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 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 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.
This process of upheaval of some parts of the earth, accompanied with subsidence in other parts, is one which cannot be accounted for by any natural laws with which we are acquainted. It is in all probability the result of a series of changes which are taking place in the interior of the earth, but of which we know nothing at all. It is in the commencement of this series of changes that we trace that direct interference of the Creator--which is indicated by the command, "Let the waters under the firmament be gathered together into one place, and let the dry land appear." We have not, however, any means of ascertaining how long a period elapsed before the process of upheaval reached the point at which the land would rise above the surface of the ocean.
The Lower Laurentian rocks are remarkable in another way. There is little doubt that traces of life, the earliest yet known, occur in them. They include a bed of limestone varying in thickness from 700 to 1500 feet. In all probability limestone, wherever it occurs, is an animal product, though in many cases all traces of its organization have been lost by exposure to heat. This particular bed appears to have been formed by a very lowly creature, which in organization was akin to the foraminifera, of which large quant.i.ties are now known to exist at the bottom of the Atlantic. It differed from them, however, in one respect--the individuals were connected together, as is the case now with many varieties of the coral animal. No notice of this first appearance of life is found in the Mosaic Record, nor, for reasons already given, was it possible that any mention of it should be made.
The rocks which come next to the Laurentian in the order of time are those known as the Cambrian. They are so called because they const.i.tute a large portion of the mountains of North Wales, and it was there that their characteristics were first carefully studied by Professor Sedgwick. In one of the strata of this formation--the Harlech Grit--what are known as "ripple-marks" are found, proving that parts of these rocks at the time of their deposition formed a sea-beach, and that consequently at this time, at the latest, the dry land had emerged from the ocean. In these rocks there are also decided traces of Volcanic Action, which seem to indicate the existence of a Volcano similar to the recent "Graham"s Island." At this point a considerable advance in animal life is found. The fossils comprise several corals, varieties of mollusca, and a cla.s.s of crustaceans peculiar to the very early rocks--the trilobites.
On the Cambrian rocks rest the formations known as Silurian, from the fact that they were first thoroughly examined in South Wales (Siluria) by Sir E. Murchison. In these rocks many fresh varieties of invertebrate fossils are found, and the vertebrata make their first appearance, numerous remains of fishes having been discovered. The earliest specimen was found in the Lower Ludlow beds at Leintwardine, while the Upper Ludlow formation contains an extensive bed composed almost entirely of fish-bones. Immediately above this bed are found what seem to be traces of land-plants, in the shape of the spores of a cryptogamous plant.
The Silurian rocks are succeeded by rocks which present two distinct characters, but are probably contemporaneous, the Devonian and the old Red Sandstone. The former seem to have been deposited in the bed of the sea, while the latter is a fresh-water formation. In these decided remains of land plants are found, of which about 200 species have at present been discovered. The old Red Sandstone is also peculiarly rich in fossil fish. The first signs of coal appear in this series of rocks, but on a very small scale.
We now come to what are known as the Carboniferous rocks, of which the lower series is known as the mountain limestone, and above it come the "coal measures," containing numerous beds of coal, sometimes of great thickness. These beds have resulted entirely from the decomposition, under peculiar circ.u.mstances, of an enormous development of terrestrial vegetation. They seem to have originated in vast swamps, subject to occasional flooding, and to alternate movements of upheaval and subsidence. On these swamps there must have existed for ages a vegetation of whose luxuriance the richest tropical jungles of the present time can give us no idea. They tell the tale of a time when the temperature of the earth, was uniformly high (since coal fields are found in high northern lat.i.tudes), when the atmosphere was charged with moisture, and probably contained a large proportion of carbonic acid. In the coal measures we come upon the first traces of land animals. Several remains of reptiles have been found, as well as footprints left on the soft mud or sand of a riverbank or sea- beach. There seems to be no doubt that they were left by lung- breathing animals.
The carboniferous strata form the second of our landmarks. They seem to point to the fulfilment of the command that the earth, should bring forth vegetation. There is, however, one point which requires some notice. The Mosaic account, as we read it in our English Bibles, seems to be limited to phanerogamous plants-- gra.s.s, the herb yielding seed, and the fruit-tree yielding fruit.
