features of the belt are generally lost, and the outline is at all times smoothed in appearance by that peculiarity of vision which makes all bright objects seen on a dark background appear somewhat larger than they actually are. (This is _really_ due to a fringing, as it were, of the image on the retina of the eye.) But though the depression might not be recognisable, it would exist, and, as we shall presently see, it might be detected in another way than by being actually seen. When the clouds formed which concealed the spot,--we do not know how quickly, but certainly in less than thirty-four minutes,--the depression, had the spot been at the edge, would have been removed. This change, however, like the existence of the depression, would doubtless not have been discernible by ordinary vision.

Fig. 25. Fig. 26.
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Now, let us consider the second observation mentioned above.

Fig. 27. Fig. 28. Fig. 29.
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On Thursday, June 26, 1828, the second satellite of Jupiter was about to make a passage across the planet's face. It was observed, just before this passage or transit began, in the position shown in fig. 27 by the late Admiral Smyth. He was using an excellent telescope. It gradually made its entry, looking for a few minutes like a small white mountain on the edge of the planet, and finally disappeared. The reader must understand that the moon was not hiding itself behind the planet, but was on _this_ side of it, and simply lost to view because its brightness was the same, or very nearly the same, as that of the planet's edge. (Its place is shown in fig. 28, but of course the little dark ring was not so seen.) "At least 12 or 13 minutes must have elapsed," says Smyth, "when, accidentally turning to Jupiter again, to my astonishment I perceived the same satellite outside the disc," as shown in fig. 29. It remained visible there for at least four minutes, and then suddenly vanished. To show that the observation was not due to any local or personal peculiarity, it is only necessary to mention that two other astronomers, Mr. Maclear at Biggleswade, and Dr. Pearson at South Kilworth, observed the same extraordinary behaviour of Jupiter's second satellite. The three telescopes are thus described by Admiral Smyth,--

Mr. Maclear's, 3⅓ inches in opening, 3½ feet long;
Dr. Pearson's, 6¾ inches in opening, 12 feet long;
Adm. Smyth's, 3½ inches in opening, 5 feet long;

all good observing telescopes. Now, of course, the satellite did not really stop. Nothing short of a miracle could have stopped the satellite, or, if the satellite could have stopped, have set it travelling on again as usual. For the satellite did not lose one mile, or change its velocity by the thousandth part of a mile per hour or even per annum.

But suppose such a change had taken place at the edge of Jupiter as we have seen would certainly have taken place there if the changes affecting the spot which South saw had occurred to a region at the edge, as in fig. 26, instead of the middle, as in fig. 25. Then Smyth's observation would be perfectly explained. We require, indeed, to suppose the change occurring in a different order, the outer cloud-layer being in the first instance well-developed and very rapidly becoming dissipated, so that the outline which had at first been at its usual level, was very rapidly depressed to the inner cloud-layer. But, of course, if the rapid formation of clouds by condensation can occur on Jupiter, so also can the rapid dissipation occur, especially at that particular part where Smyth saw the satellite behave so strangely. For that part is being carried, by the planet's swift rotation, into sunlight, and the extra heat to which it is thus exposed might readily effect the dissipation of widely extended cloud strata, supposing the temperature near that critical value at which clouds form or are dissipated.

Here, then, is an explanation of a phenomenon which otherwise seems utterly inexplicable. The explanation requires only that a process like one which has been observed to occur on Jupiter's disc should occur at a part of his surface forming at the moment a portion of his outline. If we had never known of such changes as South and other observers have noted in the markings of Jupiter, we should be compelled by Smyth's observation to admit their possibility. If we had never known of Smyth's observation we should be compelled by South's to admit that such a change of outline as is indicated by Smyth's observation must be possible,--must, in fact, occur whenever cloud-masses form or are dissipated over wide areas at the apparent edge of the planet. When we have both forms of evidence it seems altogether unreasonable to entertain any further doubt on this point.

But Smyth's observation, thus interpreted, indicates an enormous distance between the outer and inner cloud-layers which formed the planet's edge near the satellite in figs. 28 and 29 respectively. I find after making every possible allowance for errors in his estimate of time, not taken it would seem from his observatory clock, that the distance separating these cloud-layers cannot have been less than 3,500 miles, or not far from half the diameter of our earth. It is the startling nature of this result which perhaps deters many from accepting the explanation of Smyth's observation here advanced. But there is no other explanation. The satellite cannot have stopped in its course; Jupiter cannot have shifted his place bodily; the satellite was on this side of the planet,--therefore no effects of the planet's atmosphere on the line of sight from the planet can help us; three observers at different stations saw the phenomenon,--therefore neither effects of our earth's atmosphere nor personal peculiarities can account for the strange phenomenon. "Explanation is set at defiance," says Webb; "demonstrably neither in the atmosphere of the earth nor Jupiter, where and what could have been the cause?" The explanation I have advanced is the only possible answer to this question.

I might occupy twenty times the space here available to me in detailing various other phenomena all pointing in the same way,--that is, all tending to show that Jupiter is a planet glowing with intense heat, surrounded by a deep cloud-laden atmosphere, intensely hot in its lower portions, but not necessarily so in the parts we see, and undergoing changes (consequences of heat) of a stupendous nature, such as the small heat of the remote sun, which shines on Jupiter with less than the 27th part of the heat we receive, could not by any possibility produce. But partly because space will not permit, partly because most of these phenomena have been described in my "Orbs Around Us," and "Other Worlds," I content myself by describing a singular observation recently made, which, with South's and Smyth's, seems to place the theory I have advanced beyond the possibility of doubt or cavil.