Chapter VIII…

Chapter VIII

PLANETS AND SATELLITES--(continued)

"The analogy between Mars and the earth is perhaps by far the greatest in the whole solar system." So Herschel wrote in 1783,[965] and so we may safely say to-day, after six score further years of scrutiny. The circumstance lends a particular interest to inquiries into the physical habitudes of our exterior planetary neighbour.

Fontana first caught glimpses, at Naples in 1636 and 1638,[966] of dusky stains on the ruddy disc of Mars. They were next seen by Hooke and Cassini in 1666, and this time with sufficient distinctness to serve as indexes to the planet's rotation, determined by the latter as taking place in a period of twenty-four hours forty minutes.[967] Increased confidence was given to this result through Maraldi's precise verification of it in 1719.[968] Among the spots observed by him, he distinguished two as stable in position, though variable in size. They were of a peculiar character, showing as bright patches round the poles, and had already been noticed during sixty years back. A current conjecture of their snowy nature obtained validity when Herschel connected their fluctuations in extent with the progress of the Martian seasons. The inference of frozen precipitations could scarcely be resisted when once it was clearly perceived that the shining polar zones did actually by turns diminish and grow with the alternations of summer and winter in the corresponding hemisphere.

This, it may be said, was the opening of our acquaintance with the state of things prevailing on the surface of Mars. It was accompanied by a steady assertion, on Herschel's part, of permanence in the dark markings, notwithstanding partial obscurations by clouds and vapours floating in a "considerable but moderate atmosphere." Hence the presumed inhabitants of the planet were inferred to "probably enjoy a situation in many respects similar to ours."[969]

Schroeter, on the other hand, went altogether wide of the truth as regards Mars. He held that the surface visible to us is a mere shell of drifting cloud, deriving a certain amount of apparent stability from the influence on evaporation and condensation of subjacent but unseen areographical features;[970] and his opinion prevailed with his contemporaries. It was, however, rejected by Kunowsky in 1822, and finally overthrown by Beer and Maedler's careful studies during five consecutive oppositions, 1830-39. They identified at each the same dark spots, frequently blurred with mists, especially when the local winter prevailed, but fundamentally unchanged.[971] In 1862 Lockyer established a "marvellous agreement" with Beer and Maedler's results of 1830, leaving no doubt as to the complete fixity of the main features, amid "daily, nay, hourly," variations of detail through transits of clouds.[972] On seventeen nights of the same opposition, F. Kaiser of Leyden obtained drawings in which nearly all the markings noted in 1830 at Berlin reappeared, besides spots frequently seen respectively by Arago in 1813, by Herschel in 1783, and one sketched by Huygens in 1672 with a writing-pen in his diary.[973] From these data the Leyden observer arrived at a period of rotation of 24h. 37m. 22.62s., being just one second shorter than that deduced, exclusively from their own observations, by Beer and Maedler. The exactness of this result was practically confirmed by the inquiries of Professor Bakhuyzen of Leyden.[974] Using for a middle term of comparison the disinterred observations of Schroeter, with those of Huygens at one, and of Schiaparelli at the other end of an interval of 220 years, he was enabled to show, with something like certainty, that the time of rotation (24h. 37m. 22.735s.) ascribed to Mars by Mr. Proctor[975] in reliance on a drawing executed by Hooke in 1666, was too long by nearly one-tenth of a second. The minuteness of the correction indicates the nicety of care employed. Nor employed vainly; for, owing to the comparative antiquity of the records available in this case, an almost infinitesimal error becomes so multiplied by frequent repetition as to produce palpable discrepancies in the positions of the markings at distant dates. Hence Bakhuyzen's period of 24h. 37m. 22.66s. is undoubtedly of a precision unapproached as regards any other heavenly body save the earth itself.

Two facts bearing on the state of things at the surface of Mars were, then, fully acquired to science in or before the year 1862. The first was that of the seasonal fluctuations of the polar spots; the second, that of the general permanence of certain dark gray or greenish patches, perceived with the telescope as standing out from the deep yellow ground of the disc. That these varieties of tint correspond to the real diversities of a terraqueous globe, the "ripe cornfield"[976] sections representing land, the dusky spots and streaks, oceans and straits, has long been the prevalent opinion. Sir J. Herschel in 1830 led the way in ascribing the redness of the planet's light to an inherent peculiarity of soil.[977] Previously it had been assimilated to our sunset glows rather than to our red sandstone formations--set down, that is, to an atmospheric stoppage of blue rays. But the extensive Martian atmosphere, implicitly believed in on the strength of some erroneous observations by Cassini and Roemer in the seventeenth century, vanished before the sharp occultation of a small star in Leo, witnessed by Sir James South in 1822;[978] and Dawes's observation in 1865,[979] that the ruddy tinge is deepest near the central parts of the disc, certified its non-atmospheric origin. The absolute whiteness of the polar snow-caps was alleged in support of the same inference by Sir William Huggins in 1867.[980]

All recent operations tend to show that the atmosphere of Mars is much thinner than our own. This was to have been expected a priori, since the same proportionate mass of air would on his smaller globe form a relatively sparse covering.[981] Besides, gravity there possesses less than four-tenths its force here, so that this sparser covering would weigh less, and be less condensed, than if it enveloped the earth. Atmospheric pressure would accordingly be of about two and a quarter, instead of fifteen terrestrial pounds per square inch. This corresponds with what the telescope shows us. It is extremely doubtful whether any features of the earth's actual surface could be distinguished by a planetary spectator, however well provided with optical assistance. Professor Langley's inquiries[982] led him to conclude that fully twice as much light is absorbed by our air as had previously been supposed--say 40 per cent. of vertical rays in a clear sky. Of the sixty reaching the earth, less than a quarter would be reflected even from white sandstone; and this quarter would again pay heavy toll in escaping back to space. Thus not more than perhaps ten or twelve out of the original hundred sent by the sun would, under the most favourable circumstances, and from the very centre of the earth's disc, reach the eye of a Martian or lunar observer. The light by which he views our world is, there is little doubt, light reflected from the various strata of our atmosphere, cloud or mist-laden or serene, as the case may be, with an occasional snow-mountain figuring as a permanent white spot.

This consideration at once shows us how much more tenuous the Martian air must be, since it admits of topographical delineations of the Martian globe. The clouds, too, that form in it seem in general to be rather of the nature of ground-mists than of heavy cumulus.[983] Occasionally, indeed, durable and extensive strata become visible. During the latter half of October, 1894, for instance, a region as large as Europe remained apparently cloud-covered. Yet most recent observers are unable to detect the traces of aqueous absorption in the Martian spectrum noted by Huggins in 1867[984] and by Vogel in 1873.[985] Campbell vainly looked for them,[986] visually in 1894, spectrographically in 1896; Keeler was equally unsuccessful;[987] Jewell[988] holds that they could, with present appliances, only be perceived if the atmosphere of Mars were much richer in water-vapour than that of the earth. There can be little doubt, however, that its supply is about the minimum adequate to the needs of a living, and perhaps a life-nuturing planet.

The climate of Mars seems to be unexpectedly mild. Its theoretical mean temperature, taking into account both distance from the sun and albedo, is 34 deg. C. below freezing.[989] Yet its polar snows are both less extensive and less permanent than those on the earth. The southern white hood, noticed by Schiaparelli in 1877 to have survived the summer only as a small lateral patch, melted completely in 1894. Moreover, Mr. W. H. Pickering observed with astonishment the disappearance, in the course of thirty-three days of June and July, 1892, of 1,600,000 square miles of southern snow.[990] Curiously enough, the initial stage of shrinkage in the white calotte was marked by its division into two unequal parts, as if in obedience to the mysterious principle of duplication governing so many Martian phenomena.[991] Changes of the hues associated respectively with land and water accompanied in lower latitudes, and were thought to be occasioned by floods ensuing upon this rapid antarctic thaw. It is true that scarcity of moisture would account for the scantiness and transitoriness of snowy deposits easily liquefied because thinly spread. But we might expect to see the whole wintry hemisphere, at any rate, frost-bound, since the sun radiates less than half as much heat on Mars as on the earth. Water seems, nevertheless, to remain, as a rule, uncongealed everywhere outside the polar regions. We are at a loss to imagine by what beneficent arrangement the rigorous conditions naturally to be looked for can be modified into a climate which might be found tolerable by creatures constituted like ourselves.

Martian topography may be said to form nowadays a separate sub-department of descriptive astronomy. The amount of detail become legible by close scrutiny on a little disc which, once in fifteen years, attains a maximum of about 1/5000 the area of the full moon, must excite surprise and might provoke incredulity. Spurious discoveries, however, have little chance of holding their own where there are so many competitors quite as ready to dispute as to confirm.

The first really good map of Mars was constructed in 1869 by Proctor from drawings by Dawes. Kaiser of Leyden followed in 1872 with a representation founded upon data of his own providing in 1862-64; and Terby, in his valuable Areographie, presented to the Brussels Academy in 1873[992] a careful discussion of all important observations from the time of Fontana downwards, thus virtually adding to knowledge by summarising and digesting it. The memorable opposition of September 5, 1877, marked a fresh epoch in the study of Mars. While executing a trigonometrical survey (the first attempted) of the disc, then of the unusual size of 25" across, G. V. Schiaparelli, director of the Milan Observatory, detected a novel and curious feature. What had been taken for Martian continents were found to be, in point of fact, agglomerations of islands, separated from each other by a network of so-called "canals" (more properly channels).[993] These are obviously extensions of the "seas," originating and terminating in them, and sharing their gray-green hue, but running sometimes to a length of three or four thousand miles in a straight line, and preserving throughout a nearly uniform breadth of about sixty miles. Further inquiries have fully substantiated the discovery made at the Brera Observatory. The "canals" of Mars are an actually existent and permanent phenomenon. An examination of the drawings in his possession showed M. Terby that they had been seen, though not distinctively recognised, by Dawes, Secchi, and Holden; several were independently traced out by Burton at the opposition of 1879; all were recovered by Schiaparelli himself in 1879 and 1881-82; and their indefinite multiplication resulted from Lovell's observations in 1894 and 1896.

When the planet culminated at midnight, and was therefore in opposition, December 26, 1881, its distance was greater, and its apparent diameter less than in 1877, in the proportion of sixteen to twenty-five. Its atmosphere was, however, more transparent, and ours of less impediment to northern observers, the object of scrutiny standing considerably higher in northern skies. Never before, at any rate, had the true aspect of Mars come out so clearly as at Milan, with the 8-3/4-inch Merz refractor of the observatory, between December, 1881, and February, 1882. The canals were all again there, but this time they were--in as many as twenty cases--seen in duplicate. That is to say, a twin-canal ran parallel to the original one at an interval of 200 to 400 miles.[994]

We are here brought face to face with an apparently insoluble enigma. Schiaparelli regards the "germination" of his canals as a periodical phenomenon depending on the Martian seasons. It is, assuredly, not an illusory one, since it was plainly apparent, during the opposition of 1886, to MM. Perrotin and Thollon at Nice,[995] and to the former, using the new 30-inch refractor of that observatory, in 1888; Mr. A. Stanley Williams, with the help of only a 6-1/2-inch reflector, distinctly perceived in 1890 seven of the duplicate objects noted at Milan,[996] and the Lick observations, both of 1890 and of 1892, together with the drawings made at Flagstaff and Mexico during the last favourable oppositions of the nineteenth century, brought unequivocal confirmation to the accuracy of Schiaparelli's impressions.[997] Various conjectures have been hazarded in explanation of this bizarre appearance. The difficulty of conceiving a physical reality corresponding to it has suggested recourse to an optical rationale. Proctor regarded it as an effect of diffraction;[998] Stanislas Meunier, of oblique reflection from overlying mist-banks;[999] Flammarion considers it possible that companion-canals might, under special circumstances, be evoked by refraction as a kind of mirage.[1000] But none of these speculations are really admissible, when all the facts are taken into account. The view that the canals of Mars are vast rifts due to the cooling of the globe, is recommended by the circumstance that they tend to follow great circles; nevertheless, it would break down if, as Schiaparelli holds, the fluctuations in their visibility depend upon actual obliterations and re-emergencies. Fantastic though the theory of their artificial origin appear, it is held by serious astronomers. Its vogue is largely due to Mr. Lowell's ingenious advocacy. He considers the Martian globe to be everywhere intersected by an elaborate system of irrigation-works, rendered necessary by a perennial water-famine, relieved periodically by the melting of the polar snows. Nor does he admit the existence of oceans, or lakes. What have been taken for such are really tracts covered with vegetation, the bright areas intermixed with them representing sandy deserts. And it is noteworthy in this connection that Professor Barnard obtained in 1894,[1001] with the great Lick refractor, "suggestive and impressive views" disclosing details of light and shade on the gray-green patches so intricate and minute as almost to preclude the supposition of their aqueous nature.

The closeness of the terrestrial analogy has thus of late been much impaired. Even if the surface of Mars be composed of land and water, their distribution must be of a completely original type. The interlacing everywhere of continents with arms of the sea (if that be the correct interpretation of the visual effects) implies that their levels scarcely differ;[1002] and Schiaparelli carries most observers with him in holding that their outlines are not absolutely constant, encroachments of dusky upon bright tints suggesting extensive inundations.[1003] The late N. E. Green's observations at Madeira in 1877 indicated, on the other hand, a rugged south polar region. The contour of the snow-cap not only appeared indented, as if by valleys and promontories, but brilliant points were discerned outside the white area, attributed to isolated snow-peaks.[1004] Still more elevated, if similarly explained, must be the "ice island" first seen in a comparatively low latitude by Dawes in January, 1865.

On August 4, 1892, Mars stood opposite to the sun at a distance of only 34,865,000 miles from the earth. In point of vicinity, then, its situation was scarcely less favourable than in 1877. The low altitude of the planet, however, practically neutralised this advantage for northern observers, and public expectation, which had been raised to the highest pitch by the announcements of sensation-mongers, was somewhat disappointed at the "meagreness" of the news authentically received from Mars. Valuable series of observations were, nevertheless, made at Lick and Arequipa; and they unite in testifying to the genuine prevalence of surface-variability, especially in certain regions of intermediate tint, and perhaps of the "crude consistence" of "boggy Syrtes, neither sea, nor good dry land." Professor Holden insisted on the "enormous difficulties in the way of completely explaining the recorded phenomena by terrestrial analogies";[1005] Mr. W. H. Pickering spoke of "conspicuous and startling changes." They, however, merely overlaid, and partially disguised, a general stability. Among the novelties detected by Mr. Pickering were a number of "lakes," or "oases" (in Lowell's phraseology), under the aspect of black dots at the junctions of two or more canals;[1006] and he, no less than the Lick astronomers and M. Perrotin at Nice,[1007] observed brilliant clouds projecting beyond the terminator, or above the limb, while carried round by the planet's rotation. They seemed to float at an altitude of at least twenty miles, or about four times the height of terrestrial cirrus; but this was not wonderful, considering the low power of gravity acting upon them. Great capital was made in the journalistic interest out of these imaginary signals from intelligent Martians, desirous of opening communications with (to them) problematical terrestrial beings. Similar effects had, however, been seen before by Mr. Knobel in 1873, by M. Terby in 1888, and at the Lick Observatory in 1890; and they were discerned again with particular distinctness by Professor Hussey at Lick, August 27, 1896.[1008]

The first photograph of Mars was taken by Gould at Cordoba in 1879. Little real service in planetary delineation has, it is true, been so far rendered by the art, yet one achievement must be recorded to its credit. A set of photographs obtained by Mr. W. H. Pickering on Wilson's Peak, California, April 9, 1890, showed the southern polar cap of Mars as of moderate dimensions, but with a large dim adjacent area. Twenty-four hours later, on a corresponding set, the dim area was brilliantly white. The polar cap had become enlarged in the interim, apparently through a wide-spreading snow-fall, by the annexation of a territory equal to that of the United States. The season was towards the close of winter in Mars. Never until then had the process of glacial extension been actually (it might be said) superintended in that distant globe.

Mars was gratuitously supplied with a pair of satellites long before he was found actually to possess them. Kepler interpreted Galileo's anagram of the "triple" Saturn in this sense; they were perceived by Micromegas on his long voyage through space; and the Laputan astronomers had even arrived at a knowledge, curiously accurate under the circumstances, of their distances and periods. But terrestrial observers could see nothing of them until the night of August 11, 1877. The planet was then within one month of its second nearest approach to the earth during the last century; and in 1845 the Washington 26-inch refractor was not in existence.[1009] Professor Asaph Hall, accordingly, determined to turn the conjecture to account for an exhaustive inquiry into the surroundings of Mars. Keeping his glaring disc just outside the field of view, a minute attendant speck of light was "glimpsed" August 11. Bad weather, however, intervened, and it was not until the 16th that it was ascertained to be what it appeared--a satellite. On the following evening a second, still nearer to the primary, was discovered, which, by the bewildering rapidity of its passages hither and thither, produced at first the effect of quite a crowd of little moons.[1010]

Both these delicate objects have since been repeatedly observed, both in Europe and America, even with comparatively small instruments. At the opposition of 1884, indeed, the distance of the planet was too great to permit of the detection of both elsewhere than at Washington. But the Lick equatoreal showed them, July 18, 1888, when their brightness was only 0.12 its amount at the time of their discovery; so that they can now be followed for a considerable time before and after the least favourable oppositions.

The names chosen for them were taken from the Iliad, where "Deimos" and "Phobos" (Fear and Panic) are represented as the companions in battle of Ares. In several respects, they are interesting and remarkable bodies. As to size, they may be said to stand midway between meteorites and satellites. From careful photometric measures executed at Harvard in 1877 and 1879, Professor Pickering concluded their diameters to be respectively six and seven miles.[1011] This is on the assumption that they reflect the same proportion of the light incident upon them that their primary does. But it may very well be that they are less reflective, in which case they would be more extensive. The albedo of Mars is put by Mueller at 0.27; his surface, in other words, returns 27 per cent. of the rays striking it. If we put the albedo of his satellites equal to that of our moon, 0.17, their diameters will be increased from 6 and 7 to 7-1/2 and 9 miles, Phobos, the inner one, being the larger. Mr. Lowell, however, formed a considerably larger estimate of their dimensions.[1012] It is interesting to note that Deimos, according to Professor Pickering's very distinct perception, does not share the reddish tint of Mars.

Deimos completes its nearly circular revolutions in thirty hours eighteen minutes, at a distance from the surface of its ruling body of 12,500 miles; Phobos traverses an elliptical orbit[1013] in seven hours thirty-nine minutes twenty-two seconds, at a distance of only 3,760 miles. This is the only known instance of a satellite circulating faster than its primary rotates, and is a circumstance of some importance as regards theories of planetary development. To a Martian spectator the curious effect would ensue of a celestial object, seemingly exempt from the general motion of the sphere, rising in the west, setting in the east, and culminating twice, or even thrice a day; which, moreover, in latitudes above 69 deg. north or south, would be permanently and altogether hidden by the intervening curvature of the globe.

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The detection of new members of the solar system has come to be one of the most ordinary of astronomical events. Since 1846 no single year has passed without bringing its tribute of asteroidal discovery. In the last of the seventies alone, a full score of miniature planets were distinguished from the thronging stars amid which they seem to move; 1875 brought seventeen such recognitions; their number touched a minimum of one in 1881; it rose in 1882, and again in 1886, to eleven; dropped to six in 1889, and sprang up with the aid of photography to twenty-seven in 1892. That high level has since, on an average, been maintained; and on January 1, 1902, nearly 500 asteroids were recognised as revolving between the orbits of Mars and Jupiter. Of these, considerably more than one hundred are claimed by one investigator alone--Dr. Max Wolf of Heidelburg; M. Charlois of Nice comes second with 102; while among the earlier observers Palisa of Vienna contributed 86, and C. H. F. Peters of Clinton (N. Y.), whose varied and useful career terminated July 19, 1890, 52 to the grand total. The construction by Chacornac and his successors at Paris, and more recently by Peters at Clinton, of ecliptical charts showing all stars down to the thirteenth and fourteenth magnitudes respectively, rendered the picking out of moving objects above that brightness a mere question of time and diligence. Both, however, are vastly economised by the photographic method. Tedious comparisons of the sky with charts are no longer needed for the identification of unrecorded, because simulated stars. Planetary bodies declare themselves by appearing upon the plate, not in circular, but in linear form. Their motion converts their images into trails, long or short according to the time of exposure. The first asteroid (No. 323) thus detected was by Max Wolf, December 22, 1891.[1014] Eighteen others were similarly discovered in 1892, by the same skilful operator; and ten more through Charlois's adoption at Nice of the novel plan now in exclusive use for picking up errant light-specks. Far more onerous than the task of their discovery is that of keeping them in view once discovered--of tracking out their paths, ixing their places, and calculating the disturbing effects upon them of the mighty Jovian mass. These complex operations have come to be centralised at Berlin under the superintendence of Professor Tietjen, and their results are given to the public through the medium of the Berliner Astronomisches Jahrbuch.

