and Distances, that the centripetal force decreased in the reciprocal proportion of the squares of the distances; and having one day met Sir Christopher Wren and Dr. Hookc, the latter affirmed that he had demonstrated upon that principle all the laws of the celestial motions. Dr. Halley confessed that his attempts were unsuccessful, and Sir Christopher, in order to encourage the inquiry, offered to present a book of forty shillings' value to either of the two philosophers who should, in the space of two months, bring him a convincing demonstration of it. Hooke persisted in the declaration that he possessed the method, but avowed it to be his intention to conceal it for some time. He promised, however, to show it to Sir Christopher; but there is every reason to believe that this promise was never fulfilled.

In August, 1684, Dr. Halley went to Cambridge for the express purpose of consulting Newton on this interesting subject. Newton assured him that he had brought this demonstration to perfection, and promised him a copy of it. This copy was received in November by the Doctor, who made a second visit to Cambridge, in order to induce its author to have it inserted in the register book of the Society. On the 10th of December, Dr. Halley announced to the Society, that he had seen at Cambridge Mr. Newton's treatise De Molu Corporum, which he had promised to send to the Society to be entered upon their register; and Dr. Halley was desired to unite with Mr. Paget, Master of the Mathematical School in Christ's Hospital, in reminding Mr. Newton of his promise "for securing the invention to himself till such time as he can be at leisure to publish it." On the 25th of February, Mr. Aston, the Secretary, communicated a letter from Mr. Newton, in which he expressed his willingness "to enter in the register his notions about motion, and his intentions to fit them suddenly for the press." The progress

of his work was, however, interrupted by a visit of five or eix weeks which he made in Lincolnshire; but he proceeded with such diligence on his return, that he was able to transmit the manuscript to London before the end of April. This manuscript, entitled Philosophies Naturalis Frincipia Mathematica, and dedicated to the Society, was presented by Dr. Vincent on the 28th of April, 1686, when Sir John Hoskins, the vice-president, and the particular friend of Dr. Hooke, was in the chair. Dr. Vincent passed a just encomium on the novelty and dignity of the subject; and another member added, that " Mr. Newton had carried the thing so far, that there was no more to be added." To these remarks the vice-president replied, that the method "was so much the more to be prized as it was both invented and perfected at the same time." Dr. Hooke took offence at these remarks, and blamed Sir John for not having mentioned "what he had discovered to him;" but the vice-president did not seem to recollect any such communication, and the consequence of this discussion was, that " these two, who till then were the most inseparable cronies, have since scarcely seen one another, and are utterly fallen out." After the breaking up of the meeting, the Society adjourned to the coffee-house, where Dr. Hooke stated that he not only had made the same discovery, but had given the first hint of it to Newton.

An account of these proceedings was communicated to Newton through two different channels. In a letter dated May 22nd, Dr. Halley wrote to him " that Mr. Hooke had some pretensions upon the invention of the rule of the decrease of gravity being reciprocally as the squares of the distances from the centre. He says you had the notion from him, though ho owns the demonstration of the curves generated thereby to be wholly your own. How much of this is so you know best, as likewise what you have to do in this matter. Only Mr. Hooke seems to expect you will make some mention of him in the preface, which it is possible you may see reason to prefix."

This communication from Dr. Halloy induced our author, on the 20th of June, to address a long letter to him, in which he gives a minute and able refutation of Hookc's claims; but before this letter was dispatched, another correspondent, who had received his information from one of the members that were present, informed Newton "that Hooke made a great stir, pretending that he had all from him, and desiring they would see that he had justice done him." This fresh charge seems to have ruffled the tranquillity of Newton; and he accordingly added an angry and satirical postscript, in which he treats Hooke with little ceremony, and goes so far as to conjecture that Hooke might have acquired his knowledge of the law from a letter of his own to Huygens, directed to Oldenburg, and dated January 14th, 1672-3. "My letter to Hugenius was directed to Mr. Oldenburg, who used to keep the originals. His papers came into Mr. Hooke's possession. Mr. Hooke, knowing my hand, might have the curiosity to look into that letter, and there take the notion of comparing the forces of the planets arising from their circular motion; and so what he wrote to me afterwards about the rate of gravity, might be nothing but the fruit of my own garden."

In replying to this letter, Dr. Hallcy assured him that Hooke's " manner of claiming the discovery had been represented to him in worse colours than it ought, and that he neither made public application to the Society for justice, nor pretended that you had all from him." The effect of this assurance was to make Newton regret that he had written the angry postscript to his letter; and in replying to Halley on the 14th of July, 1686, he not only expresses his regret, but recounts the different new ideas which he had acquired from Hooke's correspondence, and suggests it

as the best method "of compromising the present dispute," to add a scholium, in which Wren, Hooke, and Halley are acknowledged to have independently deduced the law of gravity from the second law of Kepler.*

At the meeting of the 28th of April, at which the manuscript of the Principia was presented to the Royal Society, it was agreed that the printing of it should be referred to the Council; that a letter of thanks should be written to its author; and at a meeting of the Council on the 19th of May, it was resolved that the MS. should be printed at the Society's expense, and that Dr. Halley should superintend it while going through the press. These resolutions were communicated by Dr. Halley in a letter dated the 22nd of May; and in Newton's reply on the 20th of June, already mentioned, he makes the following observations: "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 afterwards 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 law-suits 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 two first books without the third, will not so well bear the title of Philosophies Natwalis Principia Matltematica; and therefore I had altered it to this, Do Motu Corponm

Libri duo. But, after second thoughts, I retain the former title. T will help the sale of the book, which I ought not to diminish now 'tis yours." K' In replying to this letter on the 29 th of 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 2nd of March, 1686-7. The third book was also transmitted, and presented on the 6th of April, and the whole 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—the first treating of their motions in free space, and the second of their motions in a resisting medium; whilst the third bears the title, On the System of the World. The first two books contain the