But the effects of the new element thus called into operation would not be limited to the production of air and water. It is estimated that oxygen constitutes, by weight, nearly half of the solid crust of the earth. It forms a part of every rock and of every metallic ore. The second day, then, must have been a period of intense chemical action, resulting from the introduction of this powerful agent.
But (2) what is the meaning of the division of the waters which are above the firmament from the waters which were under the firmament? At present all the water contained in the atmosphere, in the shape of vapour and clouds, is so insignificant in comparison with that vast volume of water which not only fills the ocean, but also permeates the solid earth, that such a notice of it seems unaccountable. Mr. Goodwin, indeed, maintains that there was an ancient belief, not only that the firmament was a solid vault, but that on it there rested another ocean, at least as copious as that with which we are acquainted. [Footnote: Essays and Reviews, p. 220] In support of this assertion he brings forward the phrase, "The windows of heaven were opened" (Gen, VII. 11) and other similar expressions. But such phrases as this evidently belong to the same class as the fanciful names so often given to the clouds in the hymns of the Rig Veda. Both expressions evidently point to a time when figurative language, if no longer a necessity, was at all events a common and favourite form of speech, and was understood by all. Dr. Whewell [Footnote: Plurality of Worlds, chap. x. Section 5.] has put forward the curious notion that when the creation of the interior planets was completed, there remained a superfluity of water, which was gathered up into the four exterior planets. But the only fact in favour of such an hypothesis is the close correspondence between the apparent density of these planets and that of water. Now, as will be seen immediately, there is strong reason to believe that the true density of these planets is much greater than their apparent diameters would seem to indicate; so that the one solitary ground on which the suggestion rests vanishes when it is examined. Apart from this, however, the suggestion that there would be any superfluous material when the work of creation was finished, is a very strange one. Neither of these views, then, can be accepted as giving a satisfactory meaning to the text.
Astronomical investigations however, which have been carried on with great diligence during the last four winters, and which are still being continued with unremitting interest, have brought to light phenomena which seem to be in remarkable correspondence with the state of things spoken of in the text. It has already been noticed that the eight greater planets at present known to us are divided into two groups of four by the intervening belt of minor planets. These two groups have totally distinct characteristics. In density, magnitude, and length, of day the members of each group differ little from each other, while the two groups differ very widely. The moon is the only satellite as yet known in the inner group. The planets of the outer group are attended by at least seventeen satellites.
Of these outer planets Jupiter, from his great brilliancy, specially attracts observation, while from his comparative proximity to the earth we are enabled to examine him much more satisfactorily than we can Saturn, Uranus, or Neptune. Two facts with reference to him have long been well known, the one, that the polar compression in his case is much greater than it is in any of the interior planets, so that when seen through a telescope of very moderate power his disc is evidently elliptical, while the compression of the interior planets can only be detected by the most delicate micrometrical measurements—the other, that his apparent surface is always crossed by several alternating belts of light and shade, which though subject to constant changes of detail, always preserve the same general character. Until recently the generally received theory was that these belts consisted of clouds, raised by the heat of the sun, and arranged in zones under the influence of winds similar in character to, and produced by the same causes as, the trade-winds which blow over our own oceans. This view, however, has been shown by Mr. Proctor to be untenable. [Footnote: See a paper by Mr. Proctor in the Monthly Packet for October, 1870.]
About forty years ago, a very remarkable phenomenon was observed simultaneously, but independently, by three astronomers, Admiral Smyth, Mr. Maclean, and Mr. Pearson, who were watching a transit of Jupiter's second satellite from stations several miles apart. Admiral Smyth's account of it is as follows:—"On Thursday, the 26th of June, 1828, the moon being nearly full, and the evening extremely fine, I was watching the second satellite of Jupiter as it gradually approached to transit the disc of the planet. My instrument was an excellent refractor of 3 3/4 inches aperture, and five feet focal length, with a power of one hundred. The satellite appeared in contact at about half-past ten, and for some minutes remained on the edge of the limb, presenting an appearance not unlike that of the lunar mountains which come into view during the first quarter of the moon, until it finally disappeared on the body of the planet. At least twelve or thirteen minutes must have elapsed when, accidentally turning to Jupiter again, I perceived the same satellite outside the disc. It was in the same position as to being above a line with the lower belt, where it remained distinctly visible for at least four minutes, and then suddenly vanished." A somewhat similar phenomenon, but of shorter duration, was witnessed by Messrs. Gorton and Wray, during an occultation of the same satellite, April 26, 1863. In this case the satellite reappeared after passing behind the apparent disc of the planet. So lately as 1868 this phenomenon was regarded as inexplicable. [Footnote: Webb's Celestial Objects, p. 141.]