The cui bono? however, began to be agitated. Was it worth while to maintain a staff of astronomers for the sole purpose of keeping hold over the identity of the innumerable component particles of a cosmical ring? The prospect, indeed, of all but a select few of the asteroids being thrown back by their contemptuous captors into the sea of space seemed so imminent that Professor Watson provided by will against the dereliction of the twenty-two discovered by himself. But the fortunes of the whole family improved through the distinction obtained by one of them. On August 14, 1898, the trail of a rapidly-moving, star-like object of the eleventh magnitude imprinted itself on a plate exposed by Herr Witt at the Urania Observatory, Berlin. Its originator proved to be unique among asteroids. "Eros" is, in sober fact,

'one of those mysterious stars Which hide themselves between the Earth and Mars,'

divined or imagined by Shelley.[1015] True, several of its congeners invade the Martian sphere at intervals; but the proper habitat of Eros is within that limit, although its excursions transcend it. In other words, its mean distance from the sun is about 135, as compared with the Martian distance of 141 million miles. Further, its orbit being so fortunately circumstanced as to bring it once in sixty-seven years within some 15 millions of miles of the earth, it is of extraordinary value to celestial surveyors. The calculation of its movements was much facilitated by detections, through a retrospective search,[1016] of many of its linear images among the star-dots on the Harvard plates.[1017] The little body--which can scarcely be more than twenty miles in diameter--shows peculiarities of behaviour as well as of position. Dr. von Oppolzer, in February, 1901,[1018] announced it to be extensively and rapidly variable. Once in 2 hours 38 minutes it lost about three-fourths of its light,[1019] but these fluctuations quickly diminished in range, and in the beginning of May ceased altogether.[1020] Evidently, then, they depend upon the situation of the asteroid relatively to ourselves; and, so far, events lent countenance to M. Andre's eclipse hypothesis, since mutual occultations of the supposed planetary twins could only take place when the plane of their revolutions passed through the earth, and this condition would be transitory. Yet the recognition in Eros of an "Algol asteroid" seems on other grounds inadmissible;[1021] nor until the phenomenon is conspicuously renewed--as it probably will be at the opposition of 1903--can there be much hope of finding its appropriate rationale.

The crowd of orbits disclosed by asteroidal detections invites attentive study. D'Arrest remarked in 1851,[1022] when only thirteen minor planets were known, that supposing their paths to be represented by solid hoops, not one of the thirteen could be lifted from its place without bringing the others with it. The complexity of interwoven tracks thus illustrated has grown almost in the numerical proportion of discovery. Yet no two actually intersect, because no two lie exactly in the same plane, so that the chances of collision are at present nil. There is only one case, indeed, in which it seems to be eventually possible. M. Lespiault has pointed out that the curves traversed by "Fides" and "Maia" approach so closely that a time may arrive when the bodies in question will either coalesce or unite to form a binary system.[1023]

The maze threaded by the 500 asteroids contrasts singularly with the harmoniously ordered and rhythmically separated orbits of the larger planets. Yet the seeming confusion is not without a plan. The established rules of our system are far from being totally disregarded by its minor members. The orbit of Pallas, with its inclination of 34 deg. 42', touches the limit of departure from the ecliptic level; the average obliquity of the asteroidal paths is somewhat less than that of the sun's equator;[1024] their mean eccentricity is below that of the curve traced out by Mercury, and all without exception are pursued in the planetary direction--from west to east.

The zone in which these small bodies travel is about three times as wide as the interval separating the earth from the sun. It extends perilously near to Jupiter, and dovetails into the sphere of Mars.

Their distribution is very unequal. They are most densely congregated about the place where a single planet ought, by Bode's Law, to revolve; it may indeed be said that only stragglers from the main body are found more than fifty million miles within or without a mean distance from the sun 2.8 times that of the earth. Significant gaps, too, occur where some force prohibitive of their presence would seem to be at work. The probable nature of that force was suggested by the late Professor Kirkwood, first in 1866, when the number of known asteroids was only eighty-eight, and again with more confidence in 1876, from the study of a list then run up to 172.[1025] It appears that these bare spaces are found just where a revolving body would have a period connected by a simple relation with that of Jupiter. It would perform two or three circuits to his one, five to his two, nine to his five, and so on. Kirkwood's inference was that the gaps in question were cleared of asteroids by the attractive influence of Jupiter. For disturbances recurring time after time--owing to commensurability of periods--nearly at the same part of the orbit, would have accumulated until the shape of that orbit was notably changed. The body thus displaced would have come in contact with other cosmical particles of the same family with itself--then, it may be assumed, more evenly scattered than now--would have coalesced with them, and permanently left its original track. In this way the regions of maximum perturbation would gradually have become denuded of their occupants.

We can scarcely doubt that this law of commensurability has largely influenced the present distribution of the asteroids. But its effects must have been produced while they were still in an unformed, perhaps a nebular condition. In a system giving room for considerable modification through disturbance, the recurrence of conjunctions with a dominating mass at the same orbital point need not involve instability.[1026] On the whole, the correspondence of facts with Kirkwood's hypothesis has not been impaired by their more copious collection.[1027] Some chasms of secondary importance have indeed been bridged; but the principal stand out more conspicuously through the denser scattering of orbits near their margins. Nor is it doubtful that the influence of Jupiter in some way produced them. M. de Freycinet's study of the problem they present[1028] has, however, led him to the conclusion that they existed ab origine, thus testifying rather to the preventive than to the perturbing power of the giant planet.

The existence, too, of numerous asteroidal pairs travelling in approximately coincident tracks, must date from a remote antiquity. They result, Professor Kirkwood[1029] believed, from the divellent action of Jupiter upon embryo pigmy planets, just as comets moving in pursuit of one another are a consequence of the sundering influence of the sun.

Leverrier fixed, in 1853,[1030] one-fourth of the earth's mass as the outside limit for the combined masses of all the bodies circulating between Mars and Jupiter; but it is far from probable that this maximum is at all nearly approached. M. Berberich[1031] held that the moon would more than outweigh the whole of them, a million of the lesser bodies shining like stars of the twelfth magnitude being needed, according to his judgment, to constitute her mass. And M. Niesten estimated that the whole of the 216 asteroids discovered up to August, 1880, amounted in volume to only 1/4000th of our globe,[1032] and we may safely add--since they are tolerably certain to be lighter, bulk for bulk, than the earth--that their proportionate mass is smaller still. A fairly concordant result was published in 1895 by Mr. B. M. Roszel.[1033] He found that the lunar globe probably contains forty times, the terrestrial globe 3,240 times the quantity of matter parcelled out among the first 311 minor planets. The actual size of a few of them may now be said to be known. Professor Pickering, from determinations of light-intensity, assigned to Vesta a diameter of 319 miles, to Pallas 167, to Juno 94, down to twelve and fourteen for the smaller members of the group.[1034] An albedo equal to that of Mars was assumed as the basis of the calculation. Moreover, Professor G. Mueller[1035] of Potsdam examined photometrically the phases of seven among them, of which four--namely, Vesta, Iris, Massalia, and Amphitrite--were found to conform precisely to the behaviour of Mars as regards light-change from position, while Ceres, Pallas, and Irene varied after the manner of the moon and Mercury. The first group were hence inferred to resemble Mars in physical constitution, nature of atmosphere, and reflective capacity; the second to be moon-like bodies.

Finally, Professor Barnard, directly measuring with the Yerkes refractor the minute discs presented by the original quartette, obtained the following authentic data concerning them:[1036] Diameter of Ceres, 477 miles, albedo = 0.18; diameter of Pallas, 304 miles, albedo = 0.23; diameter of Vesta, 239 miles, albedo = 0.74; diameter of Juno, 120 miles, albedo = 0.45. Thus, the rank of premier asteroid proves to belong to Ceres, and to have been erroneously assigned to Vesta in consequence of its deceptive brilliancy. What kind of surface this indicates, it is hard to say. The dazzling whiteness of snow can hardly be attributed to bare rock; yet the dynamical theory of gases--as Dr. Johnstone Stoney pointed out in 1867[1037]--prohibits the supposition that bodies of insignificant gravitative power can possess aerial envelopes. Even our moon, it is calculated, could not permanently hold back the particles of oxygen, nitrogen, or water-gas from escaping into infinite space; still less, a globe one thousand times smaller. Vogel's suspicion of an air-line in the spectrum of Vesta[1038] has, accordingly, not been confirmed.

* * * * *

Crossing the zone of asteroids on our journey outward from the sun, we meet with a group of bodies widely different from the "inferior" or terrestrial planets. Their gigantic size, low specific gravity, and rapid rotation, obviously from the first threw the "superior" planets into a class apart; and modern research has added qualities still more significant of a dissimilar physical constitution. Jupiter, a huge globe 86,000 miles in diameter, stands pre-eminent among them. He is, however, only primus inter pares; all the wider inferences regarding his condition may be extended, with little risk of error, to his fellows; and inferences in his case rest on surer grounds than in the case of the others, from the advantages offered for telescopic scrutiny by his comparative nearness.

Now the characteristic modern discovery concerning Jupiter is that he is a body midway between the solar and terrestrial stages of cosmical existence--a decaying sun or a developing earth, as we choose to put it--whose vast unexpended stores of internal heat are mainly, if not solely, efficient in producing the interior agitations betrayed by the changing features of his visible disc. This view, impressed upon modern readers by Mr. Proctor's popular works, was anticipated in the last century. Buffon wrote in his Epoques de la Nature (1778):[1039]--"La surface de Jupiter est, comme l'on sait, sujette a des changemens sensibles, qui semblent indiquer que cette grosse planete est encore dans un etat d'inconstance et de bouillonnement."

Primitive incandescence, attendant, in his fantastic view, on planetary origin by cometary impacts with the sun, combined, he concluded, with vast bulk to bring about this result. Jupiter has not yet had time to cool. Kant thought similarly in 1785;[1040] but the idea did not commend itself to the astronomers of the time, and dropped out of sight until Mr. Nasmyth arrived at it afresh in 1853.[1041] Even still, however, terrestrial analogies held their ground. The dark belts running parallel to the equator, first seen at Naples in 1630, continued to be associated--as Herschel had associated them in 1781--with Jovian trade-winds, in raising which the deficient power of the sun was supposed to be compensated by added swiftness of rotation. But opinion was not permitted to halt here.

In 1860 G. P. Bond of Cambridge (U.S.) derived some remarkable indications from experiments on the light of Jupiter.[1042] They showed that fourteen times more of the photographic rays striking it are reflected by the planet than by our moon, and that, unlike the moon, which sends its densest rays from the margin, Jupiter is brightest near the centre. But the most perplexing part of his results was that Jupiter actually seemed to give out more light than he received. Bond, however, rightly considered his data too uncertain for the support of so bold an assumption as that of original luminosity, and, even if the presence of native light were proved, thought that it might emanate from auroral clouds of the terrestrial kind. The conception of a sun-like planet was still a remote, and seemed an extravagant one.

Only since it was adopted and enforced by Zoellner in 1865,[1043] can it be regarded as permanently acquired to science. The rapid changes in the cloud-belts both of Jupiter and Saturn, he remarked, attest a high internal temperature. For we know that all atmospheric movements on the earth are sun-heat transformed into motion. But sun-heat at the distance of Jupiter possesses but 1/27, at that of Saturn 1/100 of its force here. The large amount of energy, then, obviously exerted in those remote firmaments must have some other source, to be found nowhere else than in their own active and all-pervading fires, not yet banked in with a thick solid crust.

The same acute investigator dwelt, in 1871,[1044] on the similarity between the modes of rotation of the great planets and of the sun, applying the same principles of explanation to each case. The fact of this similarity is undoubted. Cassini[1045] and Schroeter both noticed that markings on Jupiter travelled quicker the nearer they were to his equator; and Cassini even hinted at their possible assimilation to sun-spots.[1046] It is now well ascertained that, as a rule (not without exceptions), equatorial spots give a period some 5-1/2 minutes shorter than those in latitudes of about 30 deg. But, as Mr. Denning has pointed out,[1047] no single period will satisfy the observations either of different markings at the same epoch, or of the same markings at different epochs. Accelerations and retardations, depending upon processes of growth or change, take place in very much the same kind of way as in solar maculae, inevitably suggesting similarity of origin.

The interesting query as to Jupiter's surface incandescence has been studied since Bond's time with the aid of all the appliances furnished to physical inquirers by modern inventiveness, yet without bringing to it a categorical reply. Zoellner in 1865, Mueller in 1893, estimated his albedo at 0.62 and 0.75 respectively, that of fresh-fallen snow being 0.78, and of white paper 0.70.[1048] But the disc of Jupiter is by no means purely white. The general ground is tinged with ochre; the polar zones are leaden or fawn coloured; large spaces are at times stained or suffused with chocolate-browns and rosy hues. It is occasionally seen ruled from pole to pole with dusky bars, and is never wholly free from obscure markings. The reflection, then, by it, as a whole, of about 70 per cent. of the rays impinging upon it, might well suggest some original reinforcement.

Nevertheless, the spectroscope gives little countenance to the supposition of any considerable permanent light-emission. The spectrum of Jupiter, as examined by Huggins, 1862-64, and by Vogel, 1871-73, shows the familiar Fraunhofer rays belonging to reflected sunlight. But it also shows lines of native absorption. Some of these are identical with those produced by the action of our own atmosphere, especially one or more groups due to aqueous vapours; others are of unknown origin; and it is remarkable that one among the latter--a strong band in the red--agrees in position with a dark line in the spectra of some ruddy stars.[1049] There is, besides, a general absorption of blue rays, intensified--as Le Sueur observed at Melbourne in 1869[1050]--in the dusky markings, evidently through an increase of depth in the atmospheric strata traversed by the light proceeding from them.

All these observations, however (setting aside the stellar line as of doubtful significance), point to a cool planetary atmosphere. One spectrograph, it is true, taken by Dr. Henry Draper, September 27, 1879,[1051] seemed to attest the action of intrinsic light; but the peculiarity was referred by Dr. Vogel, with convincing clearness, to a flaw in the film.[1052] So far, then, native emissions from any part of Jupiter's diversified surface have not been detected; and, indeed, the blackness of the shadows cast by his satellites on his disc sufficiently proves that he sends out virtually none but reflected light.[1053] This conclusion, however, by no means invalidates that of his high internal temperature.

The curious phenomena attending Jovian satellite-transits may be explained, partly as effects of contrast, partly as due to temporary obscurations of the small discs projected on the large disc of Jupiter. At their first entry upon its marginal parts, which are several times less luminous than those near the centre, they invariably show as bright spots, then usually vanish as the background gains lustre, to reappear, after crossing the disc, thrown into relief, as before, against the dusky limb. But instances are not rare, more especially of the third and fourth satellites standing out, during the entire middle part of their course, in such inky darkness as to be mistaken for their own shadows. The earliest witness of a "black transit" was Cassini, September 2, 1665; Roemer in 1677, and Maraldi in 1707 and 1713, made similar observations, which have been multiplied in recent years. In some cases the process of darkening has been visibly attended by the formation, or emergence into view, of spots on the transiting body, as noted by the two Bonds at Harvard, March 18, 1848.[1054] The third satellite was seen by Dawes, half dark, half bright, when crossing Jupiter's disc, August 21, 1867;[1055] one-third dark by Davidson of California, January 15, 1884, under the same circumstances;[1056] and unmistakably spotted, both on and off the planet, by Schroeter, Secchi, Dawes, and Lassell.

The first satellite sometimes looks dusky, but never absolutely black, in travelling over the disc of Jupiter. The second appears uniformly white--a circumstance attributed by Dr. Spitta[1057] to its high albedo. The singularly different aspects, even during successive transits, of the third and fourth satellites, are connected by Professor Holden[1058] with the varied luminosity of the segments of the planetary surface they are projected upon, and W. H. Pickering inclines to the same opinion; but fluctuations in their own brightness[1059] may be a concurrent cause. Herschel concluded in 1797 that, like our moon, they always turn the same face towards their primary, and as regards the outer satellite, Engelmann's researches in 1871, and C. E. Burton's in 1873, made this almost certain; while both for the third and fourth Jovian moons it was completely assured by W. H. Pickering's and A. E. Douglass's observations at Arequipa in 1892,[1060] and at Flagstaff in 1894-95.[1061] Strangely enough, however, the interior members of the system have preserved a relatively swift rotation, notwithstanding the enormous checking influence upon it of Jove-raised tides.

All the satellites are stated, on good authority, to be more or less egg-shaped. On September 8, 1890, Barnard saw the first elongated and bisected by a bright equatorial belt, during one of its dark transits;[1062] and his observation, repeated August 3, 1891, was completely verified by Schaeberle and Campbell, who ascertained, moreover, that the longer axis of the prolate body was directed towards Jupiter's centre.[1063] The ellipticity of its companions was determined by Pickering and Douglass; indeed, that of No. 3 had long previously been noticed by Secchi.[1064] No. 3 also shows equatorial stripes, perceived in 1891 by Schaeberle and Campbell,[1065] and evident later to Pickering and Douglass;[1066] nor need we hesitate to admit as authentic their records of similar, though less conspicuous markings on the other satellites. A constitution analogous to that of Jupiter himself was thus unexpectedly suggested; and Vogel's detection of lines--or traces of lines--in their spectra, agreeing with absorption-rays derived from their primary, lends support to the conjecture that they possess gaseous envelopes similar to his.

The system of Jupiter, as it was discovered by Galileo, and investigated by Laplace, appeared in its outward aspect so symmetrical, and displayed in its inner mechanism such harmonious dynamical relations, that it might well have been deemed complete. Nevertheless, a new member has been added to it. Near midnight on September 9, 1892, Professor Barnard discerned with the Lick 36-inch "a tiny speck of light," closely following the planet.[1067] He instantly divined its nature, watched its hurried disappearance in the adjacent glare, and made sure of the reality of his discovery on the ensuing night. It was a delicate business throughout, the Liliputian luminary subsiding into invisibility before the slightest glint of Jovian light, and tarrying, only for brief intervals, far enough from the disc to admit of its exclusion by means of an occulting plate. The new satellite is estimated to be of the thirteenth stellar magnitude, and, if equally reflective of light with its next neighbour, Io (satellite No. 1), its diameter must be about one hundred miles. It revolves at a distance of 112,500 miles from Jupiter's centre, and of 68,000 from his bulging equatorial surface. Its period of 11h. 57m. 23s. is just two hours longer than Jupiter's period of rotation, so that Phobos still remains a unique example of a secondary body revolving faster than its primary rotates. Jupiter's innermost moon conforms in its motions strictly, indeed inevitably, to the plane of his equatorial protuberance, following, however, a sensibly elliptical path the major axis of which is in rapid revolution.[1068] Its very insignificance raises the suspicion that it may not prove solitary. Possibly it belongs to a zone peopled by asteroidal satellites. More than fifteen thousand such small bodies could be furnished out of the materials of a single full-sized satellite spoiled in the making. But we must be content for the present to register the fact without seeking to penetrate the meaning of its existence. Very high and very fine telescopic power is needed for its perception. Outside the United States, it has been very little observed. The only instruments in this country successfully employed for its detection are, we believe, Dr. Common's 5-foot reflector and Mr. Newall's 25-inch refractor.

In the course of his observations on Jupiter at Brussels in 1878, M. Niesten was struck with a rosy cloud attached to a whitish zone beneath the dark southern equatorial band.[1069] Its size was enormous. At the distance of Jupiter, its measured dimensions of 13" by 3" implied a real extension in longitude of 30,000, in latitude of something short of 7,000 miles. The earliest record of its appearance seems to be by Professor Pritchett, director of the Morrison Observatory (U.S.), who figured and described it July 9, 1878.[1070] It was again delineated August 9, by Tempel at Florence.[1071] In the following year it attracted the wonder and attention of almost every possessor of a telescope. Its colour had by that time deepened into a full brick-red, and was set off by contrast with a white equatorial spot of unusual brilliancy. During three ensuing years these remarkable objects continued to offer a visible and striking illustration of the compound nature of the planet's rotation. The red spot completed a circuit in nine hours fifty-five minutes thirty-six seconds; the white spot in about five and a half minutes less. Their relative motion was thus no less than 260 miles an hour, bringing them together in the same meridian at intervals of forty-four days ten hours forty-two minutes. Neither, however, preserved continuously the same uniform rate of travel. The period of each had lengthened by some seconds in 1883, while sudden displacements, associated with the recovery of lustre after recurrent fadings, were observed in the position of the white spot,[1072] recalling the leap forward of a reviving sun-spot. Just the opposite effect attended the rekindling of the companion object. While semi-extinct, in 1882-84, it lost little motion; but a fresh access of retardation was observed by Professor Young[1073] in connection with its brightening in 1886. This suggests very strongly that the red spot is fed from below. A shining aureola of "faculae," described by Bredichin at Moscow, and by Lohse at Potsdam, as encircling it in September, 1879,[1074] was held to strengthen the solar analogy.

