Part 7 (1/2)
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 ma.s.ses 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. Ma.s.ses 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 uprus.h.i.+ng 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 ma.s.ses 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 pa.s.s 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.
a.s.suming 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 a.s.sume 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 circ.u.mference of its...o...b..t is 2,671,000 miles. The satellite travels through this...o...b..t 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.
But next, the atmosphere of Jupiter is probably of very great density. Dr. Huggins states that he has observed in the spectrum of Jupiter ”three or four strong lines, one of them coincident with a strong line in the earth's atmosphere.” [Footnote: Lecture at Manchester, November 16, 1870.] Strong lines mark increased density in the absorbent medium, and lines. .h.i.therto un.o.bserved indicate new elements. It is therefore probable that the atmosphere of Jupiter is not only much more dense than that of the earth, but also contains some elements--which are absent from the latter. When with this fact we connect the very great extent of the atmosphere, it will be evident that the pressure at the surface of the planet will be enormous, and from this we can form an estimate of the intensity of the forces which must be at work in the interior of the planet, to project jets of vapour through such an atmosphere to so great a height.
The link which connects Jupiter with the earth, in the second stage of its existence, is the mention by Moses of the ”waters which were above the firmament.” Viewed in the light of the present condition of the earth such a notice seems unaccountable.
But if the earth at that time were in a condition similar to that in which Jupiter appears to be now, the water in the atmosphere or above the firmament would be a very important element in any description that might be given of it. It is in fact most probable that all the water (in the strict sense of the word) then in existence would be in a state of vapour, and that the waters which were under the firmament were the molten materials which afterwards formed rocks and ores, since, as has been already noticed, the word is the only one which could be employed to describe fluids in general.
We may now try to form some idea of the probable state of the earth at this period. Its centre would be occupied by a fused ma.s.s, in which were blended all the more intractable solid const.i.tuents of the present world. This would be surrounded by an atmosphere of very great height and density, containing not only all the present const.i.tuents of air, but also all, or nearly all, the water, and all the more volatile of the metals and other elements. Carbonic acid, to a very large extent, would probably be present, and a very considerable proportion of the oxygen which now exists in combination with various bases, and forms by weight so large a proportion of the solid crust of the world.
Owing to the intense heat, chemical combinations would readily be formed between the ingredients of the fused ma.s.s and the other elements which existed in the form of vapour, and thus the earliest of the vast variety of existing minerals would be elaborated. The volumes of steam which floated in the upper regions of the atmosphere would rapidly part with their heat by radiation into s.p.a.ce, and would descend towards the surface of the earth in the form of rain. At first probably, and for a long time, they would not reach the surface, but as they approached it would be again converted into vapour, and re-ascend to pa.s.s again and again through the same process. But by this means the intense heat of the nucleus would be gradually conveyed away, till the cooling reached a point at which some of the superficial materials would a.s.sume a solid form. It is by no means certain what is the true primary rock--for a long time it was almost universally a.s.sumed to be granite, since granite is uniformly found underlying the oldest sedimentary rocks that are known. But as these rocks have been forced from their original position and tilted up, the underlying stratum may probably be of later date than the upper ones, since it was the elevating agent. So that we can have no certain knowledge on this point, since the earliest sedimentary strata, wherever they retain their original position, must be at a depth far below the reach of man. If, however, Sir C. Kyell's view of the conditions requisite for the formation of granite are correct, these conditions [Footnote: Student's Geology, chap. x.x.xi.]--heat, moisture, and enormous pressure--would all be present at the surface of the nucleus. Some kind of solid floor must have been formed before the next stage could be reached, at which it would be possible for water to exist in a fluid state. This, however, would be possible at a much higher temperature than at present, owing to the enormous atmospheric pressure. It is possible now, by artificial means, to raise water, nearly if not quite, to a red heat, without the formation of steam, and the pressure of the atmosphere in the case supposed would, in all probability, be much greater than any which we can now apply under the conditions necessary for heating the water.
