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.

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.

Now, let us consider the second observation mentioned above.

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,—

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.

Mr. Todd of Adelaide has recently obtained for his observatory a fine 8-inch telescope by Mr. Cooke. With this instrument, mounted in December, 1874, he has made many valuable observations of the motions of Jupiter's satellites. Ordinarily, of course, the entry of each satellite on the planet's face and the egress therefrom, the disappearance of each satellite behind the planet or in the planet's shadow (not necessarily the same thing) and the reappearance, are effected in what may be called the normal way; and Mr. Todd's experience in this respect has been like that of other observers. But on two occasions he and his assistant, Mr. Ringwood, observed that a satellite, when passing behind the planet's edge, did not disappear at once, but remained visible as if seen through the edge, for about two minutes. The same satellite behaved thus on each occasion,—viz. the satellite nearest the planet. As this satellite travels at the rate of about 645 miles per minute, it would follow that the satellite was seen through a depth of nearly 1300 miles, or, after making all possible allowance for optical illusions, some 900 or 1000 miles. The effect of refraction cannot then be great in the air of Jupiter, to this depth below the usual limit of the upper clouds,—for otherwise the satellite would have been altogether distorted. And this very fact, that for 1000 miles or so below the highest clouds the change of atmospheric density is not sufficient to produce any noticeable refractive effects, implies that the true base of the atmosphere of Jupiter lies very far lower yet—perhaps many hundreds of miles lower.

If the reader now look again at the picture at page 201, he will understand, I think, how those great round white clouds in the chief belt,—clouds thousands of miles long and broad,—are probably hundreds of miles deep also, and float in an atmosphere still deeper.

All that we know about Jupiter, in fine, from direct observation, as well as all that we can infer respecting the past history of the solar system, tends to show that he is still an extremely young planet. He is the giant of the solar family in bulk, and probably he is far older than our earth in years; but in development he is, in all probability, the youngest of the sun's family of planets, and certainly far younger than the earth on which we live.