A Popular History of Astronomy During the Nineteenth Century Part 50

At Potsdam, the bright yellow line was perceived with astonishment by Vogel on May 31, and was next evening identified with Fraunhofer's "D."

Its character led him to infer a very considerable density in the glowing vapour emitting it.[1309] Ha.s.selberg founded an additional argument in favour of the electrical origin of cometary light on the changes in the spectrum of comet Wells.[1310] For they were closely paralleled by some earlier experiments of Wiedemann, in which the gaseous spectra of vacuum tubes were at once effaced on the introduction of metallic vapours. It seemed as if the metal had no sooner been rendered volatile by heat, than it usurped the entire office of carrying the discharge, the resulting light being thus exclusively of its production. Had simple incandescence by heat been in question, the effect would have been different; the two spectra would have been superposed without prejudice to either. Similarly, the replacement of the hydro-carbon bands in the spectrum of the comet by the sodium line proved electricity to be the exciting agent. For the increasing thermal power of the sun might, indeed, have ignited the sodium, but it could not have extinguished the hydro-carbons.

Sir William Huggins succeeded in photographing the spectrum of comet Wells by an exposure of one hour and a quarter.[1311] The result was to confirm the novelty of its character. None of the ultra-violet carbon groups were apparent; but certain bright rays, as yet unidentified, had imprinted themselves. Otherwise the spectrum was strongly continuous, uninterrupted even by the Fraunhofer lines detected in the spectrum of Tebb.u.t.t's comet. Hence it was concluded that a smaller proportion of reflected light was mingled with the native emissions of the later arrival.

All that is certainly known about the _extent_ of the orbit traversed by the first comet of 1882 is that it came from, and is now retreating towards, vastly remote depths of s.p.a.ce. An American computer[1312] found a period indicated for it of no less than 400,000 years; A. Thraen of Dingelstadt arrived at one of 3617.[1313] Both are perhaps equally insecure.

We have now to give some brief account of one of the most remarkable cometary apparitions on record, and--with the single exception of that identified with the name of Halley--the most instructive to astronomers.

The lessons learned from it were as varied and significant as its aspect was splendid; although from the circ.u.mstance of its being visible in general only before sunrise, the spectators of its splendour were comparatively few.

The discovery of a great comet at Rio Janeiro, September 11, 1882, became known in Europe through a telegram from M. Cruls, director of the observatory at that place. It had, however (as appeared subsequently), been already seen on the 8th by Mr. Finlay of the Cape Observatory, and at Auckland as early as September 3. A later, but very singularly conditioned detection, quite unconnected with any of the preceding, was effected by Dr. Common at Ealing. Since the eclipse of May 17, when a comet--named "Tewfik" in honour of the Khedive of Egypt--was caught on Dr. Schuster's photographs, entangled, one might almost say, in the outer rays of the corona, he had scrutinized the neighbourhood of the sun on the infinitesimal chance of intercepting another such body on its rapid journey thence or thither. We record with wonder that, after an interval of exactly four months, that infinitesimal chance turned up in his favour.

On the forenoon of Sunday, September 17, he saw a great comet close to, and rapidly approaching the sun. It was, in fact, then within a few hours of perihelion. Some measures of position were promptly taken; but a cloud-veil covered the interesting spectacle before mid-day was long past. Mr. Finlay at the Cape was more completely fortunate. Divided from his fellow-observer by half the world, he unconsciously finished, under a clearer sky, his interrupted observation. The comet, of which the silvery radiance contrasted strikingly with the reddish-yellow glare of the sun's margin it drew near to, was followed "continuously right into the boiling of the limb"--a circ.u.mstance without precedent in cometary history.[1314] Dr. Elkin, who watched the progress of the event with another instrument, thought the intrinsic brilliancy of the nucleus scarcely surpa.s.sed by that of the sun's surface. Nevertheless it had no sooner touched it than it vanished as if annihilated. So sudden was the disappearance (at 4h. 50m. 58s., Cape mean time), that the comet was at first believed to have pa.s.sed _behind_ the sun. But this proved not to have been the case. The observers at the Cape had witnessed a genuine transit. Nor could non-visibility be explained by equality of l.u.s.tre. For the gradations of light on the sun's disc are amply sufficient to bring out against the dusky background of the limb any object matching the brilliancy of the centre; while an object just equally luminous with the limb must inevitably show dark at the centre.

