Bailly sketches the plan of his work with a masterly hand in a few lines; he says, "It is interesting to transport one's self back to the times when Astronomy began; to observe how discoveries were connected together, how errors have got mixed up with truth, have delayed the knowledge of it, and retarded its progress; and, after having followed the various epochs and traversed every climate, finally to contemplate the edifice founded on the labours of successive centuries and of various nations." This vast plan essentially led to the minute discussion and comparison of a multitude of passages both ancient and modern. If the author had mixed up these discussions with the body of the work, he would have laboured for astronomers only. If he had suppressed all discussions, the book would have interested amateurs only. To avoid this double rock, Bailly decided on writing a connected narrative with the quintessence of the facts, and to place the proofs and the discussions of the merely conjectural parts, under the appellation of explanations in separate chapters. Bailly's History, without forfeiting the character of a serious and erudite work, became accessible to the public in general, and contributed to disseminate accurate notions of Astronomy both among literary men and among general society. When Bailly declared, in the beginning of his book, that he would go back to the very commencement of Astronomy, the reader might expect some pages of pure imagination. I know not, however, whether any body would have expected a chapter of the first volume to be entitled, _Of Antediluvian Astronomy_. The principal conclusion to which Bailly comes, after an attentive examination of all the positive ideas that antiquity has bequeathed to us is, that we find rather the ruins than the elements of a science in the most ancient Astronomy of Chaldaea, of India, and of China. After treating of certain ideas of Pluche, Bailly says, "The country of possibilities is immense, and although truth is contained therein, it is not often easy to distinguish it." Words so reasonable would authorize me to inquire whether the calculations of our fellow-labourer, intended to establish the immense antiquity of the Indian Tables, are beyond all criticism. But the question has been sufficiently discussed in a passage of _The Exposition of the System of the World_, on which it would be useless to insist here. Whatever came from the pen of M. de Laplace was always marked by the stamp of reason and of evidence. In the first lines of his magnificent work, after having remarked that "the history of Astronomy forms an essential part of the history of the human mind," Bailly observes, "that it is perhaps the true measure of man's intelligence, and a proof of what he can do with time and genius." I shall allow myself to add, that no study offers to reflecting minds more striking or more curious relations.