The conspicuous visibility of this astonishing object lasted three years. When the planet returned to opposition in 1882-83, it had faded so considerably that Ricco's uncertain glimpse of it at Palermo, May 31, 1883, was expected to be the last. It had, nevertheless, begun to recover in December, and presented to Mr. Denning in the beginning of 1886 much the same aspect as in October, 1882.[1075] Observed by him in an intermediate stage, February 25, 1885, when "a mere skeleton of its former self," it bore a striking likeness to an "elliptical ring" descried in the same latitude by Mr. Gledhill in 1869-70. This, indeed, might be called the preliminary sketch for the famous object brought to perfection ten years later, but which Mr. H. C. Russell of Sydney saw and drew still unfinished in June, 1876,[1076] before it had separated from its matrix, the dusky south tropical belt. In earlier times, too, a marking "at once fixed and transient" had been repeatedly perceived attached to the southernmost of the central belts. It gave Cassini in 1665 a rotation-period of nine hours fifty-six minutes,[1077] reappeared and vanished eight times during the next forty-three years, and was last seen by Maraldi in 1713. It was, however, very much smaller than the recent object, and showed no unusual colour.[1078]

The assiduous observations made on the "Great Red Spot" by Mr. Denning at Bristol and by Professor Hough at Chicago afforded grounds only for negative conclusions as to its nature. It certainly did not represent the outpourings of a Jovian volcano; it was in no sense attached to the Jovian soil--if the phrase have any application to that planet; it was not a mere disclosure of a glowing mass elsewhere seethed over by rolling vapours. It was, indeed, certainly not self-luminous, a satellite projected upon it in transit having been seen to show as bright as upon the dusky equatorial bands. A fundamental objection to all three hypotheses is that the rotation of the spot was variable. It did not then ride at anchor, but floated free. Some held that its surface was depressed below the average cloud-level, and that the cavity was filled with vapours. Professor Wilson, on the other hand, observing with the 16-inch equatorial of the Goodsell Observatory in Minnesota, received a persistent impression of the object "being at a higher level than the other markings."[1079] A crucial experiment on this point was proposed by Mr. Stanley Williams in 1890.[1080] A dark spot moving faster along the same parallel was timed to overtake the red spot towards the end of July. A unique opportunity hence appeared to be at hand of determining the relative vertical depths of the two formations, one of which must inevitably, it was thought, pass above the other. No forecast included a third alternative, which was nevertheless adopted by the dark spot. It evaded the obstacle in its path by skirting round its southern edge.[1081] Nothing, then, was gained by the conjunction, beyond an additional proof of the singular repellent influence exerted by the red spot over the markings in its vicinity. It has, for example, gradually carved out a deep bay for its accommodation in the gray belt just north of it. The effect was not at first steadily present. A premonitory excavation was drawn by Schwabe at Dessau, September 5, 1831, and again by Trouvelot, Barnard, and Elvins in 1879; yet there was no sign of it in the following year. Its development can be traced in Dr. Boeddicker's beautiful delineations of Jupiter, made with the Parsonstown 3-foot reflector, from 1881 to 1886.[1082] They record the belt as straight in 1881, but as strongly indented from January, 1883; and the cavity now promises to outlast the spot. So long as it survives, however, the forces at work in the spot can have lost little of their activity. For it must be remembered that the belt has a shorter rotation-period than the red spot, which, accordingly (as Mr. Elvins of Toronto has pointed out), breasts and diverts, by its interior energy, a current of flowing matter, ever ready to fill up its natural bed, and override the barrier of obstruction.

The famous spot was described by Keeler in 1889, as "of a pale pink colour, slightly lighter in the middle. Its outline was a fairly true ellipse, framed in by bright white clouds."[1083] The fading continuously in progress from 1887 was temporarily interrupted in 1891. The revival, indeed, was brief. Professor Barnard wrote in August, 1892: "The great red spot is still visible, but it has just passed through a crisis that seemingly threatened its very existence. For the past month it has been all but impossible to catch the feeblest trace of the spot, though the ever-persistent bay in the equatorial belt close north of it, and which has been so intimately connected with the history of the red spot, has been as conspicuous as ever. It is now, however, possible to detect traces of the entire spot. An obscuring medium seems to have been passing over it, and has now drifted somewhat preceding the spot."[1084]

The object is now always inconspicuous, and often practically invisible, and may be said to float passively in the environing medium.[1085] Yet there are sparks beneath the ashes. A rosy tinge faintly suffused it in April, 1900,[1086] and its absolute end may still be remote.

The extreme complexity of the planet's surface-movements has been strikingly evinced by Mr. Stanley Williams's detailed investigations. He enumerated in 1896[1087] nine principal currents, all flowing parallel to the equator, but unsymmetrically placed north and south of it, and showing scant signs of conformity to the solar rule of retardation with increase of latitude. The linear rate of the planet's equatorial rotation was spectroscopically determined by Belopolsky and Deslandres in 1895. Both found it to fall short of the calculated speed, whence an enlargement, by self-refraction, of the apparent disc was inferred.[1088]

Jupiter was systematically photographed with the Lick 36-inch telescope during the oppositions of 1890, 1891, and 1892, the image thrown on the plates (after eightfold direct enlargement) being one inch in diameter. Mr. Stanley Williams's measurements and discussion of the set for 1891 showed the high value of the materials thus collected, although much more minute details can be seen than can at present be photographed. The red spot shows as "very distinctly annular" in several of these pictures.[1089] Recently, the planet has been portrayed by Deslandres with the 62-foot Meudon refractor.[1090] The extreme actinic feebleness of the equatorial bands was strikingly apparent on his plates.

In 1870, Mr. Ranyard[1091]--whose death, December 14, 1894, was a serious loss to astronomy--acting upon an earlier suggestion of Sir William Huggins, collected records of unusual appearances on the disc of Jupiter, with a view to investigate the question of their recurrence at regular intervals. He concluded that the development of the deeper tinges of colour, and of the equatorial "port-hole" markings girdling the globe in regular alternations of bright and dusky, agreed, so far as could be ascertained, with epochs of sun-spot maximum. The further inquiries of Dr. Lohse at Bothkamp in 1873[1092] went to strengthen the coincidence, which had been anticipated a priori by Zoellner in 1871.[1093] Moreover, separate and distinct evidence was alleged by Mr. Denning in 1899 of decennial outbreaks of disturbance in north temperate regions.[1094] It may, indeed, be taken for granted that what Hahn terms the universal pulse of the solar system[1095] affects the vicissitudes of Jupiter; but the law of those vicissitudes is far from being so obviously subordinate to the rhythmical flow of central disturbance as are certain terrestrial phenomena. The great planet, being in fact himself a "semi-sun," may be regarded as an originator, no less than a recipient, of agitating influences, the combined effects of which may well appear insubordinate to any obvious law.

It is likely that Saturn is in a still earlier stage of planetary development than Jupiter. He is the lightest for his size of all the planets. In fact, he would float in water. And since his density is shown, by the amount of his equatorial bulging, to increase centrally,[1096] it follows that his superficial materials must be of a specific gravity so low as to be inconsistent, on any probable supposition, with the solid or liquid states. Moreover, the chief arguments in favour of the high temperature of Jupiter, apply, with increased force, to Saturn; so that it may be concluded, without much risk of error, that a large proportion of his bulky globe, 73,000 miles in diameter, is composed of heated vapours, kept in active and agitated circulation by the process of cooling.

His unique set of appendages has, since the middle of the last century, formed the subject of searching and fruitful inquiries, both theoretical and telescopic. The mechanical problem of the stability of Saturn's rings was left by Laplace in a very unsatisfactory condition. Considering them as rotating solid bodies, he pointed out that they could not maintain their position unless their weight were in some way unsymmetrically distributed; but made no attempt to determine the kind or amount of irregularity needed to secure this end. Some observations by Herschel gave astronomers an excuse for taking for granted the fulfilment of the condition thus vaguely postulated; and the question remained in abeyance until once more brought prominently forward by the discovery of the dusky ring in 1850.

The younger Bond led the way, among modern observers, in denying the solidity of the structure. The fluctuations in its aspect were, he asserted in 1851,[1097] inconsistent with such a hypothesis. The fine dark lines of division, frequently detected in both bright rings, and as frequently relapsing into imperceptibility, were due, in his opinion, to the real nobility of their particles, and indicated a fluid formation. Professor Benjamin Peirce of Harvard University immediately followed with a demonstration, on abstract grounds, of their non-solidity.[1098] Streams of some fluid denser than water were, he maintained, the physical reality giving rise to the anomalous appearance first disclosed by Galileo's telescope.

The mechanism of Saturn's rings, proposed as the subject of the Adams Prize, was dealt with by James Clerk Maxwell in 1857. His investigation forms the groundwork of all that is at present known in the matter. Its upshot was to show that neither solid nor fluid rings could continue to exist, and that the only possible composition of the system was by an aggregated multitude of unconnected particles, each revolving independently in a period corresponding to its distance from the planet.[1099] This idea of a satellite-formation had been, remarkably enough, several times entertained and lost sight of. It was first put forward by Roberval in the seventeenth century, again by Jacques Cassini in 1715, and with perfect definiteness by Wright of Durham in 1750.[1100] Little heed, however, was taken of these casual anticipations of a truth which reappeared, a virtual novelty, as the legitimate outcome of the most refined modern methods.

The details of telescopic observation accord, on the whole, admirably with this hypothesis. The displacements or disappearance of secondary dividing-lines--the singular striated appearance, first remarked by Short in the eighteenth century, last by Perrotin and Lockyer at Nice, March 18, 1884[1101]--show the effects of waves of disturbance traversing a moving mass of gravitating particles;[1102] the broken and changing line of the planet's shadow on the ring gives evidence of variety in the planes of the orbits described by those particles. The whole ring-system, too, appears to be somewhat elliptical.[1103]

The satellite-theory has derived unlooked-for support from photometric inquiries. Professor Seeliger pointed out in 1888[1104] that the unvarying brilliancy of the outer rings under all angles of illumination, from 0 deg. to 30 deg., can be explained from no other point of view. Nor does the constitution of the obscure inner ring offer any difficulty. For it is doubtless formed of similar small bodies to those aggregated in the lucid members of the system, only much more thinly strewn, and reflecting, consequently, much less light. It is not, indeed, at first easy to see why these sparser flights should show as a dense dark shading on the body of Saturn. Yet this is invariably the case. The objection has been urged by Professor Hastings of Baltimore. The brightest parts of these appendages, he remarked,[1105] are more lustrous than the globe they encircle; but if the inner ring consists of identical materials, possessing presumably an equal reflective capacity, the mere fact of their scanty distribution would not cause them to show as dark against the same globe. Professor Seeliger, however, replied[1106] that the darkening is due to the never-ending swarms of their separate shadows transiting the planet's disc. Sunlight is not, indeed, wholly excluded. Many rays come and go between the open ranks of the meteorites. For the dusky ring is transparent. The planet it encloses shows through it, as if veiled with a strip of crape. A beautiful illustration of its quality in this respect was derived by Professor Barnard from an eclipse of Japetus, November 1, 1889.[1107] The eighth moon remained steadily visible during its passage through the shadow of the inner ring, but with a progressive loss of lustre in approaching its bright neighbour. There was no breach of continuity. The satellite met no gap, corresponding to that between the dusky ring and the body of Saturn, through which it could shine with undiminished light, but was slowly lost sight of as it plunged into deeper and deeper gloom. The important facts were thus established, that the brilliant and obscure rings merge into each other, and that the latter thins out towards the Saturnian globe.

The meteoric constitution of these appendages was beautifully demonstrated in 1895 by Professor Keeler,[1108] then director of the Alleghany Observatory, Pittsburgh. From spectrographs taken with the slit adjusted to coincidence with the equatorial plane of the system, he determined the comparative radial velocities of its different parts. And these supply a crucial test of Clerk Maxwell's theory. For if the rings were solid, the swiftest rates of rotation should be at their outer edges, corresponding to wider circles described in the same period; while, if they are pulverulent, the inverse relation must hold good. This proved to be actually the case. The motion slowed off outward, in agreement with the diminishing speed of particles travelling freely, each in its own orbit. Keeler's result was promptly confirmed by Campbell,[1109] as well as by Deslandres and Belopolsky.

A question of singular interest, and one which we cannot refrain from putting to ourselves, is--whether we see in the rings of Saturn a finished structure, destined to play a permanent part in the economy of the system; or whether they represent merely a stage in the process of development out of the chaotic state in which it is impossible to doubt that the materials of all planets were originally merged. M. Otto Struve attempted to give a definite answer to this important query.

A study of early and later records of observations disclosed to him, in 1851, an apparent progressive approach of the inner edge of the bright ring to the planet. The rate of approach he estimated at about fifty-seven English miles a year, or 11,000 miles during the 194 years elapsed since the time of Huygens.[1110] Were it to continue, a collapse of the system must be far advanced within three centuries. But was the change real or illusory--a plausible, but deceptive inference from insecure data? M. Struve resolved to put it to the test. A set of elaborately careful micrometrical measures of the dimensions of Saturn's rings, executed by himself at Pulkowa in the autumn of 1851, was provided as a standard of future comparison; and he was enabled to renew them, under closely similar circumstances, in 1882.[1111] But the expected diminution of the space between Saturn's globe and his rings had not taken place. A slight extension in the width of the system, both outward and inward, was indeed, hinted at; and it is worth notice that just such a separation of the rings was indicated by Clerk Maxwell's theory, so that there is an a priori likelihood of its being in progress. Yet Hall's measures in 1884-87[1112] failed to supply evidence of alteration with time; and Barnard's, executed at Lick in 1894-95,[1113] showed no sensible divergence from them. Hence, much weight cannot be laid upon Huygens's drawings and descriptions, which had been held to prove conclusively a partial filling up, since 1657, of the interval between the ring and the planet.[1114]

The rings of Saturn replace, in Professor G. H. Darwin's view,[1115] an abortive satellite, scattered by tidal action into annular form. For they lie closer to the planet than is consistent with the integrity of a revolving body of reasonable bulk. The limit of possible existence for such a mass was fixed by Roche of Montpellier, in 1848,[1116] at 2.44 mean radii of its primary; while the outer edge of the ring-system is distant 2.38 radii of Saturn from his centre. The virtual discovery of its pulverulent condition dates, then, according to Professor Darwin, from 1848. He conjectures that the appendage will eventually disappear, partly through the dispersal of its constituent particles inward, and their subsidence upon the planet's surface, partly by their dispersal outward, to a region beyond "Roche's limit," where coalescence might proceed unhindered by the strain of unequal attractions. One modest satellite, revolving inside Mimas, would then be all that was left of the singular appurtenances we now contemplate with admiration.

There seems reason to admit that Kirkwood's law of commensurability has had some effect in bringing about the present distribution of the matter composing them. Here the influential bodies are Saturn's moons, while the divisions and boundaries of the rings represent the spaces where their disturbing action conspires to eliminate revolving particles. Kirkwood, in fact, showed, in 1867,[1117] that a body circulating in the chasm between the bright rings known as "Cassini's division," would have a period nearly commensurable with those of four out of the eight moons; and Meyer of Geneva subsequently calculated all such combinations, with the result of bringing out coincidences between regions of maximum perturbation and the limiting and dividing lines of the system.[1118] This is in itself a strong confirmation of the view that the rings are made up of independently revolving small bodies.

On December 7, 1876, Professor Asaph Hall discovered at Washington a bright equatorial spot on Saturn, which he followed and measured through above sixty rotations, each performed in ten hours fourteen minutes twenty-four seconds.[1119] This, he was careful to add, represented the period, not necessarily of the planet, but only of the individual spot. The only previous determination of Saturn's axial movement (setting aside some insecure estimates by Schroeter) was Herschel's in 1794, giving a period of ten hours sixteen minutes. The substantial accuracy of Hall's result was verified by Mr. Denning in 1891.[1120] In May and June of that year, ten vague bright markings near the equator were watched by Mr. Stanley Williams, who derived from them a rotation period only two seconds shorter than that determined at Washington. Nevertheless, similarly placed spots gave in 1892 and 1893 notably quicker rates;[1121] so that the task of timing the general drift of the Saturnian surface by the displacements of such objects is hampered, to an indefinite extent, by their individual proper motions.

Saturn's outermost satellite, Japetus, is markedly variable--so variable that it sends us, when brightest, just 4-1/2 times as much light as when faintest. Moreover, its fluctuations depend upon its orbital position in such a way as to make it a conspicuous telescopic object when west, a scarcely discernible one when east of the planet. Herschel's inference[1122] of a partially obscured globe turning always the same face towards its primary seems the only admissible one, and is confirmed by Pickering's measurements of the varying intensity of its light. He remarked further that the dusky and brilliant hemispheres must be so posited as to divide the disc, viewed from Saturn, into nearly equal parts; so that this Saturnian moon, even when "full," appears very imperfectly illuminated over one-half of its surface.[1123]

Zoellner estimated the albedo of Saturn at 0.51, Mueller at 0.88, a value impossibly high, considering that the spectrum includes no vestige of original emissions. Closely similar to that of Jupiter, it shows the distinctive dark line in the red (wave-length 618), which we may call the "red-star line"; and Janssen, from the summit of Etna in 1867[1124] found traces in it of aqueous absorption. The light from the ring appears to be pure reflected sunshine unmodified by original atmospheric action.[1125]

Uranus, when favourably situated, can easily be seen with the naked eye as a star between the fifth and sixth magnitudes. There is indeed, some reason to suppose that he had been detected as a wandering orb by savage "watchers of the skies" in the Pacific long before he swam into Herschel's ken. Nevertheless, inquiries into his physical habitudes are still in an early stage. They are exceedingly difficult of execution, even with the best and largest modern telescopes; and their results remain clouded with uncertainty.

It will be remembered that Uranus presents the unusual spectacle of a system of satellites travelling nearly at right angles to the plane of the ecliptic. The existence of this anomaly gives a special interest to investigations of his axial movement, which might be presumed, from the analogy of the other planets, to be executed in the same tilted plane. Yet this is far from being certainly the case.

Mr. Buffham in 1870-72 caught traces of bright markings on the Uranian disc, doubtfully suggesting a rotation in about twelve hours in a plane not coincident with that in which his satellites circulate.[1126] Dusky bands resembling those of Jupiter, but very faint, were barely perceptible to Professor Young at Princeton in 1883. Yet, though almost necessarily inferred to be equatorial, they made a considerable angle with the trend of the satellites' orbits.[1127] More distinctly by the brothers Henry, with the aid of their fine refractor, two gray parallel rulings, separated by a brilliant zone, were discerned every clear night at Paris from January to June, 1884.[1128] What were taken to be the polar regions appeared comparatively dusky. The direction of the equatorial rulings (for so we may safely call them) made an angle of 40 deg. with the satellites' line of travel. Similar observations were made at Nice by MM. Perrotin and Thollon, March to June, 1884, a lucid spot near the equator, in addition, indicating rotation in a period of about ten hours.[1129] The discrepancy was, however, considerably reduced by Perrotin's study of the planet in 1889 with the new 30-inch equatoreal.[1130] The dark bands, thus viewed to better advantage than in 1884, appeared to deviate no more than 10 deg. from the satellites' orbit-plane. No definitive results, on the other hand, were derived by Professors Holden, Schaeberle, and Keeler from their observations of Uranus in 1889-90 with the potent instrument on Mount Hamilton. Shadings, it is true, were almost always, though faintly, seen; but they appeared under an anomalous, possibly an illusory aspect. They consisted, not of parallel, but of forked bands.[1131]

Measurements of the little sea-green disc which represents to us the massive bulk of Uranus, by Young, Schiaparelli,[1132] Safarik, H. C. Wilson[1133] and Perrotin, prove it to be quite distinctly bulged. The compression at once caught Barnard's trained eye in 1894,[1134] when he undertook at Lick a micrometrical investigation of the system; and he was surprised to perceive that the major axis of the elliptical surface made an angle of about 28 deg. with the line of travel pursued by the satellites. Nothing more can be learned on this curious subject for some years, since the pole of the planet is just now turned nearly towards the earth; but Barnard's conclusion is unlikely to be seriously modified. He fixed the mean diameter of Uranus at 34,900 miles. But this estimate was materially reduced through Dr. See's elimination of irradiative effects by means of daylight measures, executed at Washington in 1901.[1135]

The visual spectrum of this planet was first examined by Father Secchi in 1869, and later, with more advantages for accuracy, by Huggins, Vogel,[1136] and Keeler.[1137] It is a very remarkable one. In lieu of the reflected Fraunhofer lines, imperceptible perhaps through feebleness of light, six broad bands of original absorption appear, one corresponding to the blue-green ray of hydrogen (F), another to the "red-star line" of Jupiter and Saturn, the rest as yet unidentified. The hydrogen band seems much too strong and diffuse to be the mere echo of a solar line, and might accordingly be held to imply the presence of free hydrogen in the Uranian atmosphere. This, however, would be difficult of reconcilement with Keeler's identification of an absorption-group in the yellow with a telluric waterband.

