In replying to this letter on the 29th June, 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 the 2d March, 1686–7. The third book was also transmitted, and presented on the 6th April, and the whole work was completed and published in the month of May, 1687. Such is a 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 endeavour to convey to the reader some idea of its contents, and of the brilliant discoveries which it disseminated over Europe. The _Principia_ consists of three books. The first and second, which occupy three-fourths of the work, are entitled, _On the Motion of Bodies_; and the third bears the title, _On the System of the World_. The first two books contain the mathematical principles of philosophy, namely, the laws and conditions of motions and forces; and they are illustrated with several philosophical scholia, which treat of some of the most general and best established points in philosophy, such as the density and resistance of bodies, spaces void of matter, and the motion of sound and light. The object of the third book is to deduce from these principles the constitution of the system of the world; and this book has been drawn up in as popular a style as possible, in order that it may be generally read.