In the winter of 1868-9 the attention of astronomers was called to the fact that rapid and extensive changes were taking place in the appearance of Jupiter's belts, and they have consequently been watched from that time with unremitting attention by astronomers furnished with telescopes of the best quality. The results of these observations are given in two very interesting papers, communicated to the Popular Science Review, by Mr. Webb. [Footnote: Popular Science Review for April, 1870, and July, 1871.] Very curious markings and variations in the depth of shade have been seen, accompanied by equally curious changes of colour. Mr. Browning compares these changes to those which are seen when a cloud of steam of varying depth and density is illuminated from behind by a strong light, as when we look through the steam escaping from the safety-valve of a locomotive at a gas-lamp immediately behind it. This appears to be the true explanation of the phenomenon. [Footnote: Popular Science Review, 1871, p. 307.] These belts are probably due to vast masses of steam, poured forth with great force from the body of the planet. As the atmosphere of Jupiter is probably of enormous depth, the rotatory velocity of its upper portions would be much greater than that of the surface of the planet, hence the steam would arrange itself in belts parallel to the equator of the planet. But this view leads us to wonderful conclusions with reference to the condition of the planet.
"Processes of the most amazing character are taking place beneath that cloudy envelope, which forms the visible surface of the planet as seen by the terrestrial observer. The real globe of the planet would seem to be intensely heated, perhaps molten, through the fierceness of the heat which pervades it. Masses of vapour streaming continually upward from the surface of this fiery globe would be gathered at once into zones because of their rapid change of distance from the centre. That which is wholly unintelligible when we regard the surface of Jupiter as swept like our earth by polar and equatorial winds, is readily interpreted when we recognize the existence of rapidly uprushing streams of vapour." [Footnote: Mr. Proctor in Monthly Packet, October, 1870.]
Supposing then that the atmosphere of Jupiter is of very great depth, and thus laden with masses of watery vapour, the effect of a sudden current of heated, but comparatively dry, air or gas would be the immediate absorption of the whole or a large portion of the vapour, and the consequent transparency of the portion of the atmosphere affected by it. We see this result continually on a small scale in our own atmosphere, when a heavy cloud comes in contact with a warm air current, and rapidly melts away, Many of the rapid changes which have been witnessed in Jupiter's appearance are readily explained if this view is admitted. Supposing such a thing to have happened near the edge of the disc, the phenomenon recorded by Admiral Smyth is at once satisfactorily explained. When the satellite appeared to pass on to the disc, and to be lost in the light of the planet, it would for some time, proportional to the depth of Jupiter's atmosphere, have behind it a background of clouds only, it would not have entered upon the actual disc of the planet. If then these clouds were suddenly absorbed, the atmosphere behind the satellite would become transparent and invisible, the background would be gone, and the satellite would reappear. In the case of the occultation witnessed by Messrs. Gorton and Wray, the satellite would at first be hidden by cloud only, and would reappear if the cloud were removed. Such seems to be the true explanation of these hitherto mysterious phenomena. That they could not have resulted from any alteration in the motions of the planet or the satellite is evident. Such an alteration would have been instantly detected, since the places of both the planet and the satellites are computed years in advance, and any such change would at once have thrown out all these computations.
Assuming that this is the true solution of the mystery, we are enabled to form an approximate estimate of the extent of the atmosphere of Jupiter. The time between the first and second disappearances does not seem to have been accurately noted. Admiral Smyth's account makes it 16 or 17 minutes; but if we estimate it at 15 minutes only, and if we further assume that the second disappearance was upon the actual disc of Jupiter, and not upon a lower stratum of clouds, we shall be safe from any risk of exaggeration. The probability seems to be that the second disappearance was caused not by the disc, but by the formation of a fresh body of cloud, as it was not gradual, as in the first instance, but sudden. We shall then only have an estimate which cannot be greater, but may be much less, than the true value.
The mean distance of the second satellite from the centre of Jupiter is in round numbers 425,000 miles, and consequently the circumference of its orbit is 2,671,000 miles. The satellite travels through this orbit in about 86 hours, which gives a horary velocity of 31,400 miles, or 7850 miles in 15 minutes. This then is the least possible depth of the atmosphere of Jupiter. [Footnote: For the direction of the motion of the satellite would be at right angles to the line of sight.] The whole diameter of Jupiter, atmosphere and all, is 85,390 miles. Deduct from this 15,700 miles for the atmosphere, and we have for the diameter of the solid nucleus rather less than 70,000 miles. The height of the atmosphere is therefore not less than three-fourteenths of the radius of the planet, and may be much greater. The extent of the atmosphere, combined with the rapidity of rotation, accounts satisfactorily for the great apparent polar compression of the planet. Another inference is that the density of the planet must exceed the ordinary estimate in the proportion of two to one.