Notwithstanding its high albedo--0.62, according to Zoellner--proof is wanting that any of the light of Uranus is inherent. Mr. Albert Taylor announced, indeed, in 1889, his detection, with Common's giant reflector, of bright flutings in its spectrum;[1138] but Professor Keeler's examination proved them to be merely contrast effects.[1139] Sir William and Lady Huggins, moreover, obtained about the same time a photograph purely solar in character. The spectrum it represented was crossed by numerous Fraunhofer lines, and by no others. It was, then, presumably composed entirely of reflected light.

* * * * *

Judging from the indications of an almost evanescent spectrum, Neptune, as regards physical condition, is the twin of Uranus, as Saturn of Jupiter. Of the circumstances of his rotation we are as good as completely ignorant. Mr. Maxwell Hall, indeed, noticed at Jamaica, in November and December, 1883, certain rhythmical fluctuations of brightness, suggesting revolution on an axis in slightly less than eight hours;[1140] but Professor Pickering reduces the supposed variability to an amount altogether too small for certain perception, and Dr. G. Mueller denies its existence in toto. It is true their observations were not precisely contemporaneous with those of Mr. Hall[1141] who believes the partial obscurations recorded by himself to have been of a passing kind, and to have suddenly ceased after a fortnight of prevalence. Their less conspicuous renewal was visible to him in November, 1884, confirming a rotation period of 7.92 hours.

It was ascertained at first by indirect means that the orbit of Neptune's satellite is inclined about 20 deg. to his equator. Mr. Marth[1142] having drawn attention to the rapid shifting of its plane of motion, M. Tisserand and Professor Newcomb[1143] independently published the conclusion that such shifting necessarily results from Neptune's ellipsoidal shape. The movement is of the kind exemplified--although with inverted relations--in the precession of the equinoxes. The pole of the satellite, owing to the pull of Neptune's equatorial protuberance, describes a circle round the pole of his equator in a retrograde direction, and in a period of over five hundred years. The amount of compression indicated for the primary body is, at the outside, 1/85; whence it can be inferred that Neptune possesses a lower rotatory velocity than the other giant planets. Direct verification of the trend theoretically inferred for the satellite's movement was obtained by Dr. See in 1899. The Washington 26-inch refractor disclosed to him, under exceptionally favourable conditions, a set of equatorial belts on the disc of Neptune, and they took just the direction prescribed by theory. Their objective reality cannot be doubted, although Barnard was unable, either with the Lick or the Yerkes telescope,[1144] to detect any definite markings on this planet. Its diameter was found by him to be 32,900 miles.

The possibility that Neptune may not be the most remote body circling round the sun has been contemplated ever since he has been known to exist. Within the last few years the position at a given epoch of a planet far beyond his orbital verge has been approximately fixed by two separate investigators.

Professor George Forbes of Edinburgh adopted in 1880 a novel plan of search for unknown members of the solar system, the first idea of which was thrown out by M. Flammarion in November, 1879.[1145] It depends upon the movements of comets. It is well known that those of moderately short periods are, for a reason already explained, connected with the larger planets in such a way that the cometary aphelia fall near some planetary orbit. Jupiter claims a large retinue of such partial dependents, Neptune owns six, and there are two considerable groups, the farthest distances of which from the sun lie respectively near 100 and 300 times that of the earth. At each of these vast intervals, one involving a period of 1,000, the other of 5,000 years, Professor Forbes maintains that an unseen planet circulates. He even computed elements for the nearer of the two, and fixed its place on the celestial sphere;[1146] but the photographic searches made for it by Dr. Roberts at Crowborough and by Mr. Wilson at Daramona proved unavailing. Undeterred by Deichmueller's discouraging opinion that cometary orbits extending beyond the recognised bounds of the solar system are too imperfectly known to serve as the basis of trustworthy conclusions,[1147] the Edinburgh Professor returned to the attack in 1901.[1148] He now sought to prove that the lost comet of 1556 actually returned in 1844, but with elements so transformed by ultra-Neptunian perturbations as to have escaped immediate identification. If so, the "wanted" planet has just entered the sign Libra, and, being larger than Jupiter, should be possible to find.

Almost simultaneously with Forbes, Professor Todd set about groping for the same object by the help of a totally different set of indications. Adams's approved method commended itself to him; but the hypothetical divagations of Neptune having scarcely yet had time to develop, he was thrown back upon the "residual errors" of Uranus. They gave him a virtually identical situation for the new planet with that derived from the clustering of cometary aphelia.[1149] Yet its assigned distance was little more than half that of the nearer of Professor Forbes's remote pair, and it completed a revolution in 375 instead of 1,000 years. The agreement in them between the positions determined, on separate grounds, for the ultra-Neptunian traveller was merely an odd coincidence; nor can we be certain, until it is seen, that we have really got into touch with it.

FOOTNOTES:

[Footnote 965: Phil. Trans., vol. lxxiv., p. 260.]

[Footnote 966: Novae Observationes, p. 105.]

[Footnote 967: Phil. Trans., vol. i., p. 243.]

[Footnote 968: Mem. de l'Ac., 1720, p. 146.]

[Footnote 969: Phil. Trans., vol. lxxiv., p. 273.]

[Footnote 970: A large work, entitled Areographische Fragmente, in which Schroeter embodied the results of his labours on Mars, 1785-1803, narrowly escaped the conflagration of 1813, and was published at Leyden in 1881.]

[Footnote 971: Beitraege, p. 124.]

[Footnote 972: Mem. R. A. Soc., vol. xxxii., p. 183.]

[Footnote 973: Astr. Nach., No. 1,468.]

[Footnote 974: Observatory, vol. viii., p. 437.]

[Footnote 975: Month. Not., vols. xxviii., p. 37; xxix., p. 232; xxxiii., p. 552.]

[Footnote 976: Flammarion, L'Astronomie, t. i., p. 266.]

[Footnote 977: Smyth, Cel. Cycle, vol. i., p. 148 (1st ed.).]

[Footnote 978: Phil. Trans., vol. cxxi., p. 417.]

[Footnote 979: Month. Not., vol. xxv., p. 227.]

[Footnote 980: Phil. Mag., vol. xxxiv., p. 75.]

[Footnote 981: Proctor, Quart. Jour. of Science, vol. x., p. 185; Maunder, Sunday Mag., January, February, March, 1882; Campbell, _Publ. Astr. Pac. Soc._, vol. vi., p. 273.]

[Footnote 982: Am. Jour. of Sc., vol. xxviii., p. 163.]

[Footnote 983: Burton, Trans. Roy. Dublin Soc., vol. i., 1880, p. 169.]

[Footnote 984: Month. Not., vol. xxvii., p. 179; Astroph. Journ., vol. i., p. 193.]

[Footnote 985: Untersuchungen ueber die Spectra der Planeten, p. 20; Astroph. Journ., vol. i., p. 203.]

[Footnote 986: Publ. Astr. Pac. Soc., vols. vi., p. 228; ix., p. 109; Astr. and Astroph., vol. xiii., p. 752; Astroph. Jour., vol. ii., p. 28.]

[Footnote 987: Ibid., vol. v., p. 328.]

[Footnote 988: Ibid., vols. i., p. 311; iii., p. 254.]

[Footnote 989: C. Christiansen, Beiblaetter, 1886, p. 532.]

[Footnote 990: Astr. and Astrophysics, vol. xi., p. 671.]

[Footnote 991: Flammarion, La Planete Mars, p. 574.]

[Footnote 992: Memoires Couronnes, t. xxxix.]

[Footnote 993: Lockyer, Nature, vol. xlvi., p. 447.]

[Footnote 994: Mem. Spettr. Italiani, t. xi., p. 28.]

[Footnote 995: Bull. Astr., t. iii., p. 324.]

[Footnote 996: Journ. Brit. Astr. Ass., vol. i., p. 88.]

[Footnote 997: Publ. Pac. Astr. Soc., vol. ii., p. 299; Percival Lowell, Mars, 1896; Annals of the Lowell Observatory, vol. ii., 1900.]

[Footnote 998: Old and New Astr., p. 545.]

[Footnote 999: L'Astronomie, t. xi., p. 445.]

[Footnote 1000: La Planete Mars, p. 588.]

[Footnote 1001: Month. Notices, vol. lvi., p. 166.]

[Footnote 1002: L'Astronomie, t. viii.]

[Footnote 1003: Astr. Nach., No. 3,271; Astr. and Astrophysics, vol. xiii., p. 716.]

[Footnote 1004: Month. Not., vol. xxxviii., p. 41; _Mem. Roy. Astr. Soc._, vol. xliv., p. 123.]

[Footnote 1005: Astr. and Astrophysics, vol. xi., p. 668.]

[Footnote 1006: Ibid., p. 850.]

[Footnote 1007: Comptes Rendus, t. cxv., p. 379.]

[Footnote 1008: Astr. Jour., No. 384; Publ. Astr. Pac. Soc., vol. vi., p. 109. Cf. Observatory vol. xvii., pp. 295-336.]

[Footnote 1009: See Mr. Wentworth Erck's remarks in Trans. Roy. Dublin Soc., vol. i., p. 29.]

[Footnote 1010: Month. Not., vol. xxxviii., p. 206.]

[Footnote 1011: Annals Harvard Coll. Obs., vol. xi., pt. ii., p. 217.]

[Footnote 1012: Young, Gen. Astr., p. 366.]

[Footnote 1013: Campbell, Publ. Pac. Astr. Soc., vol. vi., p. 270.]

[Footnote 1014: Astr. Nach., No. 3,319.]

[Footnote 1015: Witch of Atlas, stanza iii. I am indebted to Dr. Garnett for the reference.]

[Footnote 1016: Recommended by Chandler, Astr. Jour., No. 452.]

[Footnote 1017: Harvard Circulars, Nos. 36, 37, 51.]

[Footnote 1018: Astr. Nach., No. 3,687.]

[Footnote 1019: Montangerand, Comptes Rendus, March 11, 1901.]

[Footnote 1020: Pickering, Astroph. Jour., vol. xiii., p. 277.]

[Footnote 1021: Harvard Circular, No. 58.]

[Footnote 1022: Astr. Nach., No. 752.]

[Footnote 1023: L. Niesten, Annuaire, Bruxelles, 1881, p. 269.]

[Footnote 1024: According to Svedstrup (Astr. Nach., Nos. 2,240-41), the inclination to the ecliptic of the "mean asteroid's" orbit is = 6 deg.]

[Footnote 1025: Smiths. Report, 1876, p. 358; The Asteroids (Kirkwood), p. 42, 1888.]

[Footnote 1026: Tisserand, Annuaire, Paris, 1891, p. B. 15; Newcomb, Astr. Jour., No. 477; Backlund, Bull. Astr., t. xvii., p. 81; Parmentier, Bull. Soc. Astr. de France, March, 1896; Observatory, vol. xviii., p. 207.]

[Footnote 1027: Berberich, Astr. Nach., No. 3,088.]

[Footnote 1028: Bull. Astr., t. xviii., p. 39.]

[Footnote 1029: The Asteroids, p. 48; Publ. Astr. Pac. Soc., vols. ii., p. 48; iii., p. 95.]

[Footnote 1030: Comptes Rendus, t. xxxvii., p. 797.]

[Footnote 1031: Bull. Astr., t. v., p. 180.]

[Footnote 1032: Annuaire, Bruxelles, 1881, p. 243.]

[Footnote 1033: Johns Hopkins Un. Circular, January, 1895; Observatory, vol. xviii., p. 127.]

[Footnote 1034: Harvard Annals, vol. xi., part ii., p. 294.]

[Footnote 1035: Astr. Nach., Nos. 2,724-5.]

[Footnote 1036: Month. Not., vol. lxi., p. 69.]

[Footnote 1037: Astroph. Jour., vol. vii., p. 25.]

[Footnote 1038: Spectra der Planeten, p. 24.]

[Footnote 1039: Tome i., p. 93.]

[Footnote 1040: Berlinische Monatsschrift, 1785, p. 211.]

[Footnote 1041: Month. Not., vol. xiii., p. 40.]

[Footnote 1042: Mem. Am. Ac., vol. viii., p. 221.]

[Footnote 1043: Photom. Unters., p. 303.]

[Footnote 1044: Astr. Nach., No. 1,851.]

[Footnote 1045: Mem. de l'Ac., t. x., p. 514.]

[Footnote 1046: Ibid., 1692, p. 7.]

[Footnote 1047: Month. Not., vol. xliv., p. 63.]

[Footnote 1048: Photom. Unters., pp. 165, 273; Potsdam Publ., No. 30.]

[Footnote 1049: Vogel, Sp. der Planeten, p. 33, note.]

[Footnote 1050: Proc. Roy. Soc., vol. xviii., p. 250.]

[Footnote 1051: Month. Not., vol. xl., p. 433.]

[Footnote 1052: Sitzungsberichte, Berlin, 1895, ii., p. 15.]

[Footnote 1053: The anomalous shadow-effects recorded by Webb (Cel. Objects, p. 170, 4th ed.) are obviously of atmospheric and optical origin.]

[Footnote 1054: Engelmann, Ueber die Helligkeitsverhaeltnisse der Jupiterstrabanten, p. 59.]

[Footnote 1055: Month. Not., vol. xxviii., p. 11.]

[Footnote 1056: Observatory, vol. vii., p. 175.]

[Footnote 1057: Month. Not., vol. xlviii., p. 43.]

[Footnote 1058: Publ. Astr. Pac. Soc., vol. ii., p. 296.]

[Footnote 1059: Pickering failed to obtain any photometric evidence of their variability. Harvard Annals, vol. xi., p. 245.]

[Footnote 1060: Astr. and Astroph., vol. xii., pp. 194, 481.]

[Footnote 1061: Annals Lowell Obs., vol. ii., pt. i.]

[Footnote 1062: Astr. Nach., Nos. 2,995, 3,206; Month. Not., vols. li., p. 556; liv., p. 134. Barnard remains convinced that the oval forms attributed to Jupiter's satellites are illusory effects of their markings. Astr. Nach., Nos. 3,206, 3,453; Astr. and Astroph., vol. xiii., p. 272.]

[Footnote 1063: Publ. Astr. Pac. Soc., vol. iii., p. 355.]

[Footnote 1064: Astr. Nach., No. 1,017.]

[Footnote 1065: Publ. Astr. Pac. Soc., vol. iii., p. 359.]

[Footnote 1066: Astr. Nach., No. 3,432.]

[Footnote 1067: Astr. Jour., Nos. 275, 325, 367, 472; Observatory, vol. xv., p. 425.]

[Footnote 1068: Tisserand, Comptes Rendus, October 8, 1894; Cohn, Astr. Nach., No. 3,404.]

[Footnote 1069: Bull. Ac. R. Bruxelles, t. xlviii., p. 607.]

[Footnote 1070: Astr. Nach., No. 2,294.]

[Footnote 1071: Ibid., No. 2,284.]

[Footnote 1072: Denning, Month. Not., vol. xliv., pp. 64, 66; Nature, vol. xxv., p. 226.]

[Footnote 1073: Sidereal Mess., December, 1886, p. 289.]

[Footnote 1074: Astr. Nach., Nos. 2,280, 2,282.]

[Footnote 1075: Month. Not., vol. xlvi., p. 117.]

[Footnote 1076: Proc. Roy. Soc. N. S. Wales, vol. xiv., p. 68.]

[Footnote 1077: Phil. Trans., vol. i., p. 143.]

[Footnote 1078: For indications relative to the early history of the red spot, see Holden, Publ. Astr. Pac. Soc., vol. ii., p. 77; Noble, Month. Not., vol. xlvii., p. 515; A. S. Williams, Observatory, vol. xiii., p. 338.]

[Footnote 1079: Astr. and Astrophysics, vol. xi., p. 192.]

[Footnote 1080: Month. Not., vol. l., p. 520.]

[Footnote 1081: Observatory, vol. xiii., pp. 297, 326.]

[Footnote 1082: Trans. R. Dublin Soc., vol. iv., p. 271, 1889.]

[Footnote 1083: Publ. Astr. Pac. Soc., vol. ii., p. 289.]

[Footnote 1084: Astr. and Astrophysics, vol. xi., p. 686.]

[Footnote 1085: Denning, Knowledge, vol. xxiii., p. 200; Observatory, vol. xxiv., p. 312; Pop. Astr., vol. ix., p. 448; Nature, vol. lv., p. 89.]

[Footnote 1086: Williams, Observatory, vol. xxiii., p. 282.]

[Footnote 1087: Month. Not., vol. lvi., p. 143.]

[Footnote 1088: Belopolsky, Astr. Nach., No. 3,326.]

[Footnote 1089: Publ. Astr. Pac. Soc., vol. iv., p. 176.]

[Footnote 1090: Bull. Astr., 1900, p. 70.]

[Footnote 1091: Month. Not., vol. xxxi., p. 34.]

[Footnote 1092: Beobachtungen, Heft ii., p. 99.]

[Footnote 1093: Ber. Saechs. Ges. der Wiss., 1871, p. 553.]

[Footnote 1094: Month. Not., vol. lix., p. 76.]

[Footnote 1095: Beziehungen der Sonnenfleckenperiode, p. 175.]

[Footnote 1096: A. Hall, Astr. Nach., No. 2,269.]

[Footnote 1097: Astr. Jour. (Gould's), vol. ii., p. 17.]

[Footnote 1098: Ibid., p. 5.]

[Footnote 1099: On the Stability of the Motion of Saturn's Rings, p. 67.]

[Footnote 1100: Mem. de l'Ac., 1715, p. 47; Montucla, _Hist. des Math._, t. iv., p. 19; An Original Theory of the Universe, p. 115.]

[Footnote 1101: Comptes Rendus, t. xcviii., p. 718.]

[Footnote 1102: Proctor, Saturn and its System (1865), p. 125.]

[Footnote 1103: Perrotin, Comptes Rendus, t. cvi., p. 1716.]

[Footnote 1104: Abhandl. Akad. der Wiss., Munich, Bd. xvi., p. 407.]

[Footnote 1105: Smiths. Report, 1880 (Holden).]

[Footnote 1106: Quoted by Dr. E. Anding, Astr. Nach., No. 2,881.]

[Footnote 1107: Astr. and Astrophysics, vol. xi., p. 119; _Month. Not._, vol. l., p. 108.]

[Footnote 1108: Astroph. Jour., vol. i., p. 416.]

[Footnote 1109: Ibid., vol. ii., p. 127.]

[Footnote 1110: Mem. de l'Ac. Imp. (St. Petersb.), t. vii., 1853, p. 464.]

[Footnote 1111: Astr. Nach., No. 2,498.]

[Footnote 1112: Washington Observations, App. ii., p. 22]

[Footnote 1113: Month. Not., vol. lvi., p. 163.]

[Footnote 1114: T. Lewis, Observatory, vol. xviii., p. 379.]

[Footnote 1115: Harper's Magazine, June, 1889.]

[Footnote 1116: Mem. de l'Acad. de Montpellier, t. viii., p. 296, 1873.]

[Footnote 1117: Meteoric Astronomy, chap. xii. He carried the subject somewhat farther in 1871. See Observatory, vol. vi., p. 335.]

[Footnote 1118: Astr. Nach., No. 2,527.]

[Footnote 1119: Amer. Jour. of Sc., vol. xiv., p. 325.]

[Footnote 1120: Observatory, vol. xiv., p. 369.]

[Footnote 1121: Month. Not., vol. liv., p. 297.]

[Footnote 1122: Phil. Trans., vol. lxxxii., p. 14.]

[Footnote 1123: Smiths. Report, 1880.]

[Footnote 1124: Comptes Rendus, t. lxiv., p. 1304.]

[Footnote 1125: Huggins, Proc. R. Soc., vol. xlvi., p. 231; Keeler, Astr. Nach., No. 2,927; Vogel, Astroph. Jour., vol. i., p. 278.]

[Footnote 1126: Month. Not., vol. xxxiii., p. 164.]

[Footnote 1127: Astr. Nach., No 2,545.]

[Footnote 1128: Comptes Rendus, t. xcviii., p. 1419.]

[Footnote 1129: Comptes Rendus, t. xcviii., pp. 718, 967.]

[Footnote 1130: V. J. S. Astr. Ges., Jahrg. xxiv., p. 267.]

[Footnote 1131: Publ. Astr. Pac. Soc., vol. iii., p. 287.]

[Footnote 1132: Astr. Nach., No. 2,526.]

[Footnote 1133: Ibid., No. 2,730.]

[Footnote 1134: Astr. Jour., Nos. 370, 374.]

[Footnote 1135: Astr. Nach., No. 3,768.]

[Footnote 1136: Ann. der Phys., Bd. clviii., p. 470; Astroph. Jour., vol. i., p. 280.]

[Footnote 1137: Astr. Nach., No. 2,927.]

[Footnote 1138: Month. Not., vol. xlix., p. 405.]

[Footnote 1139: Astr. Nach., No. 2,927; Scheiner's Spectralanalyse, p. 221.]

[Footnote 1140: Month. Not., vol. xliv., p. 257.]

[Footnote 1141: Observatory, vol. vii., pp. 134, 221, 264.]