Now, it is a well-known fact that the great ma.s.s of the vegetation, the remains of which const.i.tute coal, consisted of cryptogamic plants, which do not produce seed, properly so called, but only spores; the distinction being that the spore contains the germ and nothing more, while in the seed the germ is provided with a store of nutriment to a.s.sist in the earlier stages of the development of the plant. What appears to be a farther discrepancy, the absence of any traces of the gra.s.ses, leads in reality to the solution of the difficulty.
The word which is translated "gra.s.s" [Hebrew script] means in reality, any fresh sprout. Now it is remarkable that Moses specifies three kinds of vegetation, with regard to two of which it is noted that they produce seed, while nothing is said of the seed of the remaining cla.s.s. Gra.s.s too, is really a herb bearing seed, and, as such would be included in the second cla.s.s, and there would have been no occasion, to mention it separately. It would appear then that the first cla.s.s consisted of seedless plants, i. e. of the cryptogamia. This conclusion is strengthened when we turn to verses 29 and 30. If the word [Hebrew script] were correctly translated "gra.s.s," we should certainly expect to find it in those verses, since the gra.s.ses contribute more to the food of both man and beast, than all the other herbaceous plants put together. This omission then, is an indication that the word, as used in this chapter, denotes a cla.s.s of plants which are not commonly employed for food, and this condition also is fulfilled in the cryptogamia.
There are then four special points in this period, of which two seem to correspond with the Mosaic record, while the other two are unnoticed in it. The two points of correspondence are the upheaval of the dry land, and the prevalence of a very abundant and luxuriant Flora. As in the case of the fifth and sixth days, the words used with reference to land plants seem to denote a period of remarkable development, rather than the first appearance. The two points unnoticed are the beginnings of animal and vegetable life. In the case of animal life the omission has already been accounted for. The beginning of vegetable life was probably contemporaneous with that of animal life, for each is necessary to the other, since the food of the animal must be prepared by the vegetable, and after being used by the former returns to a state in which it is fitted for the nourishment of the latter. As animal life commenced in the ocean, so in all probability did vegetable life, though no certain traces of it are found in the earliest rocks; but this is easily accounted for by the very perishable character of the simpler forms of algae. Like the earliest animals, the first algae were probably microscopic plants, and the omission of any mention of them was therefore inevitable.
One characteristic of cryptogamic vegetation is important for its bearing on the work of the fourth day. Almost all the phanerogamic plants are dependent for their development upon the direct light and heat of the sun. Deprived of these they either perish entirely, or make an unhealthy growth, and produce little or no fruit. But the cryptogamia, in general, thrive best when they are protected from the direct rays of the sun. They nourish in a diffused light, and with abundant atmospheric moisture. And so we find them at this time doing what seems a very important work in the progress of the world. By taking up and decomposing the excess of carbonic acid which at this time probably existed in the atmosphere, they at once purified that atmosphere, and rendered it fit for the respiration of more highly organized creatures, and laid up in the earth an invaluable store of fuel for the future use of man. The other orders of vegetation seem to have existed in very small proportions at this time, and only in their lower forms. As the conditions of the earth changed, the cryptogamia seemed to have dwindled away, while higher forms of vegetation a.s.serted their supremacy. It is not, however, improbable that a special development at a much later period is indicated by the mention in the second chapter of the formation of the garden of Eden.
SECTION 7. THE FOURTH DAY.
"And G.o.d said, Let there be luminaries in the firmament of heaven to divide between the day and the night, and let them be for signs and for seasons, and for days and for years.
"And let them be for luminaries in the firmament of heaven to give light upon the earth; and it was so.
"And G.o.d made the two luminaries, the great ones; the luminary, the great one, to rule over the day, and the luminary, the small one, to rule over the night, and also the stars.
"And G.o.d gave them in the firmament of heaven to give light upon the earth.
"And to rule over the day and over the night, and to divide between the light and between the darkness; and G.o.d saw that it was good.