The only admissible view, then, is that the bulk of the comet was of too filmy a texture, and its presumably solid nucleus too small, to intercept any noticeable part of the solar rays--a piece of information worth remembering.

PLATE III.

[Ill.u.s.tration: The Great Comet of September, 1882.

Photographed at the Royal Observatory, Cape of Good Hope]

On the following morning, the object of this unique observation showed (in Sir David Gill's words) "an astonis.h.i.+ng brilliancy as it rose behind the mountains on the east of Table Bay, and seemed in no way diminished in brightness when the sun rose a few minutes afterward. It was only necessary to shade the eye from direct sunlight with the hand at arm's length, to see the comet, with its brilliant white nucleus and dense white, sharply bordered tail of quite half a degree in length."[1315]

All over the world, wherever the sky was clear during that day, September 18, it was obvious to ordinary vision. Since 1843 nothing had been seen like it. From Spain, Italy, Algeria, Southern France, despatches came in announcing the extraordinary appearance. At Cordoba, in South America, the "blazing star near the sun" was the one topic of discourse.[1316] Moreover--and this is altogether extraordinary--the records of its daylight visibility to the naked eye extend over three days. At Reus, near Tarragona, it showed bright enough to be seen through a pa.s.sing cloud when only three of the sun's diameters from his limb, just before its final rush past perihelion on September 17; while at Carthagena in Spain, on September 19, it was kept in view during two hours before and two hours after noon, and was similarly visible in Algeria on the same day.[1317]

But still more surprising than the appearance of the body itself were the nature and relations of the path it moved in. The first rough elements computed for it by Mr. Tebb.u.t.t, Dr. Chandler, and Mr. White, a.s.sistant at the Melbourne Observatory, showed at once a striking resemblance to those of the twin comets of 1843 and 1880. This suggestive fact became known in this country, September 27, through the medium of a Dunecht circular. It was fully confirmed by subsequent inquiries, for which ample opportunities were luckily provided. The likeness was not, indeed, so absolutely perfect as in the previous case; it included some slight, though real differences; but it bore a strong and unmistakable stamp, broadly challenging explanation.

Two hypotheses only were really available. Either the comet of 1882 was an accelerated return of those of 1843 and 1880, or it was a fragment of an original ma.s.s to which they also had belonged. For the purposes of the first view the "resisting medium" was brought into full play; the opinion of its efficacy was for some time both prevalent and popular, and formed the basis, moreover, of something of a sensational panic. For a comet which, at a single pa.s.sage through the sun's atmosphere, encountered sufficient resistance to shorten its period from thirty-seven to two years and eight months, must, in the immediate future, be brought to rest on his surface; and the solar conflagration thence ensuing was represented in some quarters, with more licence of imagination than countenance from science, as likely to be of catastrophic import to the inhabitants of our little planet.

But there was a test available in 1882 which it had not been possible to apply either in 1843 or in 1880. The two bodies visible in those years had been observed only after they had already pa.s.sed perihelion;[1318]

the third member of the group, on the other hand, was accurately followed for a week before that event, as well as during many months after it. Finlay's and Elkin's observation of its disappearance at the sun's edge formed, besides, a peculiarly delicate test of its motion.

The opportunity was thus afforded, by directly comparing the comet's velocity before and after its critical plunge through the solar surroundings, of ascertaining with approximate certainty whether any considerable r.e.t.a.r.dation had been experienced in the course of that plunge. The answer distinctly given was that there had not. The computed and observed places on both sides of the sun fitted harmoniously together. The effect, if any were produced, was too small to be perceptible.

This result is, in itself, a memorable one. It seems to give the _coup de grace_ to Encke's theory--discredited, in addition, by Backlund's investigation--of a resisting medium growing rapidly denser inwards. For the perihelion distance of the comet of 1882, though somewhat greater than that of its predecessors, was nevertheless extremely small. It pa.s.sed at less than 300,000 miles of the sun's surface. But the ethereal substance long supposed to obstruct the movement of Encke's comet would there be nearly 2,000 times denser than at the perihelion of the smaller body, and must have exerted a conspicuous r.e.t.a.r.ding influence. That none such could be detected seems to argue that no such medium exists.

