ruption of the comet were noticed in the southern hemisphere. Mr. Ranvard has remarked that when the comet became visible in the southern hemisphere 'the inner duplicate structure was still visible, but the outer arcs had been dissipated:' so that the comet does not seem to have undergone any marked changes in consequence of its passage near the sun. SECTION I. WHAT IS A COMET? Complexity and extent of the question-The law of gravitation suffices to explain the movements of comets-Lacunæ in the theory; acceleration of the motion of the comets of Encke and Faye-Origin of comets; their systems-Questions relative to their physical and chemical constitution-Form of atmospheres; birth and development of tails. LET us glance back for a moment at the contents of the preceding chapters. We there find many facts accumulated, observations both interesting and instructive, phenomena whose variations suggest reflections without limit concerning the nature of the bodies to which they relate. Nevertheless, do these collected facts permit a clear and certain reply to the simple question: What is a comet? I say a simple question, for so, as a rule, it is thought to be by non-scientific people; but in reality there is no question more complex. In order to attempt to reply to it, or at least to relate what is known for certain about comets, and to pass in review the most probable conjectures on doubtful points, we must proceed methodically, and thus as it were divide the difficulty. A first natural division of the subject is at once apparent, it seems to us, from the exposition of cometary phenomena which has been made in the preceding chapters. This division includes the movements of comets, either apparent or real, all that relates to their orbits, and, in a word, the laws which govern them, not only as concerns what we may call the regular portion of their course, but in the vicissitudes and perturbations to which they are subjected by other celestial bodies. So far-in theory, at least we find no difficulty in explaining the various facts, such as the periodicity of certain comets, the disappearance of some, the nonreappearance of others, the delay or too speedy arrival of those whose epoch of return has been calculated. Gravitation is the principle that renders an account of all these facts, of all these movements; the theory of comets is, in this respect, the same as that of the planets; and, if there still remain difficulties and facts unexplained, neither the principle nor its application are for a moment doubted by any true astronomer. There are difficulties, as we have already seen. For example, we ask ourselves, under the operation of what cause does Encke's comet continually shorten its period of revolution? Is this diminution due to the influence of a resisting medium or to the action of a repulsive force? Opinions are divided; but this is no impeachment of the principle of gravitation, or the fact that the sun attracts a mass inversely as the square of its distance from his centre. There are obscurities, as, for instance, the origin of comets. That all comets belong to the solar system cannot be supposed, as certain amongst them move in hyperbolic orbits. But have all these bodies come originally from beyond the limits of the solar system? Do they form, as M. Hoek believes, groups or systems; and are we to consider the conversion of their original orbits into closed orbits as due to the disturbing action of the planetary masses? These questions are not yet decided; but, whatever the reply that science may |