"And there was evening, and there was morning, a fourth day."
This day"s work differs from that of the preceding and succeeding days, in the fact that its sphere was without the earth, which was only indirectly influenced by it, and consequently the geological records give us no direct information upon the subject, though in two points they tally with the Mosaical account. In the first place, the deposits of coal, which preceded this period, indicate a time when a nearly uniform temperature, and that a high one, prevailed throughout the world. The coal beds are found not only in tropical regions, but in very high lat.i.tudes. Not only is the vegetation of which these coalfields are the result, a.n.a.logous to that which is now found in warm climates only--(this might be the case, and yet we should not be justified in drawing the inference that the actual species of plants were tropical, for it often happens that different species of the same genus, having considerable external resemblance, are very different in their habits, some requiring tropical heat, while others flourish only in temperate climates)--but the marked feature is the astonishing luxuriance of this vegetation, which could only have been developed under the most favourable circ.u.mstances of warmth and moisture. Now the heat which any particular portion of the earth"s surface receives from the sun depends entirely upon the lat.i.tude.
hence it is impossible that a uniform high temperature could exist in a world which derived its heat wholly or chiefly from that source. Whether the high temperature which prevailed on the earth during the deposition of the coal measures was derived from internal heat it is impossible to say; it is evident that the temperature of the earth"s surface has been in past times, and perhaps is now, modified by causes which no scientific research has been enabled to detect [Footnote: Since the sun"s secular motion has been known, astronomers have suggested that the solar system has been carried through portions of s.p.a.ce having variable temperatures. Geologists, however, do not seem inclined to accept this as a sufficient reason for the phenomena observed.]. But we may safely conclude that during the third day the earth did not derive its heat from the sun. The second point, the barrenness of the geological records of this period, will be noticed hereafter.
The record of the fourth day"s work admits of two interpretations, it may describe things merely as they appeared, or as they actually occurred.
1. It is possible that the events of the fourth day may be described phenomenally--that up to this period the state of things on the earth had been to a great extent similar to that which we have reason to believe is still existing in the planet Jupiter- that the atmosphere was so charged with vapour that no direct rays from the heavenly bodies could penetrate it; but that at this time, owing to the declining heat, a great part of the aqueous const.i.tuents of this vapour had been precipitated in the form of rain, while other vapours had entered into chemical combinations with other elements to form the various minerals of the earth"s surface, and the atmosphere had become first translucent, and then transparent. While this process was going on, no direct light from the sun, supposing it to be already in existence, could penetrate the veil. Diffused light only could reach the earth"s surface, but when the atmosphere became clear the sun, moon, and stars would become visible.
Against this view several objections may be brought. In the first place, as has been already noticed, we cannot treat the account of the Creation as derived from ordinary human sources. Either it is a revelation from the Creator or it is nothing. Now we can readily admit that a man, speaking of an event which lie had witnessed, but did not understand, would describe it as it appeared to him, but we cannot admit this supposition when the work is described by the Great Artificer Himself. In the next place, the temperature of the earth"s surface must in this case have been affected by the sun, and must therefore have been more or less dependent upon lat.i.tude--and in the third place the distinction between day and night must have come into operation, whereas the narrative implies that it was yet incomplete.
2. The other possible interpretation is, that at this period the concentration of light and heat in the sun was so far completed that he became the luminary of the system, which had hitherto derived its light and heat from other sources. Probably, for a long time, the internal heat of the planets may have been so great that they were a light to themselves. This state of things, however, must have come to an end before animal or vegetable life could have existed on their surface, but other ways exist, and are in operation in other parts of the universe, by which light and heat might have been supplied independently of the sun. That light which is now gathered up in the sun might for a long time have existed as a nebulous ring, similar to the well-known Ring Nebula in Lyra. Any planets existing within such a ring would probably derive from it sufficient light and heat. Or the nebulous matter, in a luminous state, while slowly advancing to concentration, might as yet have been so diffused as to fill a s.p.a.ce in which the earth"s...o...b..t was included. In either case the earth would have received a uniform diffused light, without any alternations of night and day. It is of course impossible that we should be able to say whether there are any worlds in which such a state of things prevails at present. Up to this time, with one possible exception, [Footnote: "Sirius is accompanied by a 10 mag. star, whose existence was suspected (like that of Neptune), long before its discovery by Alvan Clark in 1861, from the irregular movements of its primary. But though it appears so small, its disturbing effects can only be accounted for on the supposition that its ma.s.s is at least half that of Sirius, in which case its light must be very faint, possibly wholly reflected." (Webb"s Celestial Objects, p. 202.)] the only worlds which the telescope has revealed to us, beyond the limits of our own system, are self-luminous. No reflected light is strong enough to make its existence perceptible at such enormous distances in the most powerful telescope which has yet been constructed.