Further evidence of a decisive kind was not wanting on the question of ident.i.ty. The "Great September Comet" of 1882 was in no hurry to withdraw itself from curious terrestrial scrutiny. It was discerned with the naked eye at Cordoba as late as March 7, 1883, and still showed in the field of the great equatoreal on June 1 as an "excessively faint whiteness."[1319] It was then about 480 millions of miles from the earth--a distance to which no other comet--not even excepting the peculiar one of 1729--had been pursued.[1320] Moreover, an arc of 340 out of the entire 360 degrees of its circuit had been described under the eyes of astronomers; so that its course came to be very well known.

That its movement is in a very eccentric ellipse, traversed in several hundred years, was ascertained.[1321] The later inquiries of Dr.

Kreutz,[1322] completed in a volume published in 1901,[1323]

demonstrated the period to be of about 800 years, while that of its predecessor in 1843 might possibly agree with it, but is much more probably estimated at 512 years. The hypothesis that they, or any of the comets a.s.sociated with them, were returns of an individual body is peremptorily excluded. They may all, however, have been separated from one original ma.s.s by the divellent action of the sun at close quarters.

Each has doubtless its own period, since each has most likely suffered r.e.t.a.r.dations or accelerations special to itself, which, though trifling in amount, would avail materially to alter the length of the major axis, while leaving the remaining elements of the common orbit virtually unchanged.[1324]

A fifth member was added to the family in 1887. On the 18th of January in that year, M. Thome discovered at Cordoba a comet reproducing with curious fidelity the lineaments of that observed in the same lat.i.tudes seven years previously. The narrow ribbon of light, contracting towards the sun, and running outward from it to a distance of thirty-five degrees; the unsubstantial head--a veiled nothingness, as it appeared, since no distinct nucleus could be made out; the quick fading into invisibility, were all accordant peculiarities, and they were confirmed by some rough calculations of its...o...b..t, showing geometrical affinity to be no less unmistakable than physical likeness. The observations secured were indeed, from the nature of the apparition, neither numerous nor over-reliable; and the earliest of them dated from a week after perihelion, pa.s.sed, almost by a touch-and-go escape, January 11. On January 27, this mysterious object could barely be discerned telescopically at Cordoba.[1325] That it belonged to the series of "southern comets" can scarcely be doubted; but the inference that it was an actual return of the comet of 1880, improbable in itself, was negatived by its non-appearance in 1894. Meyer's incorporation with this extraordinary group of the "eclipse-comet" of 1882[1326] has been approved by Kreutz, after searching examination.

The idea of cometary systems was first suggested by Thomas Clausen in 1831.[1327] It was developed by the late M. Hoek, director of the Utrecht Observatory, in 1865 and some following years.[1328] He found that in quite a considerable number of cases, the paths of two or three comets had a common point of intersection far out in s.p.a.ce, indicating with much likelihood a community of origin. This consisted, according to his surmise, in the disruption of a parent ma.s.s during its sweep round the star latest visited. Be this as it may, the fact is undoubted that numerous comets fall into groups, in which similar conditions of motion betray a pre-existent physical connection. Never before, however, had geometrical relations.h.i.+p been so notorious as between the comets now under consideration; and never before, in a comet still, it might be said, in the prime of life, had physical peculiarities tending to account for that affinity been so obvious as in the chief member of the group.

Observation of a granular structure in cometary nuclei dates far back into the seventeenth century, when Cysatus and Hevelius described the central parts of the comets of 1618 and 1652 respectively as made up of a congeries of minute stars. a.n.a.logous symptoms of a loose state of aggregation have of late been not unfrequently detected in telescopic comets, besides the instances of actual division offered by those connected with the names of Biela and Liais. The forces concerned in producing these effects seem to have been peculiarly energetic in the great comet of 1882.

The segmentation of the nucleus was first noticed in the United States and at the Cape of Good Hope, September 30. It proceeded rapidly. At Kiel, on October 5 and 7, Professor Kruger perceived two centres of condensation. A definite and progressive separation into _three_ ma.s.ses was observed by Professor Holden, October 13 and 17.[1329] A few days later, M. Tempel found the head to consist of _four_ lucid aggregations, ranged nearly along the prolongation of the caudal axis;[1330] and Dr.

Common, January 27, 1883, saw _five_ nuclei in a line "like pearls on a string."[1331] This remarkable character was preserved to the last moment of the comet's distinct visibility. It was a consequence, according to Dr. Kreutz, of violent interior action in the comet itself While close to the sun.