[Footnote 1142: Month. Not., vol. xlvi., p. 507.]

[Footnote 1143: Comptes Rendus, t. cvii., p. 804; _Astr. and Astroph._, vol. xiii., p. 291; Astr. Jour., No. 186.]

[Footnote 1144: Astr. Jour., Nos. 342, 436, 508.]

[Footnote 1145: Astr. Pop., p. 661; La Nature, January 3, 1880.]

[Footnote 1146: Proc. Roy. Soc. Edinb., vols. x., p. 429; xi., p. 89.]

[Footnote 1147: Vierteljahrsschrift. Astr. Ges., Jahrg. xxi., p. 206.]

[Footnote 1148: Proc. Roy. Soc. Edinb., vol. xxiii., p. 370; Nature, vol. lxiv., p. 524.]

[Footnote 1149: Amer. Jour. of Science, vol. xx., p. 225.]

Chapter IX

THEORIES OF PLANETARY EVOLUTION

We cannot doubt that the solar system, as we see it, is the result of some process of growth--that, during innumerable ages, the forces of Nature were at work upon its materials, blindly modelling them into the shape appointed for them from the beginning by Omnipotent Wisdom. To set ourselves to inquire what that process was may be an audacity, but it is a legitimate, nay, an inevitable one. For man's implanted instinct to "look before and after" does not apply to his own little life alone, but regards the whole history of creation, from the highest to the lowest--from the microscopic germ of an alga or a fungus to the visible frame and furniture of the heavens.

Kant considered that the inquiry into the mode of origin of the world was one of the easiest problems set by Nature; but it cannot be said that his own solution of it was satisfactory. He, however, struck out in 1755 a track which thought still pursues. In his Allgemeine Naturgeschichte the growth of sun and planets was traced from the cradle of a vast and formless mass of evenly diffused particles, and the uniformity of their movements was sought to be accounted for by the unvarying action of attractive and repulsive forces, under the dominion of which their development was carried forward.

In its modern form, the "Nebular Hypothesis" made its appearance in 1796.[1150] It was presented by Laplace with diffidence, as a speculation unfortified by numerical buttresses of any kind, yet with visible exultation at having, as he thought, penetrated the birth-secret of our system. He demanded, indeed, more in the way of postulates than Kant had done. He started with a sun ready made,[1151] and surrounded with a vast glowing atmosphere, extending into space out beyond the orbit of the farthest planet, and endowed with a slow rotatory motion. As this atmosphere or nebula cooled, it contracted; and as it contracted, its rotation, by a well-known mechanical law, became accelerated. At last a point arrived when tangential velocity at the equator increased beyond the power of gravity to control, and equilibrium was restored by the separation of a nebulous ring revolving in the same period as the generating mass. After a time, the ring broke up into fragments, all eventually reunited in a single revolving and rotating body. This was the first and farthest planet.

Meanwhile the parent nebula continued to shrink and whirl quicker and quicker, passing, as it did so, through successive crises of instability, each resulting in, and terminated by, the formation of a planet, at a smaller distance from the centre, and with a shorter period of revolution than its predecessor. In these secondary bodies the same process was repeated on a reduced scale, the birth of satellites ensuing upon their contraction, or not, according to circumstances. Saturn's ring, it was added, afforded a striking confirmation of the theory of annular separation,[1152] and appeared to have survived in its original form in order to throw light on the genesis of the whole solar system; while the four first discovered asteroids offered an example in which the debris of a shattered ring had failed to coalesce into a single globe.

This scene of cosmical evolution was a characteristic bequest from the eighteenth century to the nineteenth. It possessed the self-sufficing symmetry and entireness appropriate to the ideas of a time of renovation, when the complexity of nature was little accounted of in comparison with the imperious orderliness of the thoughts of man. Since its promulgation, however, knowledge has transgressed many boundaries, and set at naught much ingenious theorising. How has it fared with Laplace's sketch of the origin of the world? It has at least not been discarded as effete. The groundwork of speculation on the subject is still furnished by it. It is, nevertheless, admittedly inadequate. Of much that exists it gives no account, or an erroneous one. The march of events certainly did not everywhere--even if it did anywhere--follow the exact path prescribed for it. Yet modern science attempts to supplement, but scarcely ventures to supersede it.

Thought has, in many directions, been profoundly modified by Mayer's and Joule's discovery, in 1842, of the equivalence between heat and motion. Its corollary was the grand idea of the "conservation of energy," now one of the cardinal principles of science. This means that, under the ordinary circumstances of observation, the old maxim ex nihilo nihil fit applies to force as well as to matter. The supplies of heat, light, electricity, must be kept up, or the stream will cease to flow. The question of the maintenance of the sun's heat was thus inevitably raised; and with the question of maintenance that of origin is indissolubly connected.

Dr. Julius Robert Mayer, a physician residing at Heilbronn, was the first to apply the new light to the investigation of what Sir John Herschel had termed the "great secret." He showed that if the sun were a body either simply cooling or in a state of combustion, it must long since have "gone out." Had an equal mass of coal been set alight four or five centuries after the building of the Pyramid of Cheops, and kept burning at such a rate as to supply solar light and heat during the interim, only a few cinders would now remain in lieu of our undiminished glorious orb. Mayer looked round for an alternative. He found it in the "meteoric hypothesis" of solar conservation.[1153] The importance in the economy of our system of the bodies known as falling stars was then (in 1848) beginning to be recognised. It was known that they revolved in countless swarms round the sun; that the earth daily encountered millions of them; and it was surmised that the cone of the zodiacal light represented their visible condensation towards the attractive centre. From the zodiacal light, then, Mayer derived the store needed for supporting the sun's radiations. He proved that, by the stoppage of their motion through falling into the sun, bodies would evolve from 4,600 to 9,200 times as much heat (according to their ultimate velocity) as would result from the burning of equal masses of coal, their precipitation upon the sun's surface being brought about by the resisting medium observed to affect the revolutions of Encke's comet. There was, however, a difficulty. The quantity of matter needed to keep, by the sacrifice of its movement, the hearth of our system warm and bright would be very considerable. Mayer's lowest estimate put it at 94,000 billion kilogrammes per second, or a mass equal to that of our moon bi-annually. But so large an addition to the gravitating power of the sun would quickly become sensible in the movement of the bodies dependent upon him. Their revolutions would be notably accelerated. Mayer admitted that each year would be shorter than the previous one by a not insignificant fraction of a second, and postulated an unceasing waste of substance, such as Newton had supposed must accompany emission of the material corpuscles of light, to neutralise continual reinforcement.

Mayer's views obtained a very small share of publicity, and owned Mr. Waterston as their independent author in this country. The meteoric, or "dynamical," theory of solar sustentation was expounded by him before the British Association in 1853. It was developed with his usual ability by Lord Kelvin, in the following year. The inflow of meteorites, he remarked, "is the only one of all conceivable causes of solar heat which we know to exist from independent evidence."[1154] We know it to exist, but we now also know it to be entirely insufficient. The supplies presumed to be contained in the zodiacal light would be quickly exhausted; a constant inflow from space would be needed to meet the demand. But if moving bodies were drawn into the sun at anything like the required rate, the air, even out here at ninety-three millions of miles distance, would be thick with them; the earth would be red-hot from their impacts;[1155] geological deposits would be largely meteoric;[1156] to say nothing of the effects on the mechanism of the heavens. Lord Kelvin himself urged the inadmissibility of the "extra-planetary" theory of meteoric supply on the very tangible ground that, if it were true, the year would be shorter now, actually by six weeks, than at the opening of the Christian era. The "intra-planetary" supply, however, is too scanty to be anything more than a temporary makeshift.

The meteoric hypothesis was naturally extended from the maintenance of the sun's heat to the formation of the bodies circling round him. The earth--no less doubtless than the other planets--is still growing. Cosmical matter in the shape of falling stars and aerolites, to the amount, adopting Professor Newton's estimate, of 100 tons daily, is swept up by it as it pursues its orbital round. Inevitably the idea suggested itself that this process of appropriation gives the key to the life-history of our globe, and that the momentary streak of fire in the summer sky represents a feeble survival of the glowing hailstorm by which in old times it was fashioned and warmed. Mr. E. W. Brayley supported this view of planetary production in 1864,[1157] and it has recommended itself to Haidinger, Helmholtz, Proctor, and Faye. But the negative evidence of geological deposits appears fatal to it.

The theory of solar energy now generally regarded as the true one was enounced by Helmholtz in a popular lecture in 1854. It depends upon the same principle of the equivalence of heat and motion which had suggested the meteoric hypothesis. But here the movement surrendered and transformed belongs to the particles, not of any foreign bodies, but of the sun itself. Drawn together from a wide ambit by the force of their own gravity, their fall towards the sun's centre must have engendered a vast thermal store, of which 453/454 are computed to be already spent. Presumably, however, this stream of reinforcement is still flowing. In the very act of parting with heat, the sun develops a fresh stock. His radiations, in short, are the direct result of shrinkage through cooling. A diminution of the solar diameter by 380 feet yearly would just suffice to cover the present rate of emission, and would for ages remain imperceptible with our means of observation, since, after the lapse of 6,000 years, the lessening of angular size would scarcely amount to one second.[1158] But the process, though not terminated, is strictly a terminable one. In less than five million years, the sun will have contracted to half its present bulk. In seven million more, it will be as dense as the earth. It is difficult to believe that it will then be a luminous body.[1159] Nor can an unlimited past duration be admitted. Helmholtz considered that radiation might have gone on with its actual intensity for twenty-two, Langley allows only eighteen million years. The period can scarcely be stretched, by the most generous allowances, to double the latter figure. But this is far from meeting the demands of geologists and biologists.

An attempt was made in 1881 to supply the sun with machinery analogous to that of a regenerative furnace, enabling it to consume the same fuel over and over again, and so to prolong indefinitely its beneficent existence. The inordinate "waste" of energy, which shocks our thrifty ideas, was simultaneously abolished. The earth stops and turns variously to account one 2,250-millionth part of the solar radiations; each of the other planets and satellites takes a proportionate share; the rest, being all but an infinitesmal fraction of the whole, is dissipated through endless space, to serve what purpose we know not. Now, on the late Sir William Siemens's plan, this reckless expenditure would cease; the solar incomings and outgoings would be regulated on approved economic principles, and the inevitable final bankruptcy would be staved off to remote ages.

But there was a fatal flaw in its construction. He imagined a perpetual circulation of combustible materials, alternately surrendering and regaining chemical energy, the round being kept going by the motive force of the sun's rotation.[1160] This, however, was merely to perch the globe upon a tortoise, while leaving the tortoise in the air. The sun's rotation contains a certain definite amount of mechanical power--enough, according to Lord Kelvin, if directly converted into heat, to keep up the sun's emission during 116 years and six days--a mere moment in cosmical time. More economically applied, it would no doubt go farther. Its exhaustion would, nevertheless, under the most favourable circumstances, ensue in a comparatively short period.[1161] Many other objections equally unanswerable have been urged to the "regenerative" hypothesis, but this one suffices.

Dr. Croll's collision hypothesis[1162] is less demonstrably unsound, but scarcely less unsatisfactory. By the mutual impact of two dark masses rushing together with tremendous speed, he sought to provide the solar nebula with an immense original stock of heat for the reinforcement of that subsequently evolved in the course of its progressive contraction. The sun, while still living on its capital, would thus have a larger capital to live on, and the time-demands of the less exacting geologists and biologists might be successfully met. But the primitive event, assumed for the purpose of dispensing them from the inconvenience of "hurrying up their phenomena," is not one that a sane judgment can readily admit to have ever, in point of actual fact, happened.

There remains, then, as the only intelligible rationale of solar sustentation, Helmholtz's shrinkage theory. And this has a very important bearing upon the nebular view of planetary formation; it may, in fact, be termed its complement. For it involves the idea that the sun's materials, once enormously diffused, gradually condensed to their present volume with development of heat and light, and, it may plausibly be added, with the separation of dependent globes. The data furnished by spectrum analysis, too, favour the supposition of a common origin for sun and planets by showing their community of substance; while gaseous nebulae present examples of vast masses of tenuous vapour, such as our system may plausibly be conjectured to have primitively sprung from.

But recent science raises many objections to the details, if it supplies some degree of confirmation to the fundamental idea of Laplace's cosmogony. The detection of the retrograde movement of Neptune's satellite made it plain that the anomalous conditions of the Uranian world were due to no extraordinary disturbance, but to a systematic variety of arrangement at the outskirts of the solar domain. So that, were a trans-Neptunian planet discovered, we should be fully prepared to find it rotating, and surrounded by satellites circulating from east to west. The uniformity of movement, upon the probabilities connected with which the French geometer mainly based his scheme, thus at once vanishes.

The excessively rapid revolution of the inner Martian moon is a further stumbling-block. On Laplace's view, no satellite can revolve in a shorter time than its primary rotates; for in its period of circulation survives the period of rotation of the parent mass which filled the sphere of its orbit at the time of giving it birth. And rotation quickens as contraction goes on; therefore, the older time of axial rotation should invariably be the longer. This obstacle can, however, as we shall presently see, be turned.

More serious is one connected with the planetary periods, pointed out by Babinet in 1861.[1163] In order to make them fit in with the hypothesis of successive separation from a rotating and contracting body, certain arbitrary assumptions have to be made of fluctuations in the distribution of the matter forming that body at the various epochs of separation.[1164] Such expedients usually merit the distrust which they inspire. Primitive and permanent irregularities of density in the solar nebula, such as Miss Young's calculations suggest,[1165] do not, on the other hand, appear intrinsically improbable.

Again, it was objected by Professor Kirkwood in 1869[1166] that there could be no sufficient cohesion in such an enormously diffused mass as the planets are supposed to have sprung from to account for the wide intervals between them. The matter separated through the growing excess of centrifugal speed would have been cast off, not by rarely recurring efforts, but continually, fragmentarily, pari passu with condensation and acceleration. Each wisp of nebula, as it found itself unduly hurried, would have declared its independence, and set about revolving and condensing on its own account. The result would have been a meteoric, not a planetary system.

Moreover, it is a question whether the relative ages of the planets do not follow an order just the reverse of that concluded by Laplace. Professor Newcomb holds the opinion that the rings which eventually constituted the planets divided from the main body of the nebula almost simultaneously, priority, if there were any, being on the side of the inner and smaller ones;[1167] while in M. Faye's cosmogony,[1168] the retrograde motion of the systems formed by the two outer planets is ascribed--on grounds, it is true, of dubious validity--to their comparatively late origin.

This ingenious scheme was designed, not merely to complete, but to supersede that of Laplace, which, undoubtedly, through the inclusion by our system of oppositely directed rotations, forfeits its claim simply and singly to account for the fundamental peculiarities of its structure.

M. Faye's leading contention is that, under the circumstances assumed by Laplace, not the two outer planets alone, but the whole company must have been possessed of retrograde rotation. For they were formed--ex hypothesi--after the sun; central condensation had reached an advanced stage when the rings they were derived from separated; the principle of inverse squares consequently held good, and Kepler's Laws were in full operation. Now, particles circulating in obedience to these laws can only--since their velocity decreases outward from the centre of attraction--coalesce into a globe with a backward axial movement. Nor was Laplace blind to this flaw in his theory; but his effort to remove it, though it passed muster for the best part of a century,[1169] was scarcely successful. His planet-forming rings were made to rotate all in one piece, their outer parts thus necessarily travelling at a swifter linear rate than their inner parts, and eventually uniting, equally of necessity, into a forward-spinning body. The strength of cohesion involved may, however, safely be called impossible, especially when it is considered that nebulous materials were in question.

The reform proposed by M. Faye consists in admitting that all the planets inside Uranus are of pre-solar origin--that they took globular form in the bosom of a nearly homogeneous nebula, revolving in a single period, with motion accelerated from centre to circumference, and hence agglomerating into masses with a direct rotation. Uranus and Neptune owe their exceptional characteristics to their later birth. When they came into existence, the development of the sun was already far advanced, central force had acquired virtually its present strength, unity of period had been abolished by its predominance, and motion was retarded outward.

Thus, what we may call the relative chronology of the solar system is thrown once more into confusion. The order of seniority of the planets is now no easier to determine than the "Who first, who last?" among the victims of Hector's spear. For M. Faye's arrangements, notwithstanding the skill with which he has presented them, cannot be unreservedly accepted. The objections to them, thoughtfully urged by M. C. Wolf[1170] and Professor Darwin,[1171] are grave. Not the least so is his omission to take account of an agency of change presently to be noticed.

A further valuable discussion of the matter was published by M. du Ligondes in 1897.[1172] His views are those of Faye, modified to disarm the criticisms they had encountered; and special attention may be claimed for his weighty remark that each planet has a life-history of its own, essentially distinct from those of the others, and, despite original unity, not to be confounded with them. The drift of recent investigations seems, indeed, to be to find the embryonic solar system already potentially complete in the parent nebula, like the oak in an acorn, and to relegate detailed explanations of its peculiarities to the dim pre-nebular fore-time.

We now come to a most remarkable investigation--one, indeed, unique in its profession to lead us back with mathematical certainty towards the origin of a heavenly body. We refer to Professor Darwin's inquiries into the former relations of the earth and moon.[1173]

They deal exclusively with the effects of tidal friction, and primarily with those resulting, not from oceanic, but from "bodily" tides, such as the sun and moon must have raised in past ages on a liquid or viscous earth. The immediate effect of either is, as already explained, to destroy the rotation of the body on which the tide is raised, as regards the tide-raising body, bringing it to turn always the same face towards its disturber. This, we can see, has been completely brought about in the case of the moon. There is, however, a secondary or reactive effect. Action is always mutual. Precisely as much as the moon pulls the terrestrial tidal wave backward, the tidal wave pulls the moon forward. But pulling a body forward in its orbit implies the enlargement of that orbit; in other words, the moon is, as a consequence of tidal friction, very slowly receding from the earth. This will go on (other circumstances remaining unchanged) until the lengthening day overtakes the more tardily lengthening month, when each will be of about 1,400 hours.[1174] A position of what we may call tidal equilibrium between earth and moon will (apart from disturbance by other bodies) then be attained.

If, however, it be true that, in the time to come, the moon will be much farther from us, it follows that in the time past she was much nearer to us than she now is. Tracing back her history by the aid of Professor Darwin's clue, we at length find her revolving in a period of somewhere between two and four hours, almost in contact with an earth rotating just at the same rate. This was before tidal friction had begun its work of grinding down axial velocity and expanding orbital range. But the position was not one of stable equilibrium. The slightest inequality must have set on foot a series of uncompensated changes. If the moon had whirled the least iota faster than the earth spun she must have been precipitated upon it. Her actual existence shows that the trembling balance inclined the other way. By a second or two to begin with, the month exceeded the day; the tidal wave crept ahead of the moon; tidal friction came into play, and our satellite started on its long spiral journey outward from the parent globe. This must have occurred, it is computed, at least fifty-four million years ago.

That this kind of tidal reactive effect played its part in bringing the moon into its present position, and is still, to some slight extent, at work in changing it, there can be no doubt whatever. An irresistible conjecture carried the explorer of its rigidly deducible consequences one step beyond them. The moon's time of revolution, when so near the earth as barely to escape contact with it, must have been, by Kepler's Law, more than two and less than two and a half hours. Now it happens that the most rapid rate of rotation of a fluid mass of the earth's average density, consistent with spheroidal equilibrium, is two hours and twenty minutes. Quicken the movement but by one second and the globe must fly asunder. Hence the inference that the earth actually did fly asunder through over-fast spinning, the ensuing disruption representing the birth-throes of the moon. It is likely that the event was hastened or helped by solar tidal disturbance.

To recapitulate. Analysis tracks backward the two bodies until it leaves them in very close contiguity, one rotating and the other revolving in approximately the same time, and that time certainly not far different from, and quite possibly identical with, the critical period of instability for the terrestrial spheroid. "Is this," Professor Darwin asks, "a mere coincidence, or does it not rather point to the break-up of the primeval planet into two masses in consequence of a too rapid rotation?"[1175]

We are tempted, but are not allowed to give an unqualified assent. Mr. James Nolan of Victoria has made it clear that the moon could not have subsisted as a continuous mass under the powerful disruptive strain which would have acted upon it when revolving almost in contact with the present surface of the earth; and Professor Darwin, admitting the objection, concedes to our satellite, in its initial stage, the alternative form of a flock of meteorites.[1176] But such a congregation must have been quickly dispersed, by tidal action, into a meteoric ring. The same investigator subsequently fixed 6,500 miles from centre to centre as the minimum distance at which the moon could have revolved in its entirety; and he concluded it "necessary to suppose that, after the birth of a satellite, if it takes place at all in this way, a series of changes occur which are quite unknown."[1177] The evidence, however, for the efficiency of tidal friction in bringing about the actual configuration of the lunar-terrestrial system is not invalidated by this failure to penetrate its natal mystery. Under its influence the principal elements of that system fall into interdependent mutual relations. It connects, casually and quantitatively, the periods of the moon's revolution and of the earth's rotation, the obliquity of the ecliptic, the inclination and eccentricity of the lunar orbit. All this can scarcely be accidental.

