I have great pleasure in introducing to English readers M. Guillemin's valuable and interesting work on comets. When rapid progress has been made in any branch of science, it is generally very difficult for anyone, who has not been actually concerned in the investigations in question, to obtain accurate information of the state of our knowledge; and for this reason a book, such as the present, which gives an account of the new results that we owe to very recent researches, really confers a benefit upon many persons who, though taking a strong interest in the subject, have necessarily been quite unable to follow its development in the periodical publications of English and foreign scientific societies. There is no work that at all occupies the ground covered by that of M. Guillemin; and as the subject is one which, always of high interest, has in the last few years acquired, great importance in consequence of Schiaparelli's discovery of a connexion between comets and shooting-stars, I was anxious that it should appear in our language.