"The proof you sent me I like very well. I designed the whole to consist of three books; the second was finished last summer, being short, and only wants transcribing and drawing the cuts fairly. Some new propositions I have since thought on which I can as well let alone. The third wants the theory of comets. In autumn last I spent two months in calculation to no purpose, for want of a good method, which made me afterward return to the first book and enlarge it with diverse propositions, some relating to comets, others to other things found out last winter. The third I now design to suppress. Philosophy is such an impertinently litigious lady that a man had as good be engaged in lawsuits as have to do with her. I found it so formerly, and now I can no sooner come near her again but she gives me warning. The first two books, without the third, will not so well bear the title of _Philosophies Naturalis Principia Mathematica_; and therefore I had altered it to this: _de Moti Corporum, Libri duo_. But after second thoughts I retain the former title. 'Twill help the sale of the book, which I ought not to diminish now 'tis yours." In replying to this letter on June 29th Dr. Halley regrets that our author's tranquillity should have been thus disturbed by envious rivals, and implores him in the name of the society not to suppress the third book. "I must again beg you," says he, "not to let your resentments run so high as to deprive us of your third book, wherein your applications of your mathematical doctrine to the theory of comets, and several curious experiments which, as I guess by what you write ought to compose it, will undoubtedly render it acceptable to those who will call themselves philosophers without mathematics, which are much the greater number." To these solicitations Newton seems to have readily yielded. His second book was sent to the society, and presented on March 2, 1687. The third book was also transmitted, and presented on April 6th, and the whole work was completed and published in the month of May, 1687. Such is the brief account of the publication of a work which is memorable not only in the annals of one science or of one country, but which will form an epoch in the history of the world, and will ever be regarded as the brightest page in the records of human reason. We shall endeavor to convey to the reader some idea of its contents, and of the brilliant discoveries which it disseminated over Europe.