Meanwhile, the question of the 1843 and 1880 comets was still unsettled, and it received a fresh complication by the appearance of the remarkable comet of 1882, whose transit of the sun has been already alluded to, for the orbit of this new body proved to be a reproduction, almost, but not quite exact, of those of the previous two. Astronomers were at a greater loss than ever, for if this were a return of the 1880 comet, then the conclusion followed that something was so influencing its orbit as to have shortened its period from thirty-seven to two years. The idea of the existence of some medium round the sun, capable of resisting bodies which passed through it, and thus causing them to draw closer to their centre of attraction and shortening their periods, was now revived, and it seemed as though, at its next return, this wonderful visitant must make the final plunge into the photosphere, with what consequences none could foretell. These forebodings proved to be quite baseless. The comet passed so close to the sun (within 300,000 miles of his surface), that it must have been sensibly retarded at its passage by the resisting medium, had such a thing existed; but not the slightest retardation was discernible. The comet suffered no check in its plunge through the solar surroundings, and consequently the theory of the resisting medium may be said to have received its quietus.
Computation showed that the 1882 comet followed nearly the same orbit as its predecessors; and thus we are faced by the fact of families of comets, travelling in orbits that are practically identical, and succeeding one another at longer or shorter intervals. The idea that these families have each sprung from the disruption of some much larger body seems to be most probable, and it appears to be confirmed by the fact that in the 1882 comet the process of further disruption was actually witnessed. Schmidt of Athens detected one small offshoot of the great comet, which remained visible for several days. Barnard a few days later saw at least six small nebulous bodies close to their parent, and a little later Brooks observed another. 'Thus,' as Miss Agnes Clerke remarks, 'space appeared to be strewn with the filmy débris of this beautiful but fragile structure all along the track of its retreat from the sun.'
The state of our knowledge with regard to comets may be roughly summed up. We have extreme tenuity in the whole body, even the nucleus being apparently not solid, but a comparatively loose swarm of solid particles. The nucleus, in all likelihood, shines by reflected sunlight—in part, at all events. The nebulous surroundings and tail are produced by solar action upon the matter of which the comet is composed, this action being almost certainly electrical, though heat may play some part in it. The nebulous matter appears to proceed in waves from the nucleus, and to be swept backward along the comet's track by some repellent force, probably electrical, exerted by the sun. This part of the comet's structure consists mainly of self-luminous gases, generally of the hydrocarbon type, though sodium and iron have also been traced. Comets, certainly in many cases, probably in all, suffer gradual degradation into swarms of meteors. The existence of groups of comets, each group probably the outcome of the disruption of a much larger body, is demonstrated by the fact of successive comets travelling in almost identically similar orbits. Finally, comets are all connected with the solar system, so far, at least, that they accompany that system in its journey of 400,000,000 miles a year through space. Our system does not, as it were, pick up the comets as it sweeps along upon its great journey; it carries them along with it.
A few words may be added as to cometary observation. It is scarcely likely that any very great number of amateur observers will ever be attracted by the branch of comet-hunting. The work is somewhat monotonous and laborious, and seems to require special aptitudes, and, above all, an enormous endowment of patience. Probably the true comet-hunter, like the poet, is born, not made; and it is not likely that there are, nor desirable that there should be, many individuals of the type of Messier, the 'comet-ferret.' 'Messier,' writes a contemporary, 'is at all events a very good man, and simple as a child. He lost his wife some years ago, and his attendance upon her death-bed prevented his being the discoverer of a comet for which he had been lying in wait, and which was snatched from him by Montaigne de Limoges. This made him desperate. A visitor began to offer him consolation for his recent bereavement, when Messier, thinking only of the comet, answered, "I had discovered twelve; alas! to be robbed of the thirteenth by that Montaigne!" and his eyes filled with tears. Then, recollecting that it was necessary to deplore his wife, he exclaimed, "Ah! cette pauvre femme!" and again wept for his comet.' In addition to the fact that few have reached such a degree of scientific detachment as to put a higher value upon a comet than upon the nearest of relatives, there is the further fact that the future of cometary discovery, and of the record of cometary change seems to lie almost entirely with photography, which is wonderfully adapted for the work (Plate [XXVI.]).
1 2
Photographs of Swift's Comet. By Professor E. E. Barnard.
1. Taken April 4, 1892; exposure 1 hour. 2. Taken April 6, 1892; exposure 1 hour 5 minutes.
Anyone who desires to become a comet-hunter must, in addition to the possession of the supreme requisites, patience and perseverance, provide himself with an instrument of at least 4 inches aperture, together with a good and comprehensive set of star-charts and the New General Catalogue of nebulæ with the additions which have been made to it. The reason for this latter item of equipment is the fact that many telescopic comets are scarcely to be distinguished from nebulæ, and that an accurate knowledge of the nebulous objects in the regions to be searched for comets, or at least a means of quickly identifying such objects, is therefore indispensable. The portions of the heavens which afford the most likely fields for discovery will naturally be those in the vicinity of where the sun has set at evening, or where he is about to rise in the early morning, all comets having of necessity to approach the sun more or less closely at their perihelion passage. Other parts of the heavens should not be neglected; but these are the most likely neighbourhoods.