There are some facts connected with our own system which make it appear not improbable that up to the time of which we are speaking the light which is now gathered up in the sun was diffused over a s.p.a.ce in which at all events the earth"s...o...b..t was included. It is now a recognized fact that all the light of the system is not as yet wholly concentrated in the sun, as we generally recognize it, but that to some extent the sun is still a nebulous star. Under ordinary circ.u.mstances we see only that circular disc, which we usually recognize as the sun. Its surpa.s.sing brightness overpowers every thing else, whether we view it with the unaided eye or through the telescope. But when the actual disc is hidden from us by the moon in a total eclipse, other regions of light surrounding the disc, make their appearance, and in them the most wonderful processes are continually going on. The simultaneous discoveries of Messrs. Lockyer and Janssen, in 1868, have enabled some of these processes to be continuously watched when the sun is not eclipsed, but others can as yet only be seen during the few minutes (never amounting to seven) which a total eclipse lasts, so that as yet we know very little of them.
Immediately surrounding the disc of the sun, which is visible to the naked eye, is a brilliant ring of light, known now as the chromosphere or sierra. This is the region which till 1868 could be seen only during total eclipses, but can now be watched at all times by means of the spectroscope. In it symptoms of intense action are from time to time witnessed. For many years past, whenever a total eclipse occurred, there were observed on the edge of this ring certain red prominences. The spectroscope has revealed their nature. They consist chiefly of enormous volumes of hydrogen, ejected from the surface of the sun with a velocity almost inconceivable, and at the same time revolving about their axis after the fashion of a cyclone. [Footnote: Popular Science Review, January, 1872, p. 150; Look. Byer"s Lecture on the Sun, at Manchester, 1871.] A very remarkable instance of this was observed in America in September 1871, by Professor Young. A ma.s.s of incandescent hydrogen was propelled to a height of 200,000 miles above the visible disc; of these the last 100,000 miles were pa.s.sed through in 10 minutes. Such events, though not commonly on so vast a scale, are continually occurring on the surface of the sun, and they seem to be in close connexion with the magnetic phenomena occurring on the earth.
Beyond the chromosphere lies the corona. The spectroscope has not yet rendered this visible at all times, and consequently we are dependent upon the information to be obtained during the few minutes of total eclipses, when alone it is visible. Consequently during recent solar eclipses this has been the point to which the attention of astronomers has been especially devoted. The eclipse of December, 1870, decided one point, that the corona was a truly solar phenomenon, and not, as some astronomers imagined, an optical phenomenon, produced by our own atmosphere. The corona presents the appearance of nebulous light, fading as it becomes more remote from the sun, of very irregular outline, at some points not extending more than 15", at others as much as 60" or 70" from the sun"s disc, or, in other words, reaching to distances from the sun"s surface varying from 400,000 to 1,800,000 miles.
More important information has been obtained from the eclipse of December 12,1871. It is now ascertained that the corona comprises not only gaseous elements, especially hydrogen, but also solid or fluid particles, capable of giving a continuous though very faint spectrum with dark lines, indicating the existence of matter capable of reflecting light. The character of the coronal spectrum very much resembles that of the Nebula in Draco, No. 4373. The ascertained extent of the corona exceeds a million of miles above the surface of the sun, and it seems probable that the Zodiacal light is only a fainter extension of it. [Footnote: Popular Science Review, April, 1872, pp. 136-146.]