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348 SIR ISAAC NEWTON elucidation of the intricate subject of the moon's motions, which had long been a stum- bling-block to astronomers. He deduced theo- retically the two lunar inequalities known as the variation and the annual equation, and also the progression of the apogee and the re- gression of the nodes, though it was reserved for the mathematicians of the next century to complete the lunar theory. It is not known that he made any public announcement of his discoveries before February, 1685, when he sent to the royal society a paper containing his early researches on centripetal forces. Halley, who had endeavored in 1684 to calculate the law of the solar force directed to the planets moving in elliptical orbits, consulted Newton, and, finding that he had gone over the whole ground, induced him to communicate to the royal society the paper already alluded to. On its presentation Hooke raised a violent re- clamation relative to the asserted discovery of the law of gravitation. As early as 1666 he had arrived at very accurate notions of centripetal forces. He had published in 1674 " An At- tempt to prove the Motion of Earth from Ob- servations," in which, as Mr. Grant observes, he describes the general nature of gravity with remarkable clearness and accuracy. (There is a Latin translation of this work, printed in 1679.) But he had not attempted to compute the law of the variation of the force at differ- ent distances from the centre, which, as ap- plied to the elliptical orbits of the planets, was the very problem which Newton had solved. The pretensions of Hooke called forth from Newton a long letter, dated June 20, 1686, in which he recounts the progress of his researches, and intimates his resolution to sup- press the third book of his Principia rather than have his peace of mind disturbed by a controversy with envious rivals. By Halley's persuasion, however, Newton consented to let the whole appear. In April, 1686, the first book of the Principia was exhibited at the royal society; in June Halley undertook its publication at his own expense, although it involved him in considerable pecuniary risk; and it appeared the next year, bearing the following title page: Philosophies Naturalis Principia, Mathematica. Imprimatur Julii 5, 1686. Londini, 1687. It is divided into three books. The first treats of motion in free space ; the second treats chiefly of resisted motion ; and the third deduces from these the system of the world. The doctrines of the Principia, like all discoveries which tend to overthrow cherished opinions, were hotly com- bated for many years. The philosophy of Des- cartes, which the Newtonian theories at length supplanted, was predominant throughout Eu- rope ; and Voltaire said that at the time of his death Newton had not more than 20 followers outside of England. " The language of the French mathematicians," observes the author of the "History of the Inductive Sciences," "is Cartesian for almost half a century after the publication of the Principia of Newton." " The profound and intricate reasoning which Newton was compelled to adopt," says Mr. Grant, "formed a serious impediment to the early dissemination of his doctrines." The Brit- ish universities, however, early introduced the Newtonian philosophy as a subject of study. The university of St. Andrews in Scotland took the lead in 1690, followed by Cambridge in 1699, and by Oxford in 1704. Dr. Bentley made the new principles the basis of a theo- logical argument in the Boyle lectures preached in London in 1692-'3. After the publication of the Principia, Newton was content to ex- tend and develop the principles of his philos- ophy without advancing into any new fields of science ; and even these developments appear to have been based for the most part on ex- periments and observations previously made. He was elected to represent the university in parliament in 1689, and again in 1701 ; and though he was rejected in 1705, those who op- posed him acknowledged him to be " the glory of the university," but considered that he was sent to " tempt them from their duty by the great and just veneration they had for him." On the dissolution of parliament in 1690 he resumed his philosophical and mathematical studies at Cambridge. After this his health be- came impaired. In December, 1692, and Jan- uary and February, 1693, he wrote the four celebrated letters to Dr. Bentley on the exis- tence of a Deity. He was greatly affected about the beginning of 1692 by the loss of valuable manuscripts, which were consumed in his study by the upsetting of a candle. The notable story of his little dog Diamond having occa- sioned the mischief, and of Newton's remark- able equanimity on seeing what had happened, Brewster considers a fiction. In a letter to Pepys, dated Sept. 13, 1693, and one to Locke about the same time, there are evidences of loss of judgment. A fortnight after the for- mer was written, Newton told a common friend " that he had writ a very odd letter to Pepys, at which he was much concerned;" adding " that it was in a distemper that much seized his head, and that kept him awake for above five nights together." Not many days after his painful letter to Locke was written he wrote again with child-like simplicity and tenderness, asking to be forgiven for think' ing ill of him, and subscribing himself, " Your most humble and unfortunate servant.' 1 About this time began the celebrated quarrel be- tween Newton and Flamsteed, the astronomer royal, which was only terminated by Flam- steed's death (1719). They had been on terms of cordial intimacy till 1696, when a coolness began to spring up. Newton was engaged on his lunar theory, and required observations of the moon's places. Flamsteed, who at his own expense had put himself in possession of the proper instruments, was the only one who could furnish the observations. They were supplied, though not as promptly as Newton