Professor Darwin's first researches on this subject were communicated to the Royal Society, December 18, 1879. They were followed, January 20, 1881,[1178] by an inquiry on the same principles into the earlier condition of the entire solar system. The results were a warning against hasty generalisation. They showed that the lunar-terrestrial system, far from being a pattern for their development, was a singular exception among the bodies swayed by the sun. Its peculiarity resides in the fact that the moon is proportionately by far the most massive attendant upon any known planet. Its disturbing power over its primary is thus abnormally great, and tidal friction has, in consequence, played a predominant part in bringing their mutual relations into their present state.

The comparatively late birth of the moon tends to ratify this inference. The dimensions of the earth did not differ (according to our present authority) very greatly from what they now are when her solitary offspring came, somehow, into existence. This is found not to have been the case with any other of the planets. It is unlikely that the satellites of Jupiter, Saturn, or Mars (we may safely add, of Uranus or Neptune) ever revolved in much narrower orbits than those they now traverse; it is practically certain that they did not, like our moon, originate very near the present surfaces of their primaries.[1179] What follows? The tide-raising power of a body grows with vicinity in a rapidly accelerated ratio. Lunar tides must then have been on an enormous scale when the moon swung round at a fraction of its actual distance from the earth. But no other satellite with which we are acquainted occupied at any time a corresponding position. Hence no other satellite ever possessed tide-raising capabilities in the least comparable to those of the moon. We conclude once more that tidal friction had an influence here very different from its influence elsewhere. Quite possibly, however, that influence may be more nearly spent than in less advanced combinations of revolving globes. Mr. Nolan concluded in 1895[1180] that it still retains appreciable efficacy in the several domains of the outer planets. The moons of Jupiter and Saturn are, by his calculations, in course of sensible retreat, under compulsion of the perennial ripples raised by them on the surfaces of their gigantic primaries. He thus connects the interior position of the fifth Jovian satellite with its small mass. The feebleness of its tide-raising power obliged it to remain behind its companions; for there is no sign of its being more juvenile than the Galilean quartette.

The yielding of plastic bodies to the strain of unequal attractions is a phenomenon of far-reaching consequence. We know that the sun as well as the moon causes tides in our oceans. There must, then, be solar, no less than lunar, tidal friction. The question at once arises: What part has it played in the development of the solar system? Has it ever been one of leading importance, or has its influence always been, as it now is, subordinate, almost negligible? To this, too, Professor Darwin supplies an answer.

It can be stated without hesitation that the sun did not give birth to the planets, as the earth has been supposed to have given birth to the moon, by the disruption of its already condensed, though viscous and glowing mass, pushing them then gradually backward from its surface into their present places. For the utmost possible increase in the length of the year through tidal friction is one hour; and five minutes is a more probable estimate.[1181] So far as the pull of tide-waves raised on the sun by the planets is concerned, then, the distances of the latter have never been notably different from what they now are; though that cause may have converted the paths traversed by them from circles into ellipses.

Over their physical history, however, it was probably in a large measure influential. The first vital issue for each of them was--satellites or no satellites? Were they to be governors as well as governed, or should they revolve in sterile isolation throughout the aeons of their future existence? Here there is strong reason to believe that solar tidal friction was the overruling power. It is remarkable that planetary fecundity increases--at least so far outward as Saturn--with distance from the sun. Can these two facts be in any way related? In other words, is there any conceivable way by which tidal influence could prevent or impede the throwingoff of secondary bodies? We have only to think for a moment in order to see that this is precisely one of its direct results.[1182]

Tidal friction, whether solar or lunar, tends to reduce the axial movement of the body it acts upon. But the separation of satellites depends--according to the received view--upon the attainment of a disruptive rate of rotation. Hence, if solar tidal friction were strong enough to keep down the pace below this critical point, the contracting mass would remain intact--there would be no satellite-production. This, in all probability, actually occurred in the case both of Mercury and Venus. They cooled without dividing, because the solar friction-brake applied to them was too strong to permit acceleration to pass the limit of equilibrium. The complete destruction of their relative axial movement has been rendered probable by recent observations; and that the process went on rapidly is a reasonable further inference. The earth barely escaped the fate of loneliness incurred by her neighbours. Her first and only epoch of instability was retarded until she had nearly reached maturity. The late appearance of the moon accounts for its large relative size--through the increased cohesion of an already strongly condensed parent mass--and for the distinctive peculiarities of its history and influence on the producing globe.

Solar tidal friction, although it did not hinder the formation of two minute dependents of Mars, has been invoked to explain the anomalously rapid revolution of one of them. Phobos, we have seen, completes more than three revolutions while Mars rotates once. But this was probably not always so. The two periods were originally nearly equal. The difference, it is alleged, was brought about by tidal waves raised by the sun on the semi-fluid spheroid of Mars. Rotatory velocity was thereby destroyed, the Martian day slowly lengthened, and, as a secondary consequence, the period of the inner satellite, become shorter than the augmented day, began progressively to diminish. So that Phobos, unlike our moon, was in the beginning farther from its primary than now.

But here again Mr. Nolan entered a caveat. Applying the simple test of numerical evaluation, he showed that before solar tidal friction could lengthen the rotation-period of Mars by so much as one minute, Phobos should have been precipitated upon its surface.[1183] For the enormous disparity of mass between it and the sun is so far neutralised by the enormous disparity in their respective distances from Mars that solar tidal force there is only fifty times that of the little satellite. But the tidal effects of a satellite circulating quicker than its primary rotates exactly reverse those of one moving, like our moon, comparatively slowly, so that the tides raised by Phobos tend to shorten both periods. Its orbital momentum, however, is so extremely small in proportion to the rotational momentum of Mars, that any perceptible inroad upon the latter is attended by a lavish and ruinous expenditure of the former. It is as if a man owning a single five-pound note were to play for equal stakes with a man possessing a million. The bankruptcy sure to ensue is typified by the coming fate of the Martian inner satellite. The catastrophe of its fall needs to bring it about only a very feeble reactive pull compared with the friction which the sun should apply in order to protract the Martian day by one minute. And from the proportionate strength of the forces at work, it is quite certain that one result cannot take place without the other. Nor can things have been materially different in the past; hence the idea must be abandoned that the primitive time of rotation of Mars survives in the period of its inner satellite.

The anomalous shortness of the latter may, however, in M. Wolf's opinion,[1184] be explained by the "trainees elliptiques" with which Roche supplemented nebular annulation.[1185] These are traced back to the descent of separating strata from the shoulders of the great nebulous spheroid towards its equatorial plane. Their rotational velocity being thus relatively small, they formed "inner rings," very much nearer to the centre of condensation than would have been possible on the unmodified theory of Laplace. Phobos might, in this view, be called a polar offset of Mars; and the rings of Saturn are thought to own a similar origin.

Outside the orbit of Mars, solar tidal friction can scarcely be said to possess at present any sensible power. But it is far from certain that this was always so. It seems not unlikely that its influence was the overruling one in determining the direction of planetary rotation. M. Faye, as we have seen, objected to Laplace's scheme that only retrograde secondary systems could be produced by it. In this he was anticipated by Kirkwood, who, however, supplied an answer to his own objection.[1186]

Sun-raised tides must have acted with great power on the diffused masses of the embryo planets. By their means they doubtless very soon came to turn (in lunar fashion) the same hemisphere always towards their centre of motion. This amounts to saying that even if they started with retrograde rotation, it was, by solar tidal friction, quickly rendered direct.[1187] For it is scarcely necessary to point out that a planet turning an invariable face to the sun rotates in the same direction in which it revolves, and in the same period. As, with the progress of condensation, tides became feebler and rotation more rapid, the accelerated spinning necessarily proceeded in the sense thus prescribed for it. Hence the backward axial movements of Uranus and Neptune may very well be a survival, due to the inefficiency of solar tides at their great distance, of a state of things originally prevailing universally throughout the system.

The general outcome of Mr. Darwin's researches has been to leave Laplace's cosmogony untouched. He concludes nothing against it, and, what perhaps tells with more weight in the long run, has nothing to substitute for it. In one form or the other, if we speculate at all on the development of the planetary system, our speculations are driven into conformity with the broad lines of the Nebular Hypothesis--to the extent, at least, of admitting an original material unity and motive uniformity. But we can see now, better than formerly, that these supply a bare and imperfect sketch of the truth. We should err gravely were we to suppose it possible to reconstruct, with the help of any knowledge our race is ever likely to possess, the real and complete history of our admirable system. "The subtlety of nature," Bacon says, "transcends in many ways the subtlety of the intellect and senses of man." By no mere barren formula of evolution, indiscriminately applied all round, the results we marvel at, and by a fragment of which our life is conditioned, were brought forth; but by the manifold play of interacting forces, variously modified and variously prevailing, according to the local requirements of the design they were appointed to execute.

FOOTNOTES:

[Footnote 1150: Exposition du Systeme du Monde, t. ii., p. 295.]

[Footnote 1151: In later editions a retrospective clause was added admitting a prior condition of all but evanescent nebulosity.]

[Footnote 1152: Mec. Cel., lib. xiv., ch. iii.]

[Footnote 1153: Beitraege zur Dynamik des Himmels, p. 12.]

[Footnote 1154: Trans. Roy. Soc. of Edinburgh, vol. xxi., p. 66.]

[Footnote 1155: Newcomb, Pop. Astr., p. 521 (2nd ed.).]

[Footnote 1156: M. Williams, Nature, vol. iii., p. 26.]

[Footnote 1157: Comp. Brit. Almanac, p. 94.]

[Footnote 1158: Radau, Bull. Astr., t. ii., p. 316.]

[Footnote 1159: Newcomb, Pop. Astr., pp. 521-525.]

[Footnote 1160: Proc. Roy. Soc., vol. xxxiii., p. 393.]

[Footnote 1161: To this hostile argument, as urged by Mr. E. Douglas Archibald, Sir W. Siemens opposed the increase of rotative velocity through contraction (Nature, vol. xxv., p. 505). But contraction cannot restore lost momentum.]

[Footnote 1162: _Stellar Evolution, and its Relations to Geological Time_, 1889.]

[Footnote 1163: Comptes Rendus, t. lii., p. 481. See also Kirkwood, Observatory, vol. iii., p. 409.]

[Footnote 1164: Fouche, Comptes Rendus, t. xcix., p. 903.]

[Footnote 1165: Astroph. Jour., vol. xiii., p. 338.]

[Footnote 1166: Month. Not., vol. xxix., p. 96.]

[Footnote 1167: Pop. Astr., p. 257.]

[Footnote 1168: Sur l'Origine du Monde, 1884.]

[Footnote 1169: Kirkwood adverted to it in 1864, Am. Jour., vol. xxxviii., p. 1.]

[Footnote 1170: Bull. Astr., t. ii.]

[Footnote 1171: Nature, vol. xxxi., p. 506.]

[Footnote 1172: Formation Mecanique du Systeme du Monde; Bull. Astr., t. xiv., p. 313 (O. Callandreau). See also, Le Probleme Solaire, by l'Abbe Th. Moreux, 1900.]

[Footnote 1173: Phil. Trans., vol. clxxi., p. 713.]

[Footnote 1174: Mr. J. Nolan has pointed out (Nature, vol. xxxiv., p. 287) that the length of the equal day and month will be reduced to about 1,240 hours by the effects of solar tidal friction.]

[Footnote 1175: Phil. Trans., vol. clxxi., p. 835.]

[Footnote 1176: Nature, vol. xxxiii., p. 368; see also Nolan, Ibid., vol. xxxiv., p. 286.]

[Footnote 1177: Phil. Trans., vol. clxxviii., p. 422.]

[Footnote 1178: Ibid., vol. clxxii., p. 491.]

[Footnote 1179: Ibid., p. 530.]

[Footnote 1180: Satellite Evolution, Melbourne, 1895; Knowledge, vol. xviii., p. 205.]

[Footnote 1181: Phil. Trans., vol. clxxii., p. 533.]

[Footnote 1182: This was perceived by M. Ed. Roche in 1872. Mem. de l'Acad. des Sciences de Montpellier, t. viii., p. 247.]

[Footnote 1183: Nature, vol. xxxiv., p. 287.]

[Footnote 1184: Bull. Astr., t. ii., p. 223.]

[Footnote 1185: Montpellier Mems., t. viii., p. 242.]

[Footnote 1186: Amer. Jour., vol. xxxviii. (1864), p. 1.]

[Footnote 1187: Wolf, Bull. Astr., t. ii., p. 76.]

Chapter X

RECENT COMETS

On the 2nd of June, 1858, Giambattista Donati discovered at Florence a feeble round nebulosity in the constellation Leo, about one-tenth the diameter of the full moon. It proved to be a comet approaching the sun. But it changed little in apparent place or brightness for some weeks. The gradual development of a central condensation of light was the first symptom of coming splendour. At Harvard, in the middle of July, a strong stellar nucleus was seen; on August 14 a tail began to be thrown out. As the comet wanted still over six weeks of the time of its perihelion-passage, it was obvious that great things might be expected of it. They did not fail of realisation.

Not before the early days of September was it generally recognised with the naked eye, though it had been detected without a glass at Pulkowa, August 19. But its growth was thenceforward surprisingly rapid, as it swept with accelerated motion under the hindmost foot of the Great Bear, and past the starry locks of Berenice. A sudden leap upward in lustre was noticed on September 12, when the nucleus shone with about the brightness of the pole-star, and the tail, notwithstanding large foreshortening, could be traced with the lowest telescopic power over six degrees of the sphere. The appendage, however, attained its full development only after perihelion, September 30, by which time, too, it lay nearly square to the line of sight from the earth. On October 10 it stretched in a magnificent scimitar-like curve over a third and upwards of the visible hemisphere, representing a real extension in space of fifty-four million miles. But the most striking view was presented on October 5, when the brilliant star Arcturus became involved in the brightest part of the tail, and during many hours contributed, its lustre undiminished by the interposed nebulous screen, to heighten the grandeur of the most majestic celestial object of which living memories retain the impress. Donati's comet was, according to Admiral Smyth's testimony,[1188] outdone "as a mere sight-object" by the great comet of 1811; but what it lacked in splendour, it surely made up in grace, and variety of what we may call "scenic" effects.

Some of these were no less interesting to the student than impressive to the spectator. At Pulkowa, on the 16th September, Winnecke,[1189] the first director of the Strasburg Observatory, observed a faint outer envelope resembling a veil of almost evanescent texture flung somewhat widely over the head. Next evening, the first of the "secondary" tails appeared, possibly as part of the same phenomenon. This was a narrow straight ray, forming a tangent to the strong curve of the primary tail, and reaching to a still greater distance from the nucleus. It continued faintly visible for about three weeks, during part of which time it was seen in duplicate. For from the chief train itself, at a point where its curvature abruptly changed, issued, as if through the rejection of some of its materials, a second beam nearly parallel to the first, the rigid line of which contrasted singularly with the softly diffused and waving aspect of the plume of light from which it sprang. Olbers's theory of unequal repulsive forces was never more beautifully illustrated. The triple tail seemed a visible solar analysis of cometary matter.

The processes of luminous emanation going on in this body forcibly recalled the observations made on the comets of 1744 and 1835. From the middle of September, the nucleus, estimated by Bond to be under five hundred miles in diameter, was the centre of action of the most energetic kind. Seven distinct "envelopes" were detached in succession from the nebulosity surrounding the head, and after rising towards the sun during periods of from four to seven days, finally cast their material backward to form the right and left branches of the great train. The separation of these by an obscure axis--apparently as black, quite close up to the nucleus, as the sky--indicated for the tail a hollow, cone-like structure;[1190] while the repetition of certain spots and rays in the same corresponding situation on one envelope after another served to show that the nucleus--to some local peculiarity of which they were doubtless due--had no proper rotation, but merely shifted sufficiently on an axis to preserve the same aspect towards the sun as it moved round it.[1191] This observation of Bond's was strongly confirmatory of Bessel's hypothesis of opposite polarities in such bodies' opposite sides.

The protrusion towards the sun, on September 25, of a brilliant luminous fan-shaped sector completed the resemblance to Halley's comet. The appearance of the head was now somewhat that of a "bat's-wing" gaslight. There were, however, no oscillations to and fro, such as Bessel had seen and speculated upon in 1835. As the size of the nucleus contracted with approach to perihelion, its intensity augmented. On October 2, it outshone Arcturus, and for a week or ten days was a conspicuous object half an hour after sunset. Its lustre--setting aside the light derived from the tail--was, at that date, 6,300 times what it had been on June 15, though theoretically--taking into account, that is, only the differences of distance from sun and earth--it should have been only 1/33 of that amount. Here, it might be thought, was convincing evidence of the comet itself becoming ignited under the growing intensity of the solar radiations. Yet experiments with the polariscope were interpreted in an adverse sense, and Bond's conclusion that the comet sent us virtually unmixed reflected sunshine was generally acquiesced in. It was, nevertheless, negatived by the first application of the spectroscope to these bodies.

Very few comets have been so well or so long observed as Donati's. It was visible to the naked eye during 112 days; it was telescopically discernible for 275, the last observation having been made by Mr. William Mann at the Cape of Good Hope, March 4, 1859. Its course through the heavens combined singularly with the orbital place of the earth to favour curious inspection. The tail, when near its greatest development, lost next to nothing by the effects of perspective, and at the same time lay in a plane sufficiently inclined to the line of sight to enable it to display its exquisite curves to the greatest advantage. Even the weather was, on both sides of the Atlantic, propitious during the period of greatest interest, and the moon as little troublesome as possible. The volume compiled by the younger Bond is a monument to the care and skill with which these advantages were turned to account. Yet this stately apparition marked no turning-point in the history of cometary science. By its study knowledge was indeed materially advanced, but along the old lines. No quick and vivid illumination broke upon its path. Quite insignificant objects--as we have already partly seen--have often proved more vitally instructive.

Donati's comet has been identified with no other. Its path is an immensely elongated ellipse, lying in a plane far apart from that of the planetary movements, carrying it at perihelion considerably within the orbit of Venus, and at aphelion out into space to 5-1/2 times the distance from the sun of Neptune. The entire circuit occupies over 2,000 years, and is performed in a retrograde direction, or against the order of the Signs. Before its next return, about the year 4000 A.D., the enigma of its presence and its purpose may have been to some extent--though we may be sure not completely--penetrated.

On June 30, 1861, the earth passed, for the second time in the century, through the tail of a great comet. Some of our readers may remember the unexpected disclosure, on the withdrawal of the sun below the horizon on that evening, of an object so remarkable as to challenge universal attention. A golden-yellow planetary disc, wrapt in dense nebulosity, shone out while the June twilight of these latitudes was still in its first strength. The number and complexity of the envelopes surrounding the head produced, according to the late Mr. Webb,[1192] a magnificent effect. Portions of six distinct emanations were traceable. "It was as though a number of light, hazy clouds were floating round a miniature full moon." As the sky darkened the tail emerged to view.[1193] Although in brightness and sharpness of definition it could not compete with the display of 1858, its dimensions proved to be extraordinary. It reached upwards beyond the zenith when the head had already set. By some authorities its extreme length was stated at 118 deg., and it showed no trace of curvature. Most remarkable, however, was the appearance of two widely divergent rays, each pointing towards the head, though cut off from it by sky-illumination, of which one was seen by Mr. Webb, and both by Mr. Williams at Liverpool, a quarter of an hour before midnight. There seems no doubt that Webb's interpretation was the true one, and that these beams were, in fact, "the perspective representation of a conical or cylindrical tail, hanging closely above our heads, and probably just being lifted up out of our atmosphere."[1194] The cometary train was then rapidly receding from the earth, so that the sides of the "outspread fan" of light shown by it when we were right in the line of its axis must have appeared (as they did) to close up in departure. The swiftness with which the visually opened fan shut proved its vicinity; and, indeed, Mr. Hind's calculations showed that we were not so much near as actually within its folds at that very time.

Already M. Liais, from his observations at Rio de Janeiro, June 11 to 14, and Mr. Tebbutt, by whom the comet was discovered in New South Wales on May 13, had anticipated such an encounter, while the former subsequently proved that it must have occurred in such a way as to cause an immersion of the earth in cometary matter to a depth of 300,000 miles.[1195] The comet then lay between the earth and the sun at a distance of about fourteen million miles from the former; its tail stretched outward just along the line of intersection of its own with the terrestrial orbit to an extent of fifteen million miles; so that our globe, happening to pass at the time, found itself during some hours involved in the flimsy appendage.

No perceptible effects were produced by the meeting; it was known to have occurred by theory alone. A peculiar glare in the sky, thought by some to have distinguished the evening of June 30, was, at best, inconspicuous. Nor were there any symptoms of unusual electric excitement. The Greenwich instruments were, indeed, disturbed on the following night, but it would be rash to infer that the comet had art or part in their agitation.