There were, however, other curious proofs of a disaggregative tendency in this body. On October 9, Schmidt discovered at Athens a nebulous object 4 south-west of the great comet, and travelling in the same direction. It remained visible for a few days, and, from Oppenheim's and Hind's calculations, there can be little doubt that it was really the offspring by fission of the body it accompanied.[1332] This is rendered more probable by the unexampled spectacle offered, October 14, to Professor Barnard, then of Nashville, Tennessee, of _six or eight_ distinct cometary ma.s.ses within 6 south by west of the comet's head, none of which reappeared on the next opportunity for a search.[1333] A week later, however, one similar object was discerned by Mr. W. R.

Brooks, in the opposite direction from the comet. Thus s.p.a.ce appeared to be strewn with the filmy debris of this beautiful but fragile structure all along the track of its retreat from the sun.

Its tail was only equalled (if it were equalled) in length by that of the comet of 1843. It extended in s.p.a.ce to the vast distance of 200 millions of miles from the head; but, so imperfectly were its proportions displayed to terrestrial observers, that it at no time covered an arc of the sky of more than 30. This apparent extent was attained, during a few days previous to September 25, by a faint, thin, rigid streak, noticed only by a few observers--by Elkin at the Cape Observatory, Eddie at Grahamstown, and Cruls at Rio Janeiro. It diverged at a low angle from the denser curved train, and was produced, according to Bredikhine,[1334] by the action of a repulsive force twelve times as strong as the counter-pull of gravity. It belonged, that is, to type 1; while the great bifurcate appendage, obvious to all eyes, corresponded to the lower rate of emission characteristic of type 2. This was remarkable for the perfect definiteness of its termination, for its strongly-forked shape, and for its unusual permanence. Down to the end of January, 1883, its length, according to Schmidt's observations, was still 93 million miles; and a week later it remained visible to the naked eye, without notable abridgment.

Most singular of all was an anomalous extension of the appendage _towards_ the sun. During the greater part of October and November, a luminous "tube" or "sheath," of prodigious dimensions, seemed to surround the head, and project in a direction nearly opposite to that of the usual outpourings of attentuated matter. (See Plate III.) Its diameter was computed by Schmidt to be, October 15, no less than four million miles, and it was described by Cruls as a "truncated cone of nebulosity," stretching 3 or 4 sunwards.[1335] This, and the entire anterior part of the comet, were again surrounded by a thin, but enormously voluminous paraboloidal envelope, observed by Schiaparelli for a full month from October 19.[1336] There can be little doubt that these abnormal effluxes were a consequence of the tremendous physical disturbance suffered at perihelion; and it is worth remembering that something a.n.a.logous was observed in the comet of 1680 (Newton's), also noted for its excessively close approach to the sun, and possibly moving in a related orbit. The only plausible hypothesis as to the mode of their production is that of an opposite state of electrification in the particles composing the ordinary and extraordinary appendages.

The spectrum of the great comet of 1882 was, in part, a repet.i.tion of that of its immediate predecessor, thus confirming the inference that the previously unexampled sodium-blaze was in both a direct result of the intense solar action to which they were exposed. But the D line was, this time, not seen alone. At Dunecht, on the morning of September 18, Drs. Copeland and J. G. Lohse succeeded in identifying six brilliant rays in the green and yellow with as many prominent iron-lines;[1337] a very significant addition to our knowledge of cometary const.i.tution, and one which lent countenance to Bredikhine's a.s.sumption of various kinds of matter issuing from the nucleus with velocities inversely as their atomic weights. All the lines equally showed a slight displacement, indicating a recession from the earth of the radiating body at the rate of 37 to 46 miles a second. A similar observation, made by M. Thollon at Nice on the same day, gave emphatic sanction to the spectroscopic method of estimating movement in the line of sight. Before anything was as yet known of the comet's path or velocity, he announced, from the position of the double sodium-line alone, that at 3 p.m. on September 18 it was increasing its distance from our planet by from 61 to 76 kilometres per second.[1338] M. Bigourdan's subsequent calculations showed that its actual swiftness of recession was at that moment 73 kilometres.

Changes in the inverse order to those seen in the spectrum of comet Wells soon became apparent. In the earlier body, carbon bands had died out with _approach_ to perihelion, and had been replaced by sodium emissions; in its successor, sodium emissions became weakened and disappeared with _retreat_ from perihelion, and found their subst.i.tute in carbon bands. Professor Ricc was, in fact, able to infer, from the sequence of prismatic phenomena, that the comet had already pa.s.sed the sun; thus establis.h.i.+ng a novel criterion for determining the position of a comet in its...o...b..t by the varying quality of its radiations.

