During recent years the subject of Comets has received little attention by astronomers, apart from the routine work of observation and computation of orbits. The theory of their origin has been almost completely neglected (of necessity, in the absence of hypotheses), and the obscurity attaching to the whole subject of Comets as a cosmogonical problem had come to be accepted as yet another of the numerous mysteries of astronomy. It has been one of the principal successes of the New Cosmology that, without having any idea of an attack on the cometary problem in view, nevertheless one of the fundamental processes discovered in connexion with stellar evolution has been found to lead quite naturally to a straightforward, and indeed a necessary, explanation of the presence of comets in the solar system, and also leads on to an understanding of many of their properties. This book represents an attempt to lay this theory before as wide a circle of astronomers as possible, in the hope that it will bring about renewed interest in the subject of Comets and thereby help to integrate astronomical theory into a united philosophical whole instead of remaining a closely guarded patchwork of disconnected, more or less taxonomic descriptions.

It has seemed to me to be more than desirable to present also an account of the observational features of Comets, which do not appear to be by any means widely known, and this information I have culled from the vast literature of the subject. I have no direct observational experience, at any rate with a telescope, and I claim no originality for this material. I express my indebtedness to the numerous authors, most of whom are no longer with us, whose papers and writings I have found so absorbingly interesting; and I hope that too much of that element of interest has not disappeared as a result of my summarization and selection of their writings. The first two chapters of this book contain this account. There follow two chapters on the theory of the formation and structure of comets, and then by way of conclusion follows a short chapter showing the relation of the work to earlier attempts at theoretical explanations. An Appendix gives numerous references to the literature of Comets, but is not claimed to be exhaustive.