The perihelion-passage of this body occurred June 11, 1861; and its orbit has been shown by M. Kreutz of Bonn, from a very complete investigation founded on observations extending over nearly a year, to be an ellipse traversed in a period of 409 years.[1196]

Towards the end of August, 1862, a comet became visible to the naked eye high up in the northern hemisphere, with a nucleus equalling in brightness the lesser stars of the Plough and a feeble tail 20 deg. in length. It thus occupied quite a secondary position among the members of its class. It was, nevertheless, a splendid object in comparison with a telescopic nebulosity discovered by Tempel at Marseilles, December 19, 1865. This, the sole comet of 1866, slipped past perihelion, January 11, without pomp of train or other appendages, and might have seemed hardly worth the trouble of pursuing. Fortunately, this was not the view entertained by observers and computers; since upon the knowledge acquired of the movements of these two bodies has been founded one of the most significant discoveries of modern times. The first of them is now styled the comet (1862 iii.) of the August meteors, the second (1866 i.) that of the November meteors. The steps by which this curious connection came to be ascertained were many, and were taken in succession by a number of individuals. But the final result was reached by Schiaparelli of Milan, and remains deservedly associated with his name.

The idea prevalent in the eighteenth century as to the nature of shooting stars was that they were mere aerial ignes fatui--inflammable vapours accidentally kindled in our atmosphere. But Halley had already entertained the opinion of their cosmical origin; and Chladni in 1794 formally broached the theory that space is filled with minute circulating atoms, which, drawn by the earth's attraction, and ignited by friction in its gaseous envelope, produce the luminous effects so frequently witnessed.[1197] Acting on his suggestion, Brandes and Benzenberg, two students at the University of Gottingen, began in 1798 to determine the heights of falling stars by simultaneous observations at a distance. They soon found that they move with planetary velocities in the most elevated regions of our atmosphere, and by the ascertainment of this fact laid a foundation of distinct knowledge regarding them. Some of the data collected, however, served only to perplex opinion, and even caused Chladni temporarily to renounce his. Many high authorities, headed by Laplace in 1802, declared for the lunar-volcanic origin of meteorites; but thought on the subject was turbid, and inquiry seemed only to stir up the mud of ignorance. It needed one of those amazing spectacles, at which man assists, no longer in abject terror for his own frail fortunes, but with keen curiosity and the vivid expectation of new knowledge, to bring about a clarification.

On the night of November 12-13, 1833, a tempest of falling stars broke over the earth. North America bore the brunt of its pelting. From the Gulf of Mexico to Halifax, until daylight with some difficulty put an end to the display, the sky was scored in every direction with shining tracks and illuminated with majestic fireballs. At Boston the frequency of meteors was estimated to be about half that of flakes of snow in an average snowstorm. Their numbers, while the first fury of their coming lasted, were quite beyond counting; but as it waned, a reckoning was attempted, from which it was computed, on the basis of that much diminished rate, that 240,000 must have been visible during the nine hours they continued to fall.[1198]

Now there was one very remarkable feature common to the innumerable small bodies which traversed, or were consumed in our atmosphere that night. They all seemed to come from the same part of the sky. Traced backward, their paths were invariably found to converge to a point in the constellation Leo. Moreover, that point travelled with the stars in their nightly round. In other words, it was entirely independent of the earth and its rotation. It was a point in inter-planetary space.

The effective perception of this fact[1199] amounted to a discovery, as Olmsted and Twining, who had "simultaneous ideas" on the subject, were the first to realize. Denison Olmsted was then Professor of Mathematics in Yale College. He showed early in 1834[1200] that the emanation of the showering meteors from a fixed "radiant" proved their approach to the earth along nearly parallel lines, appearing to diverge by an effect of perspective; and that those parallel lines must be sections of orbits described by them round the sun and intersecting that of the earth. For the November phenomenon was now seen to be a periodical one. On the same night of the year 1832, although with less dazzling and universal splendour than in America in 1833, it had been witnessed over great part of Europe and in Arabia. Olmsted accordingly assigned to the cloud of cosmical particles (or "comet," as he chose to call it), by terrestrial encounters with which he supposed the appearances in question to be produced, a period of about 182 days; its path a narrow ellipse, meeting, near its farthest end from the sun, the place occupied by the earth on November 12.

Once for all, then, as the result of the star-fall of 1833, the study of luminous meteors became an integral part of astronomy. Their membership of the solar system was no longer a theory or a conjecture--it was an established fact. The discovery might be compared to, if it did not transcend in importance, that of the asteroidal group. "C'est un nouveau monde planetaire," Arago wrote,[1201] "qui commence a se reveler a nous."

Evidences of periodicity continued to accumulate. It was remembered that Humboldt and Bonpland had been the spectators at Cumana, after midnight on November 12, 1799, of a fiery shower little inferior to that of 1833, and reported to have been visible from the equator to Greenland. Moreover, in 1834 and some subsequent years, there were waning repetitions of the display, as if through the gradual thinning-out of the meteoric supply. The extreme irregularity of its distribution was noted by Olbers in 1837, who conjectured that we might have to wait until 1867 to see the phenomenon renewed on its former scale of magnificence.[1202] This was the first hint of a thirty-three or thirty-four year period.

The falling stars of November did not alone attract the attention of the learned. Similar appearances were traditionally associated with August 10 by the popular phrase in which they figured as "the tears of St. Lawrence." But the association could not be taken on trust from mediaeval authority. It had to be proved scientifically, and this Quetelet of Brussels succeeded in doing in December, 1836.[1203]

A second meteoric revolving system was thus shown to exist. But its establishment was at once perceived to be fatal to the "cosmical cloud" hypothesis of Olmsted. For if it be a violation of probability to attribute to one such agglomeration a period of an exact year, or sub-multiple of a year, it would be plainly absurd to suppose the movements of two or more regulated by such highly artificial conditions. An alternative was proposed by Adolf Erman of Berlin in 1839.[1204] No longer in clouds, but in closed rings, he supposed meteoric matter to revolve round the sun. Thus the mere circumstance of intersection by a meteoric of the terrestrial orbit, without any coincidence of period, would account for the earth meeting some members of the system at each annual passage through the "node" or point of intersection. This was an important step in advance, yet it decided nothing as to the forms of the orbits of such annular assemblages; nor was it followed up in any direction for a quarter of a century.

Hubert A. Newton took up, in 1864,[1205] the dropped thread of inquiry. The son of a mathematical mother, he attained, at the age of twenty-five, to the dignity of Professor of Mathematics in Yale University, and occupied the post until his death in 1896. The diversion of his powers, however, from purely abstract studies stimulated their effective exercise, and constituted him one of the founders of meteoric astronomy.

A search through old records carried the November phenomenon back to the year 902 A.D., long distinguished as "the year of the stars." For in the same night in which Taormina was captured by the Saracens, and the cruel Aghlabite tyrant Ibrahim ibn Ahmed died "by the judgment of God" before Cosenza, stars fell from heaven in such abundance as to amaze and terrify beholders far and near. This was on October 13, and recurrences were traced down through the subsequent centuries, always with a day's delay in about seventy years. It was easy, too, to derive from the dates a cycle of 33-1/4 years, so that Professor Newton did not hesitate to predict the exhibition of an unusually striking meteoric spectacle on November 13-14, 1866.[1206]

For the astronomical explanation of the phenomena, recourse was had to a method introduced by Erman of computing meteoric orbits. It was found, however, that conspicuous recurrences every thirty-three or thirty-four years could be explained on the supposition of five widely different periods, combined with varying degrees of extension in the revolving group. Professor Newton himself gave the preference to the shortest--of 354-1/2 days, but indicated the means of deciding with certainty upon the true one. It was furnished by the advancing motion of the node, or that day's delay of the November shower every seventy years, which the old chronicles had supplied data for detecting. For this is a strictly measurable effect of gravitational disturbance by the various planets, the amount of which naturally depends upon the course pursued by the disturbed bodies. Here the great mathematical resources of Professor Adams were brought to bear. By laborious processes of calculation, he ascertained that four out of Newton's five possible periods were entirely incompatible with the observed nodal displacement, while for the fifth--that of 33-1/4 years--a perfectly harmonious result was obtained.[1207] This was the last link in the chain of evidence proving that the November meteors--or "Leonids," as they had by that time come to be called--revolve round the sun in a period of 33.27 years, in an ellipse spanning the vast gulf between the orbits of the earth and Uranus, the group being so extended as to occupy nearly three years in defiling past the scene of terrestrial encounters. But before it was completed in March, 1867, the subject had assumed a new aspect and importance.

Professor Newton's prediction of a remarkable star-shower in November, 1866, was punctually fulfilled. This time, Europe served as the main target of the celestial projectiles, and observers were numerous and forewarned. The display, although, according to Mr. Baxendell's memory,[1208] inferior to that of 1833, was of extraordinary impressiveness. Dense crowds of meteors, equal in lustre to the brightest stars, and some rivalling Venus at her best,[1209] darted from east to west across the sky with enormous apparent velocities, and with a certain determinateness of aim, as if let fly with a purpose, and at some definite object.[1210] Nearly all left behind them trains of emerald green or clear blue light, which occasionally lasted many minutes, before they shrivelled and curled up out of sight. The maximum rush occurred a little after one o'clock on the morning of November 14, when attempts to count were overpowered by frequency. But during a previous interval of seven minutes five seconds, four observers at Mr. Bishop's observatory at Twickenham reckoned 514, and during an hour 1,120.[1211] Before daylight the earth had fairly cut her way through the star-bearing stratum; the "ethereal rockets" had ceased to fly.

This event brought the subject of shooting stars once more vividly to the notice of astronomers. Schiaparelli had, indeed, been already attracted by it. The results of his studies were made known in four remarkable letters, addressed, before the close of the year 1866, to Father Secchi, and published in the Bulletino of the Roman Observatory.[1212] Their upshot was to show, in the first place, that meteors possess a real velocity considerably greater than that of the earth, and travel, accordingly, to enormously greater distances from the sun along tracks resembling those of comets in being very eccentric, in lying at all levels indifferently, and in being pursued in either direction. It was next inferred that comets and meteors equally have an origin foreign to the solar system, but are drawn into it temporarily by the sun's attraction, and occasionally fixed in it by the backward pull of some planet. But the crowning fact was reserved for the last. It was the astonishing one that the August meteors move in the same orbit with the bright comet of 1862--that the comet, in fact, is but a larger member of the family named "Perseids" because their radiant point is situated in the constellation Perseus.

This discovery was quickly capped by others of the same kind. Leverrier published, January 21, 1867,[1213] elements for the November swarm, founded on the most recent and authentic observations; at once identified by Dr. C. F. W. Peters of Altona with Oppolzer's elements for Tempel's comet of 1866.[1214] A few days later, Schiaparelli, having recalculated the orbit of the meteors from improved data, arrived at the same conclusion; while Professor Weiss of Vienna pointed to the agreement between the orbits of a comet which had appeared in 1861 and of a star-shower found to recur on April 20 (Lyraids), as well as between those of Biela's comet and certain conspicuous meteors of November 28.[1215]

These instances do not seem to be exceptional. The number of known or suspected accordances of cometary tracks with meteor streams contained in a list drawn up in 1878[1216] by Professor Alexander S. Herschel (who has made the subject peculiarly his own) amounts to seventy-six; although the four first detected still remain the most conspicuous, and perhaps the only absolutely sure examples of a relation as significant as it was, to most astronomers, unexpected.

There had, indeed, been anticipatory ideas. Not that Kepler's comparison of shooting stars to "minute comets," or Maskelyne's "forse risultera che essi sono comete," in a letter to the Abate Cesaris, December 12, 1774,[1217] need count for much. But Chladni, in 1819,[1218] considered both to be fragments or particles of the same primitive matter, irregularly scattered through space as nebulae; and Morstadt of Prague suggested about 1837[1219] that the meteors of November might be dispersed atoms from the tail of Biela's comet, the path of which is cut across by the earth near that epoch. Professor Kirkwood, however, by a luminous intuition, penetrated the whole secret, so far as it has yet been made known. In an article published, or rather buried, in the Danville Quarterly Review for December, 1861, he argued, from the observed division of Biela, and other less noted instances of the same kind, that the sun exercises a "divellent influence" on the nuclei of comets, which may be presumed to continue its action until their corporate existence (so to speak) ends in complete pulverisation. "May not," he continued, "our periodic meteors be the debris of ancient but now disintegrated comets, whose matter has become distributed round their orbits?"[1220]

The gist of Schiaparelli's discovery could not be more clearly conveyed. For it must be borne in mind that with the ultimate destiny of comets' tails this had nothing to do. The tenuous matter composing them is, no doubt, permanently lost to the body from which it emanated; but science does not pretend to track its further wanderings through space. It can, however, state categorically that these will no longer be conducted along the paths forsaken under solar compulsion. From the central, and probably solid parts of comets, on the other hand, are derived the granules by the swift passage of which our skies are seamed with periodic fires. It is certain that a loosely agglomerated mass (such as cometary nuclei most likely are) must gradually separate through the unequal action of gravity on its various parts--through, in short, solar tidal influence. Thenceforward its fragments will revolve independently in parallel orbits, at first as a swarm, finally--when time has been given for the full effects of the lagging of the slower moving particles to develop--as a closed ring. The first condition is still, more or less, that of the November meteors; those of August have already arrived at the second. For this reason, Leverrier pronounced, in 1867, the Perseid to be of older formation than the Leonid system. He even assigned a date at which the introduction of the last-named bodies into their present orbit was probably effected through the influence of Uranus. In 126 A.D. a close approach must have taken place between the planet and the parent comet of the November stars, after which its regular returns to perihelion, and the consequent process of its disintegration, set in. Though not complete, it is already far advanced.

The view that meteorites are the dust of decaying comets was now to be put to a definite test of prediction. Biela's comet had not been seen since its duplicate return in 1852. Yet it had been carefully watched for with the best telescopes; its path was accurately known; every perturbation it could suffer was scrupulously taken into account. Under these circumstances, its repeated failure to come up to time might fairly be thought to imply a cessation from visible existence. Might it not, however, be possible that it would appear under another form--that a star-shower might have sprung from and would commemorate its dissolution?

An unusually large number of falling stars were seen by Brandes, December 6, 1798. Similar displays were noticed in the years 1830, 1838, and 1847, and the point from which they emanated was shown by Heis at Aix-la-Chapelle to be situated near the bright star Gamma Andromedae.[1221] Now this is precisely the direction in which the orbit of Biela's comet would seem to lie, as it runs down to cut the terrestrial track very near the place of the earth at the above dates. The inference was, then, an easy one, that the meteors were pursuing the same path with the comet; and it was separately arrived at, early in 1867, by Weiss, D'Arrest, and Galle.[1222] But Biela travels in the opposite direction to Tempel's comet and its attendant "Leonids"; its motion is direct, or from west to east, while theirs is retrograde. Consequently, the motion of its node is in the opposite direction too. In other words, the meeting-place of its orbit with that of the earth retreats (and very rapidly) along the ecliptic instead of advancing. So that if the "Andromedes" stood in the supposed intimate relation to Biela's comet, they might be expected to anticipate the times of their recurrence by as much as a week in half a century. All doubt as to the fact may be said to have been removed by Signor Zezioli's observation of the annual shower in more than usual abundance at Bergamo, November 30, 1867.

The missing comet was next due at perihelion in the year 1872, and the probability was contemplated by both Weiss and Galle of its being replaced by a copious discharge of falling stars. The precise date of the occurrence was not easily determinable, but Galle thought the chances in favour of November 28. The event anticipated the prediction by twenty-four hours. Scarcely had the sun set in Western Europe on November 27, when it became evident that Biela's comet was shedding over us the pulverised products of its disintegration. The meteors came in volleys from the foot of the Chained Lady, their numbers at times baffling the attempt to keep a reckoning. At Moncalieri, about 8 p.m., they constituted (as Father Denza said[1223]) a "real rain of fire." Four observers counted, on an average, four hundred each minute and a half; and not a few fireballs, equalling the moon in diameter, traversed the sky. On the whole, however, the stars of 1872, though about equally numerous, were less brilliant than those of 1866; the phosphorescent tracks marking their passage were comparatively evanescent and their movements sluggish. This is easily understood when we remember that the Andromedes overtake the earth, while the Leonids rush to meet it; the velocity of encounter for the first class of bodies being under twelve, for the second above forty-four miles a second. The spectacle was, nevertheless, magnificent. It presented itself successively to various parts of the earth, from Bombay and the Mauritius to New Brunswick and Venezuela, and was most diligently and extensively observed. Here it had well-nigh terminated by midnight.[1224]

It was attended by a slight aurora, and although Tacchini had telegraphed that the state of the sun rendered some show of polar lights probable, it has too often figured as an accompaniment of star-showers to permit the coincidence to rank as fortuitous. Admiral Wrangel was accustomed to describe how, during the prevalence of an aurora on the Siberian coast, the passage of a meteor never failed to extend the luminosity to parts of the sky previously dark;[1225] and an enhancement of electrical disturbance may well be associated with the flittings of such cosmical atoms.

A singular incident connected with the meteors of 1872 has now to be recounted. The late Professor Klinkerfues, who had observed them very completely at Gottingen, was led to believe that not merely the debris strewn along its path, but the comet itself must have been in immediate proximity to the earth during their appearance.[1226] If so, it might be possible, he thought, to descry it as it retreated in the diametrically opposite direction from that in which it had approached. On November 30, accordingly, he telegraphed to Mr. Pogson, the Madras astronomer, "Biela touched earth November 27; search near Theta Centauri"--the "anti-radiant," as it is called, being situated close to that star. Bad weather prohibited observation during thirty-six hours, but when the rain clouds broke on the morning of December 2, there a comet was, just in the indicated position. In appearance it might have passed well enough for one of the Biela twins. It had no tail, but a decided nucleus, and was about 45 seconds across, being thus altogether below the range of naked-eye discernment. It was again observed December 3, when a short tail was perceptible; but overcast skies supervened, and it has never since been seen. Its identity accordingly remains in doubt. It seems tolerably certain, however, that it was not the lost comet, which ought to have passed that spot twelve weeks earlier, and was subject to no conceivable disturbance capable of delaying to that extent its revolution. On the other hand, there is the strongest likelihood that it belonged to the same system[1227]--that it was a third fragment, torn from the parent-body of the Andromedes at a period anterior to our first observations of it.

In thirteen years Biela's comet (or its relics) travels nearly twice round its orbit, so that a renewal of the meteoric shower of 1872 was looked for on the same day of the year 1885, the probability being emphasised by an admonitory circular from Dunecht. Astronomers were accordingly on the alert, and were not disappointed. In England, observation was partially impeded by clouds; but at Malta, Palermo, Beyrout, and other southern stations, the scene was most striking. The meteors were both larger and more numerous than in 1872. Their numbers in the densest part of the drift were estimated by Professor Newton at 75,000 per hour, visible from one spot to so large a group of spectators that practically none could be missed. Yet each of these multitudinous little bodies was found by him to travel in a clear cubical space of which the edge measured twenty miles![1228] Thus the dazzling effect of a luminous throng was produced without jostling or overcrowding, by particles, it might almost be said, isolated in the void.

Their aspect was strongly characteristic of the Andromede family of meteors. "They invariably," Mr. Denning wrote,[1229] "traversed short paths with very slow motions, and became extinct in evolved streams of yellowish sparks." The conclusion seemed obvious "that these meteors are formed of very soft materials, which expand while incalescent, and are immediately crumbled and dissipated into exiguous dust."

The Biela meteors of 1885 did not merely gratify astronomers with a fulfilled prediction, but were the means of communicating to them some valuable information. Although their main body was cut through by the moving earth in six hours, and was not more than 100,000 miles across, skirmishers were thrown out to nearly a million miles on either side of the compact central battalions. Members of the system were, on the 26th of November, recorded by Mr. Denning at the hourly rate of about 130; and they did not wholly cease to be visible until December 1. They afforded besides a particularly well-marked example of that diffuseness of radiation previously observed in some less conspicuous displays. Their paths seemed to diverge from an area rather than from a point in the sky. They came so ill to focus that divergences of several degrees were found between the most authentically determined radiants. These incongruities are attributed by Professor Newton to the irregular shape of the meteoroids producing unsymmetrical resistance from the air, and hence causing them to glance from their original direction on entering it. Thus, their luminous tracks did not always represent (even apart from the effects of the earth's attraction) the true prolongation of their course through space.

The Andromedes of 1872 were laggards behind the comet from which they sprang; those of 1885 were its avant-couriers. That wasted and disrupted body was not due at the node until January 26, 1886, sixty days, that is, after the earth's encounter with its meteoric fragments. These are now probably scattered over more than five hundred million miles of its orbits;[1230] yet Professor Newton considers that all must have formed one compact group with Biela at the time of its close approach to Jupiter about the middle of 1841. For otherwise both comet and meteorites could not have experienced, as they seem to have done, the same kind and amount of disturbance. The rapidity of cometary disintegration is thus curiously illustrated.