Recapitulating what was learnt from the five conspicuous comets of 1880-82, we find that the leading facts acquired to science were these three. First, that comets may be met with pursuing each other, after intervals of many years, in the same, or nearly the same, track; so that ident.i.ty of orbit can no longer be regarded as a sure test of individual ident.i.ty. Secondly, that at least the outer corona may be traversed by such bodies with perfect apparent impunity. Finally, that their chemical const.i.tution is highly complex, and that they possess, in some cases at least, a metallic core resembling the meteoric ma.s.ses which occasionally reach the earth from planetary s.p.a.ce.

A group of five comets, including Halley's, own a sort of cliental dependence upon the planet Neptune. They travel out from the sun just to about his distance from it, as if to pay homage to a powerful protector, who gets the credit of their establishment as periodical visitors to the solar system. The second of these bodies to affect a looked-for return was a comet--the sixteenth within ten years--discovered by Pons, July 20, 1812, and found by Encke to revolve in an elliptic orbit, with a period of nearly 71 years. It was not, however, until September 1, 1883, that Mr. Brooks caught its reappearance; it pa.s.sed perihelion January 25, and was last seen June 2, 1884. At its brightest, it had the appearance of a second magnitude star, furnished with a poorly developed double tail, and was fairly conspicuous to the naked eye in Southern Europe, from December to March. One exceptional feature distinguished it. Its fluctuations in form and luminosity were unprecedented in rapidity and extent. On September 21, Dr. Chandler[1339] observed it at Harvard as a very faint, diffused nebulosity, with slight central condensation. On the next night, there was found in its place a bright star of the eighth magnitude, scarcely marked out, by a bare trace of environing haze, from the genuine stars it counterfeited. The change was attended by an eight-fold augmentation of light, and was proved by Schiaparelli's confirmatory observations[1340] to have been accomplished within a few hours. The stellar disguise was quickly cast aside. The comet appeared on September 23 as a wide nebulous disc, and soon after faded down to its original dimness. Its distance from the sun was then no less than 200 million miles, and its spectrum showed nothing unusual.

These strange variations recurred slightly on October 15, and with marked emphasis on January 1, when they were witnessed with amazement, and photometrically studied by Muller of Potsdam.[1341] The entire cycle this time was run through in less than four hours--the comet having, in that brief s.p.a.ce, condensed, with a vivid outburst of light, into a seeming star, and the seeming star having expanded back again into a comet. Scarcely less transient, though not altogether similar, changes of aspect were noted by M. Perrotin,[1342] January 13 and 19, 1884. On the latter date, the continuous spectrum given by a reddish-yellow disc surrounding the true nucleus seemed intensified by bright knots corresponding to the rays of sodium.

A comet discovered by Mr. Sawerthal at the Royal Observatory, Cape of Good Hope, February 19, 1888, distinguished itself by blazing up, on May 19, to four or five times its normal brilliancy, at the same time throwing out from the head two l.u.s.trous lateral branches.[1343] These had, on June 1, spread backward so as to join the tail, with an effect like the playing of a fountain; ten or eleven days later, they had completely disappeared, leaving the comet in its former shape and insignificance. Its abrupt display of vitality occurred two full months after perihelion.

On the morning of July 7, 1889, Mr. W. R. Brooks, of Geneva, New York, eminent as a successful comet-hunter, secured one of his customary trophies. The faint object in question was moving through the constellation Cetus, and turned out to be a member of Jupiter's numerous family of comets, revolving round the sun in a period of seven years.

Its past history came then, to a certain extent, within the scope of investigation, and proved to have been singularly eventful; nor had the body escaped scatheless from the vicissitudes to which it had been exposed. Observing from Mount Hamilton, August 2 and 5, Professor Barnard noticed this comet (1889, v.) to be attended in its progress through s.p.a.ce by four _outriders_, "The two brighter companions" (the fainter pair survived a very short time) "were perfect miniatures,"

Professor Barnard tells us,[1344] "of the larger comet, each having a small, fairly defined head and nucleus, with a faint, hazy tail, the more distant one being the larger and less developed. The three comets were in a straight line, nearly east and west, their tails lying along this line. There was no connecting nebulosity between these objects, the tails of the two smaller not reaching each other, or the large comet. To all appearance they were absolutely independent comets." Nevertheless, Spitaler, at Vienna, in the early days of August, perceived, as it were, a thin coc.o.o.n of nebulosity woven round the entire trio.[1345] One of them faded from view September 5; the other actually outshone the original comet on August 31, but was plainly of inferior vitality. It was last seen by Barnard on November 25, with the thirty-six inch refractor, while its primary afforded an observation for position with the twelve-inch, March 20, 1890.[1346] A cause for the disruption it had presumably undergone had, before then, been plausibly a.s.signed.

