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