It is probable that at this point the close of the second day must be placed: but the indications of the narrative do not enable us to fix it with any degree of certainty. As, however, from this point a new series of processes would commence, and those processes are in intimate connexion with the first of the two developments ascribed to the third day, the period when water could first maintain a fluid form on the earth's surface, seems to present the most probable line of demarcation.
SECTION 6. THE THIRD DAY.
”And G.o.d said, Let the waters under the Heaven be gathered together in one place, and let the dry land appear; and it was so.
”And G.o.d called the dry land Earth, and the gathering together of the waters called He Seas, and G.o.d saw that it was good.
”And G.o.d said, Let the earth sprout sprouts, the herb seeding seed, and the fruit-tree yielding fruit after his kind, whose seed is in it, [Footnote: ”It” seems preferable to ”itself” here.
The same Hebrew word stands for both, but if the ”fruit-tree” be taken as the antecedent, which it must be if we translate ”itself,” there seems no meaning in the statement. If we read ”it,” the p.r.o.noun will refer to the fruit--”the tree whose seed is in its fruit”--which gives an intelligible sense.] upon the earth, and it was so.
”And the earth caused to go forth sprouts, the herb seeding seed, and the fruit-tree yielding fruit whose seed is in it, after his kind, and G.o.d saw that it was good. And there was evening, and there was morning, a third day.”
The record of the third day is a very important one, because it is the first point at which the Mosaic Record comes in contact with that other record which is written in the rocks. Up to this time we have only been able to compare the statements of Moses with conjectural views of the earliest condition of the earth, which, though they may be highly probable, are at best only conjectures.
But from this point we have to deal with a number of ascertained facts--certain landmarks stand out which enable us to fix the correspondent parts of the two narratives, and guide us to the identification and interpretation of their minor details.
The first of these landmarks is the appearance of the dry land, or, in geological language, the commencement of the process of upheaval. At the close of the second day the earth was, in all probability, as we have seen, a globe internally molten, but having a solid crust which was uniformly covered with a layer of water, and surrounded by an atmosphere which, though it had parted with some of its ingredients, was still very much more complex, more dense, and more extensive than it is at present. The newly condensed waters would rest on the surface of the primeval rock, whatever that rock might be. The internal heat conducted through it would keep the waters in a state of intense ebullition, and at the same time their surface would be agitated by violent atmospheric currents as the heated air ascended, and was replaced by cooler air from the outer regions of the atmosphere. Under these circ.u.mstances the water would dissolve or wear down portions of the newly-formed rock on which it rested. At the same time the steam, which would be continually rising from the boiling ocean, would descend from the upper regions of the atmosphere in the form of rain, and bring with it in solution considerable quant.i.ties of those elements which still existed in the form of vapour, just as rain now brings down ammonia and carbonic acid which it has absorbed in its pa.s.sage through the atmosphere. New combinations would thus be formed between the materials dissolved or abraded by the ocean and those brought down by the rain. When these combinations had reached a certain amount they would be deposited in the form of mud upon the bed of the ocean, and thus the earliest sedimentary rocks would be formed. As the temperature gradually decreased, the character of these combinations would probably be changed, and at the same time the atmosphere would be diminished in volume and density, and become more pure by the absorption of a large portion of its original const.i.tuents, which would have been incorporated into various minerals.
The earliest sedimentary rock with which we are acquainted at present is what is known as the Laurentian formation. [Footnote: The whole of the geological details in this section are taken from Sir C. Lyell's Geology for Students.] It occupies an area of 200,000 square miles north of the St. Lawrence; and is also traced into the United States and the western highlands of Scotland and some of the adjacent isles. It is divided into two sections--the Upper and Lower Laurentian. It is not certain that it is really the oldest rock; for as every sedimentary rock is formed of the debris of preceding rocks, it is very possible that all the exposed portions of some older rocks may have been decomposed and worn away; but it is the oldest yet known. The thickness of the lower portion is estimated at 20,000 feet, or nearly four miles, while the Upper Laurentian beds are 10,000 feet thick. At this point we meet with the first traces of that process of upheaval and subsidence which has ever since been going on in the earth.
The Lower Laurentian rocks had been displaced from their original horizontal position before the Upper Laurentian were deposited upon them.