The adventures of Lexell's comet have long served to exemplify the effects of Jupiter's despotic sway over such bodies. Although bright enough in 1770 to be seen with the naked eye, and ascertained to be circulating in five and a half years, it had never previously been seen, and failed subsequently to present itself. The explanation of this anomaly, suggested by Lexell, and fully confirmed by the a.n.a.lytical inquiries both of Laplace and Leverrier,[1347] was that a very close approach to Jupiter in 1767 had completely changed the character of its...o...b..t, and brought it within the range of terrestrial observation; while in 1779, after having only twice traversed its new path (at its second return it was so circ.u.mstanced as to be invisible from the earth), it was, by a fresh encounter, diverted into one entirely different. Yet the possibility was not lost sight of that the great planet, by inverting its mode of action, might undo its own work, and fling the comet once more into the inner part of the solar system. This possibility seemed to be realized by Chandler's identification of Brooks's and Lexell's comet.[1348] An exceedingly close approach to Jupiter in 1886 had, he found reason to believe, produced such extensive alterations in the elements of its motion as to bring the errant body back to our neighbourhood in 1889. But his inference, though ratified by Mr. Charles Lane Poor's preliminary calculations, proved dubious on closer inquiry, and was rendered wholly inadmissible by the circ.u.mstances attending the return of Brooks's comet in 1896.[1349] The companion-objects watched by Barnard in 1889 had by that time, perhaps, become dissipated in s.p.a.ce, for they were not redetected. They represented, in all likelihood, wreckage from a collision with Jupiter, dating, perhaps, so far back as 1791, when Mr. Lane Poor found that one of the fateful meetings to which short-period comets are especially subject had taken place.

The Lexell-Brooks case was almost duplicated by the resemblance to De Vico's lost comet of 1844[1350] of one detected November 20, 1894, by Edward, son of Lewis Swift. Schulhof[1351] announced the ident.i.ty, and Chandler,[1352] under reserve, vouched for it. Had the comet continued to pursue the track laboriously laid down for it at Boston, and shown itself at the due epoch in 1900, its individuality might have been considered a.s.sured; but the formidable vicegerent of the sun once more interposed, and, in 1897, swept it out of the terrestrial range of view.

Hence the recognition remains ambiguous.

On the morning of March 7, 1892, Professor Lewis Swift discovered the brightest comet that had been seen by northern observers since 1882.

About the time of perihelion, which occurred on April 6, it was conspicuous, as it crossed the celestial equator from Aquarius towards Pegasus, with a nucleus equal to a third magnitude star, and a tail twenty degrees long. This tail was multiple, and multiple in a most curiously variable manner. It divided up into many thin nebulous streaks, the number and relative l.u.s.tre of which underwent rapid and marked changes. Their permanent record on Barnard's and W. H.

Pickering's plates marked a noteworthy advance in cometary photography.

Plate IV. reproduces two of the Lick pictures, taken with a six-inch camera, on April 5 and 7 respectively, with, in each case, an exposure of about one hour. The tail is in the first composed of three main branches, the middle one having sprung out since the previous morning, and the branches are, in their turn, split up into finer rays, to the number of perhaps a dozen in all. In the second a very different state of things is exhibited. "The southern component," Professor Barnard remarked, "which was the brightest on the 5th, had become diffused and fainter, while the middle tail was very bright and broad. Its southern side, which was the best defined, was wavy in numerous places, the tail appearing as if disturbing currents were flowing at right angles to it.

