In 1679 Newton’s attention was again called to the theory of gravitation, and by a fuller investigation of the conditions of elliptical motion, he was confirmed in the opinion that the phenomena of the planets were referable to an attractive force in the sun, of which the intensity varied in the inverse proportion of the square of the distance. The difficulty about the amount of the moon’s motion remained, but it was shortly to be removed. In 1679 Picard effected a new measurement of a degree of the earth’s surface, and Newton heard of the result at a meeting of the Royal Society in June, 1682. He immediately returned home to repeat his former calculation with these new data. Every step of the process made it more probable that the discrepance which had so long perplexed him would wholly disappear: and so great was his excitement at the prospect of entire success that he was unable to proceed with the calculation, and intrusted its completion to a friend. The triumph was perfect, and he found the theory of his youth sufficient to explain all the great phenomena of nature. From this time Newton devoted unremitting attention to the development of his system, and a period of nearly two years was entirely absorbed by it. In 1684 the outline of the mighty work was finished; yet it is likely that it would still have remained unknown, had not Halley, who was himself on the track of some part of the discovery, gone to Cambridge in August of that year to consult Newton about some difficulties he had met with. Newton communicated to him a treatise De Motu Corporum, which afterwards, with some additions, formed the first two books of the Principia. Even then Halley found it difficult to persuade him to communicate the treatise to the Royal Society, but he finally did so in April, 1686, with a desire that it should not immediately be published, as there were yet many things to complete. Hooke, whose unwearied ingenuity had guessed at the true law of gravity, immediately claimed to himself the honour of the discovery; how unjustly it is needless to say, for the merit consisted not in the conjecture but the demonstration. Newton was inclined in consequence to prevent the publication of the work, or at least of the third part, De Mundi Systemate, in which the mathematical conclusions of the former books were applied to the system of the universe. Happily his reluctance was overcome, and the whole work was published in May, 1687. Its doctrines were too novel and surprising to meet with immediate assent; but the illustrious author at once received the tribute of admiration for the boldness which had formed, and the skill which had developed his theory, and he lived to see it become the common philosophical creed of all nations.