The influence of such a result upon such a mind may be more easily conceived than described. The whole material universe was spread out before him; the sun with all his attending planets; the planets with all their satellites; the comets wheeling in every direction in their eccentric orbits; and the systems of the fixed stars stretching to the remotest limits of space. All the varied and complicated movements of the heavens, in short, must have been at once presented to his mind as the necessary result of that law which he had established in reference to the earth and the moon. After extending this law to the other bodies of the system, he composed a series of propositions on the motion of the primary planets about the sun, which were sent to London about the end of 1683, and were soon afterward communicated to the Royal Society. About this period other philosophers had been occupied with the same subject. Sir Christopher Wren had many years before endeavored to explain the planetary motions "by the composition of a descent toward the sun, and an impressed motion; but he at length gave it over, not finding the means of doing it." In January, 1683-1684, Dr. Halley had concluded from Kepler's law of the periods 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. Hooke, 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 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 December 10th Dr. Halley announced to the society that he had seen at Cambridge Newton's treatise _De Motu 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 Newton of his promise, "for securing the invention to himself till such time as he can be at leisure to publish it."