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 lustre 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 nebulæ. 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 passing through the nucleus, and Pickering, who formed a similar opinion on independent
1 2
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
grounds, assigned 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 Brédikhine'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 analogous 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.
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 substituted 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 nebulæ[1362]—scattered débris 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 circumstance could only be reconciled with the view that the bodies thus moving were casual visitors from outer space, by making, as Laplace did, the tacit assumption 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 passively 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 space 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 anticipated existence of a relationship 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.
We conclude, then, that the "cosmical current" which bears the solar system towards its unknown goal carries also with it nebulous masses of undefined extent, and at an undefined remoteness, fragments detached from which, continually entering the sphere of the sun's attraction, flit across our skies under the form of comets. These are, however, almost certainly so far strangers to our system that they had no part in the long processes of development by which its present condition was attained. They are, perhaps, survivals of an earlier, and by us scarcely and dimly conceivable state of things, when the swirling chaos from which sun and planets were, by a supreme edict, to emerge, had not as yet separately begun to be.