A short-lived persuasion that the missing heavenly body itself had been recovered, was created by Mr. Edwin Holms's discovery, at London, November 6, 1892, of a tolerably bright, tailless comet, just in a spot which Biela's comet must have traversed in approaching the intersection of its orbit with that of the earth. A hasty calculation by Berberich assigned elements to the newcomer seeming not only to ratify the identity, but to promise a quasi-encounter with the earth on November 21. The only effect of the prediction, however, was to raise a panic among the negroes of the Southern States of America. The comet quietly ignored it, and moved away from instead of towards the appointed meeting-place. Its projection, then, on the night of its discovery, upon a point of the Biela-orbit was by a mere caprice of chance. North America, nevertheless, was visited on November 23 by a genuine Andromede shower. Although the meteors were less numerous than in 1885, Professor Young estimated that 30,000, at the least, of their orange fire-streaks came, during five hours, within the range of view at Princeton.[1231] Bredikhine estimated the width of the space containing them at about 2,700,000 miles.[1232] The anticipation of their due time by four days implied--if they were a prolongation of the main Biela group, the nucleus of which passed the spot of encounter five months previously--a recession of the node since 1885 by no less than three degrees. Unless, indeed, Mr. Denning were right in supposing the display to have proceeded from "an associated branch of the main swarm through which we passed in 1872 and 1885."[1233] The existence of separated detachments of Biela meteors, due to disturbing planetary action, was contemplated as highly probable by Schiaparelli.[1234] Such may have been the belated flights met with in 1830, 1838, 1841, and 1847, and such the advance flight plunged through in 1892. A shower looked for November 23, 1899, did not fall, and no further display from this quarter is probable until November 17, 1905, although one is possible a year earlier.[1235]

The Leonids, through the adverse influence of Jupiter and Saturn, inflicted upon multitudes of eager watchers a still more poignant disappointment. A dense part of the swarm, having nearly completed a revolution since 1866, should, travelling normally, have met the earth November 15, 1899; in point of fact, it swerved sunward, and the millions of meteorites which would otherwise have been sacrificed for the illumination of our skies escaped a fiery doom. The contingency had been forecast in the able calculations of Dr. Johnstone Stoney and Dr. A. M. W. Downing,[1236] superintendent of the Nautical Almanac Office; but the verification scarcely compensated the failure. Nor was the situation retrieved in the following years. Only ragged fringes of the great tempest-cloud here and there touched our globe. As the same investigators warned us to expect, the course of the meteorites had been not only rendered sinuous by perturbation, but also broken and irregular. We can no longer count upon the Leonids. Their glory, for scenic purposes, is departed. The comet associated with them also evaded observation. Although it doubtless kept its tryst with the sun in the spring of 1899, the attendant circumstances were too unfavourable to allow it to be seen from the earth.[1237] By an almost fantastic coincidence, nevertheless, a faint comet was photographed, November 14, 1898,[1238] by Dr. Chase, of the Yale College Observatory, close to the Leonid radiant, whither a "meteorograph" was directed with a view to recording trails left by precursors of the main Leonid body. A promising start, too, was made on the same occasion with meteoric researches from sensitive plates.[1239] Indeed, Schaeberle and Colton[1240] had already, in 1896, determined the height of a Leonid by means of photographs taken at stations on different ridges of Mount Hamilton; and Professor Pickering has prosecuted similar work at Harvard, with encouraging results. Everything in this branch of science depends upon how far they can be carried. Without the meteorograph, rigid accuracy in the observation of shooting stars is unattainable, and rigid accuracy is the sine qua non for obtaining exact knowledge.

Biela does not offer the only example of cometary disruption. Setting aside the unauthentic reports of early chroniclers, we meet the "double comet" discovered by Liais at Olinda (Brazil), February 27, 1860, of which the division appeared recent, and about to be carried farther.[1241] But a division once established, separation must continually progress. The periodic times of the fragments will never be identical; one must drop a little behind the other at each revolution, until at length they come to travel in remote parts of nearly the same orbit. Thus the comet predicted by Klinkerfues and discovered by Pogson had already lagged to the extent of twelve weeks, and we shall meet instances farther on where the retardation is counted, not by weeks, but by years. Here original identity emerges only from calculation and comparison of orbits.

Comets, then, die, as Kepler wrote long ago, sicut bombyces filo fundendo. This certainty, anticipated by Kirkwood in 1861, we have at least acquired from the discovery of their generative connection with meteors. Nay, their actual materials become, in smaller or larger proportions, incorporated with our globe. It is not, indeed, universally admitted that the ponderous masses of which, according to Daubree's estimate,[1242] at least 600 fall annually from space upon the earth, ever formed part of the bodies known to us as comets. Some follow Tschermak in attributing to aerolites a totally different origin from that of periodical shooting-stars. That no clear line of demarcation can be drawn is no valid reason for asserting that no real distinction exists; and it is certainly remarkable that a meteoric fusillade may be kept up for hours without a single solid projectile reaching its destination. It would seem as if the celestial army had been supplied with blank cartridges. Yet, since a few detonating meteors have been found to proceed from ascertained radiants of shooting-stars, it is difficult to suppose that any generic difference separates them.

Their assimilation is further urged--though not with any demonstrative force--by two instances, the only two on record, of the tangible descent of an aerolite during the progress of a star-shower. On April 4, 1095, the Saxon Chronicle informs us that stars fell "so thickly that no man could count them," and adds that one of them having struck the ground in France, a bystander "cast water upon it, which was raised in steam with a great noise of boiling."[1243] And again, on November 27, 1885, while the skirts of the Andromede-tempest were trailing over Mexico, "a ball of fire" was precipitated from the sky at Mazapil, within view of a ranchman.[1244] Scientific examination proved it to be a "siderite," or mass of "nickel-iron"; its weight exceeded eight pounds, and it contained many nodules of graphite. We are not, however, authorised by the circumstances of its arrival to regard the Mazapil fragment of cosmic metal as a specimen torn from Biela's comet. In this, as in the preceding case, the coincidence of the fall with the shower may have been purely casual, since no hint is given of any sort of agreement between the tracks followed by the sample provided for curious study, and the swarming meteors consumed in the upper air.

Professor Newton's inquiries into the tracks pursued by meteorites previous to their collisions with the earth tend to distinguish them, at least specifically, from shooting-stars. He found that nearly all had been travelling with a direct movement in orbits the perihelia of which lay in the outer half of the space separating the earth from the sun.[1245] Shooting-stars, on the contrary, are entirely exempt from such limitations. The Yale Professor concluded "that the larger meteorites moving in our solar system are allied much more closely with the group of comets of short period than with the comets whose orbits are nearly parabolic." They would thus seem to be more at home than might have been expected amid the planetary family. Father Carbonelle has, moreover, shown[1246] that meteorites, if explosion-products of the earth or moon, should, with rare exceptions, follow just the kind of paths assigned to them, from data of observation, by Professor Newton. Yet it is altogether improbable that projectiles from terrestrial volcanoes should, at any geological epoch, have received impulses powerful enough to enable them, not only to surmount the earth's gravity, but to penetrate its atmosphere.

A striking--indeed, an almost startling--peculiarity, on the other hand, divides from their congeners a class of meteors identified by Mr. Denning during ten years' patient watching of such phenomena at Bristol.[1247] These are described as "meteors with stationary radiants," since for months together they seem to come from the same fixed points in the sky. Now this implies quite a portentous velocity. The direction of meteor-radiants is affected by a kind of aberration, analogous to the aberration of light. It results from a composition of terrestrial with meteoric motion. Hence, unless that of the earth in its orbit be by comparison insignificant, the visual line of encounter must shift, if not perceptibly from day to day, at any rate conspicuously from month to month. The fixity, then, of many systems observed by Mr. Denning seems to demand the admission that their members travel so fast as to throw the earth's movement completely out of the account. The required velocity would be, by Mr. Ranyard's calculation, at least 880 miles a second.[1248] But the aspect of the meteors justifies no such extravagant assumption. Their seeming swiftness is very various, and--what is highly significant--it is notably less when they pursue than when they meet the earth. Yet the "incredible and unaccountable"[1249] fact of the existence of these "long radiants," although doubted by Tisserand[1250] because of its theoretical refractoriness, must apparently be admitted. The first plausible explanation of them was offered by Professor Turner in 1899.[1251] They represent, in his view, the cumulative effects of the earth's attraction. The validity of his reasoning is, however, denied by M. Bredikhine,[1252] who prefers to regard them as a congeries of separate streams. The enigma they present has evidently not yet received its definitive solution.

The Perseids afford, on the contrary, a remarkable instance of a "shifting radiant." Mr. Denning's observations of these yellowish, leisurely meteors extend over nearly six weeks, from July 8 to August 16; the point of radiation meantime progressing no less than 57 deg. in right ascension. Doubts as to their common origin were hence freely expressed, especially by Mr. Monck of Dublin.[1253] But the late Dr. Kleiber[1254] showed, by strict geometrical reasoning, that the forty-nine radiants successively determined for the shower were all, in fact, comprised within one narrowly limited region of space. In other words, the application of the proper correction for the terrestrial movement, and the effects of attraction by which each individual shooting-star is compelled to describe a hyperbola round the earth's centre, reduces the extended line of radiants to a compact group, with the cometary radiant for its central point; the cometary radiant being the spot in the sky met by a tangent to the orbit of the Perseid comet of 1862 at its intersection with the orbit of the earth. The reality of the connection between the comet and the meteors could scarcely be more clearly proved; while the vast dimensions of the stream into which the latter are found to be diffused cannot but excite astonishment not unmixed with perplexity.

The first successful application of the spectroscope to comets was by Donati in 1864.[1255] A comet discovered by Tempel, July 4, brightened until it appeared like a star somewhat below the second magnitude, with a feeble tail 30 deg. in length. It was remarkable as having, on August 7, almost totally eclipsed a small star--a very rare occurrence.[1256] On August 5 Donati admitted its light through his train of prisms, and found it, thus analysed, to consist of three bright bands--yellow, green, and blue--separated by wider dark intervals. This implied a good deal. Comets had previously been considered, as we have seen, to shine mainly, if not wholly, by reflected sunlight. They were now perceived to be self-luminous, and to be formed, to a large extent, of glowing gas. The next step was to determine what kind of gas it was that was thus glowing in them; and this was taken by Sir William Huggins in 1868.[1257]

A comet of subordinate brilliancy, known as comet 1868 ii., or sometimes as Winnecke's, was the subject of his experiment. On comparing its spectrum with that of an olefiant-gas "vacuum tube" rendered luminous by electricity, he found the agreement exact. It has since been abundantly confirmed. All the eighteen comets tested by light analysis, between 1868 and 1880, showed the typical hydro-carbon spectrum[1258] common to the whole group of those compounds, but probably due immediately to the presence of acetylene. Some minor deviations from the laboratory pattern, in the shifting of the maxima of light from the edge towards the middle of the yellow and blue bands, have been experimentally reproduced by Vogel and Hasselberg in tubes containing a mixture of carbonic oxide with olefiant gas.[1259] Their illumination by disruptive electric discharges was, however, a condition sine qua non for the exhibition of the cometary type of spectrum. When a continuous current was employed, the carbonic oxide bands asserted themselves to the exclusion of the hydro-carbons. The distinction has great significance as regards the nature of comets. Of particular interest in this connection is the circumstance that carbonic oxide is one of the gases evolved by meteoric stones and irons under stress of heat.[1260] For it must apparently have formed part of an aeriform mass in which they were immersed at an earlier stage of their history.

PLATE II.

In a few exceptional comets the usual carbon-bands have been missed. Two such were observed by Sir William Huggins in 1866 and 1867 respectively.[1261] In each a green ray, approximating in position to the fundamental nebular line, crossed an otherwise unbroken spectrum. And Holmes's comet of 1892 displayed only a faint prismatic band devoid of any characteristic feature.[1262] Now these three might well be set down as partially effete bodies; but a brilliant comet, visible in southern latitudes in April and May, 1901, so far resembled them in the quality of its light as to give a spectrum mainly, if not purely, continuous. This, accordingly, is no symptom of decay.

The earliest comet of first-class lustre to present itself for spectroscopic examination was that discovered by Coggia at Marseilles, April 17, 1874. Invisible to the naked eye till June, it blazed out in July a splendid ornament of our northern skies, with a just perceptibly curved tail, reaching more than half way from the horizon to the zenith, and a nucleus surpassing in brilliancy the brightest stars in the Swan. Bredikhine, Vogel, and Huggins[1263] were unanimous in pronouncing its spectrum to be that of marsh or olefiant gas. Father Secchi, in the clear sky of Rome, was able to push the identification even closer than had heretofore been done. The complete hydro-carbon spectrum consists of five zones of variously coloured light. Three of these only--the three central ones--had till then been obtained from comets; owing, it was supposed, to their temperature not being high enough to develop the others. The light of Coggia's comet, however, was found to contain all five, traces of the violet band emerging June 4, of the red, July 2.[1264] Presumably, all five would show universally in cometary spectra, were the dispersed rays strong enough to enable them to be seen.

The gaseous surroundings of comets are, then, largely made up of a compound of hydrogen with carbon. Other materials are also present; but the hydro-carbon element is probably unfailing and predominant. Its luminosity is, there is little doubt, an effect of electrical excitement. Zoellner showed in 1872[1265] that, owing to evaporation and other changes produced by rapid approach to the sun, electrical processes of considerable intensity must take place in comets; and that their original light is immediately connected with these, and depends upon solar radiation, rather through its direct or indirect electrifying effects, than through its more obvious thermal power, may be considered a truth permanently acquired to science.[1266] They are not, it thus seems, bodies incandescent through heat, but glowing by electricity; and this is compatible, under certain circumstances, with a relatively low temperature.

The gaseous spectrum of comets is accompanied, in varying degrees, by a continuous spectrum. This is usually derived most strongly from the nucleus, but extends, more or less, to the nebulous appendages. In part, it is certainly due to reflected sunlight; in part, most likely, to the ignition of minute solid particles.

FOOTNOTES:

[Footnote 1188: Month. Not., vol. xix., p. 27.]

[Footnote 1189: Mem. de l'Ac. Imp., t. ii., 1859, p. 46.]

[Footnote 1190: Harvard Annals, vol. iii., p. 368.]

[Footnote 1191: Ibid., p. 371.]

[Footnote 1192: Month. Not., vol. xxii., p. 306.]

[Footnote 1193: Stothard in Ibid., vol. xxi., p. 243.]

[Footnote 1194: Intell. Observer, vol. i., p. 65.]

[Footnote 1195: Comptes Rendus, t. lxi., p. 953.]

[Footnote 1196: Smiths. Report, 1881 (Holden); Nature, vol. xxv., p. 94; Observatory, vol. xxi., p. 378 (W. T. Lynn).]

[Footnote 1197: _Ueber den Ursprung der von Pallas gefundenen Eisenmassen_, p. 24.]

[Footnote 1198: Arago, Annuaire, 1836, p. 294.]

[Footnote 1199: Humboldt had noticed the emanation of the shooting stars of 1799 from a single point, or "radiant," as Greg long afterwards termed it; but no reasoning was founded on the observation.]

[Footnote 1200: Am. Journ. of Sc., vol. xxvi., p. 132.]

[Footnote 1201: Annuaire, 1836, p. 297.]

[Footnote 1202: Ann. de l'Observ., Bruxelles, 1839, p. 248.]

[Footnote 1203: Ibid., 1837, p. 272.]

[Footnote 1204: Astr. Nach., Nos. 385, 390.]

[Footnote 1205: Am. Jour. of Sc., vol. xxxviii. (2nd ser.), p. 377.]

[Footnote 1206: Ibid., vol. xxxviii., p. 61.]

[Footnote 1207: Month. Not., vol. xxvii., p. 247.]

[Footnote 1208: Am. Jour. of Sc., vol. xliii. (2nd ser.), p. 87.]

[Footnote 1209: Grant, Month. Not., vol. xxvii., p. 29.]

[Footnote 1210: P. Smyth, Ibid., p. 256.]

[Footnote 1211: Hind, Ibid., p. 49.]

[Footnote 1212: Reproduced in Les Mondes, t. xiii.]

[Footnote 1213: Comptes Rendus, t. lxiv., p. 96.]

[Footnote 1214: Astr. Nach., No. 1,626.]

[Footnote 1215: Ibid., No. 1,632.]

[Footnote 1216: Month. Not., vol. xxxviii., p. 369.]

[Footnote 1217: Schiaparelli, Le Stelle Cadenti, p. 54.]

[Footnote 1218: Ueber Feuer-Meteore, p. 406.]

[Footnote 1219: Astr. Nach., No. 347 (Maedler); see also Boguslawski, Die Kometen, p. 98. 1857.]

[Footnote 1220: Nature, vol. vi., p. 148.]

[Footnote 1221: A. S. Herschel, Month. Not., vol. xxxii., p. 355.]

[Footnote 1222: Astr. Nach., Nos. 1,632, 1,633, 1,635.]

[Footnote 1223: Nature, vol. vii., p. 122.]

[Footnote 1224: A. S. Herschel, Report Brit. Ass., 1873, p. 390.]

[Footnote 1225: Humboldt, Cosmos, vol. i., p. 114 (Otte's trans.).]

[Footnote 1226: Month. Not., vol. xxxiii., p. 128.]

[Footnote 1227: Even this was denied by Bruhns, Astr. Nach., No. 2,054.]

[Footnote 1228: Am. Jour., vol. xxxi., p. 425.]

[Footnote 1229: Month. Not., vol. xlvi., p. 69.]

[Footnote 1230: In Schiaparelli's opinion, centuries must have elapsed while the observed amount of scattering was being produced. Le Stelle Cadenti, 1886, p. 112.]

[Footnote 1231: Astr. and Astroph., vol. xi., p. 943.]

[Footnote 1232: Bull. de l'Acad. St. Petersbourg, t. xxxv., p. 598. 1894.]

[Footnote 1233: Observatory, vol. xvi., p. 55.]

[Footnote 1234: Le Stelle Cadenti, p. 133; Rendiconti dell' Istituto Lombardo, t. iii., ser. ii., p. 23.]

[Footnote 1235: Denning, Memoirs Roy. Astr. Soc., vol. liii., p. 214; Abelmann, Astr. Nach., No. 3,516.]

[Footnote 1236: Proc. Roy. Soc., March 2, 1899; Nature, November 9, 1899.]

[Footnote 1237: Berberich, Astr. Nach., No. 3,526.]

[Footnote 1238: Elkin, Astroph. Jour., vol. ix., p. 22.]

[Footnote 1239: Elkin, Astroph. Jour., vol. x., p. 24.]

[Footnote 1240: Pop. Astr., September, 1897, p. 232.]

[Footnote 1241: Month. Not., vol. xx., p. 336.]

[Footnote 1242: Revue des deux Mondes, December 15, 1885, p. 889.]

[Footnote 1243: Palgrave, Phil. Trans., vol. cxxv., p. 175.]

[Footnote 1244: W. E. Hidden, Century Mag., vol. xxxiv., p. 534.]

[Footnote 1245: Amer. Jour. of Science, vol. xxxvi., p. i., 1888.]

[Footnote 1246: Revue des Questions Scientifiques, January, 1899, p. 194; Tisserand, Bull. Astr., t. viii., p. 460.]

[Footnote 1247: Month. Not., vol. xlv., p. 93.]

[Footnote 1248: Observatory, vol. viii., p. 4.]

[Footnote 1249: Denning, Month. Not., vol. xxxviii., p. 114.]

[Footnote 1250: Comptes Rendus, t. cix., p. 344.]

[Footnote 1251: Month. Not., vol. lix., p. 140.]

[Footnote 1252: Bull. de l'Acad. St. Petersb., t. xii., p. 95.]

[Footnote 1253: Publ. Astr. Pac. Soc., vol. iii., p. 114.]

[Footnote 1254: Month. Not., vol. lii., p. 341.]

[Footnote 1255: Astr. Nach., No. 1,488.]

[Footnote 1256: Annuaire, Paris, 1883, p. 185.]

[Footnote 1257: Phil. Trans., vol. clviii., p. 556.]

[Footnote 1258: Hasselberg, Mem. de l'Ac. Imp. de St. Petersbourg, t. xxviii. (7th ser.), No. 2, p. 66.]

[Footnote 1259: Scheiner, Die Spectralanalyse der Gestirne, p. 234. Kayser (Astr. and Astroph., vol. xiii., p. 368) refers the anomalies of the carbon-spectrum in comets wholly to instrumental sources.]

[Footnote 1260: Dewar, Proc. Roy. Inst., vol. xi., p. 541.]

[Footnote 1261: Proc. R. Soc., vol. xv., p. 5; Month. Not., vol. xxvii., p. 288.]

[Footnote 1262: Keeler, Astr. and Astrophysics, vol. xi., p. 929; Vogel, Astr. Nach., No. 3,142.]

[Footnote 1263: Proc. Roy. Soc., vol. xxiii., p. 154.]

[Footnote 1264: Hasselberg, loc. cit., p. 58.]

[Footnote 1265: Ueber die Natur der Cometen, p. 112.]

[Footnote 1266: Hasselberg, loc. cit., p. 38.]