At 42 from the head the tail made an abrupt bend towards the south, as if its current was deflected by some obstacle. In the densest portion of the tail, at the point of deflection, are a couple of dark holes, similar to those seen in some of the nebulae. The middle portion of the tail is brighter, and looks like crumpled silk in places."[1353] Next morning the southern was the prominent branch, and it was loaded, at 1 42' from the head, with a strange excrescence, suggesting the budding-out of a fresh comet in that incongruous situation.[1354] Some of these changes, Professor Barnard thought, might possibly be explained by a rotation of the tail on an axis pa.s.sing through the nucleus, and Pickering, who formed a similar opinion on independent grounds, a.s.signed about 94 hours as the period of the gyrating movement.[1355] He, moreover, determined accelerative velocities outward from the sun of definite condensations in the tail, indicating for its materials, on Bredikhine's theory, a density less than one half that of hydrogen.[1356] This conclusion applied also to Rordame's comet, which exhibited a year later phenomena a.n.a.logous to those remarked in Swift's.

Their photographic study led Professor Hussey[1357] to significant inferences as to the structure and rapid changes of cometary appendages.

PLATE IV.

[Ill.u.s.tration: Photographs of Swift's Comet.

By Professor E. E. Barnard.

No. 1. Taken April 4, 1892; exposure 1 h.

No. 2. Taken April 6, 1892; exposure 1 h. 5 m ]

Seven comets were detected in 1892, and all, strange to say, were visible together towards the close of the year.[1358] Among them was a faint object, which unexpectedly left a trail on a plate exposed by Professor Barnard to the stars in Aquila[1359] on October 12. This was the first comet actually discovered by photography, the Sohag comet having been simultaneously seen and pictured. It has a period of about six years. Holmes's comet is likewise periodical, in rather less than seven years. Its path, which is wholly comprised between the orbits of Mars and Jupiter, is less eccentric than that of any other known comet.

Subsequently to its discovery, on November 6, it underwent some curious vicissitudes. At first bright and condensed, it expanded rapidly with increasing distance from the sun (to which it had made its nearest approach on June 13), until, by the middle of December, it was barely discernible with powerful telescopes as "a feebly luminous mist on the face of the sky."[1360] But on January 16, 1893, observers in Europe and America were bewildered to find, as if subst.i.tuted for it, a yellow star of the seventh magnitude, enveloped in a thin nebulous husk, which enclosed a faint miniature tail.[1361] This condensation and recovery of light lasted in its full intensity only a couple of days. The almost evanescent faintness of Holmes's comet at its next return accounted for its invisibility previous to 1892, when it was evidently in a state of peculiar excitement. Mr. Perrine was barely able, with the Lick 36-inch, to find the vague nebulous patch which occupied its predicted place on June 10, 1899.

The origin of comets has been long and eagerly inquired into, not altogether apart from the cheering guidance of ascertained facts. Sir William Herschel regarded them as fragments of nebulae[1362]--scattered debris of embryo worlds; and Laplace approved of and adopted the idea.[1363] But there was a difficulty. No comet has yet been observed to travel in a decided hyperbola. The typical cometary orbit, apart from disturbance, is parabolic--that is to say, it is indistinguishable from an enormously long ellipse. But this circ.u.mstance could only be reconciled with the view that the bodies thus moving were casual visitors from outer s.p.a.ce, by making, as Laplace did, the tacit a.s.sumption that the solar system was at rest. His reasoning was, indeed, thereby completely vitiated, as Gauss pointed out in 1815;[1364] and the objections then urged were reiterated by Schiaparelli,[1365] who demonstrated in 1871 that a large preponderance of well-marked hyperbolic orbits should result if comets were picked up _en route_ by a swiftly-advancing sun. The fact that their native movement is practically parabolic shows it to have been wholly imparted from without. They pa.s.sively obeyed the pull exerted upon them. In other words, their condition previous to being attracted by the sun was one very nearly of relative repose.[1366] They shared, accordingly, the movement of translation through s.p.a.ce of the solar system.

This significant conclusion had been indicated, on other grounds, as the upshot of researches undertaken independently by Carrington[1367] and Mohn[1368] in 1860, with a view to ascertaining the antic.i.p.ated existence of a relations.h.i.+p between the general _lie_ of the paths of comets and the direction of the sun's journey. It is tolerably obvious that if they wander at haphazard through interstellar regions their apparitions should markedly aggregate towards the vicinity of the constellation Lyra; that is to say, we should meet considerably more comets than would overtake us, for the very same reason that falling stars are more numerous after than before midnight. Moreover, the comets met by us should be, apparently, swifter-moving objects than those coming up with us from behind; because, in the one case, our own real movement would be added to, in the other subtracted from, theirs. But nothing of all this can be detected. Comets approach the sun indifferently from all quarters, and with velocities quite independent of direction.