solution to Wren and started for Cambridge, in August 1684, to consult Newton on the subject. Without mentioning the speculations which had been made, he asked Newton what would be the curve described by a planet around the Sun on the assumption that the Sun's force diminished as the square of the distance. Newton replied promptly, "an ellipse", and on being questioned by Halley as to the reason for his answer he replied, "Why, I have calculated it." He could not, however, put his hand upon his calculation, but he promised to send it to Halley. After the latter had left Cambridge, Newton set to work to reproduce the calculation. After making a mistake and producing a different result he corrected his work and obtained his former result. Next November, Newton redeemed his promise to Halley by sending him, by the hand of Mr. Paget, a fellow of Trinity College and mathematical master of Christ's Hospital, a copy of his demonstration; and very soon afterward Halley again visited Cambridge to confer with Newton about the problem. On his return to London on 10 December 1684, he informed the Royal Society "that he had lately seen Mr. Newton at Cambridge, who had shown him a curious treatise De Motu", which at Halley's desire he promised to send to the Society to be entered upon their register. "Mr. Halley was desired to put Mr. Newton in mind of his promise for the securing this invention to himself, till he could be at leisure to publish it", and Paget was desired to join with Halley in urging Newton to do so. By the middle of February Newton had sent his paper to Aston, one of the secretaries of the Society, and in a letter to Aston dated 23 February 1685, Newton thanked him for "having entered on the register his notions about motion". This treatise De Motu was the starting point of the Principia, and was meant to be a short account of what that work was intended to embrace. It occupies twenty-four octavo pages, and consists of four theorems and seven problems, some of which are identical with some of the most important propositions of the second and third sections of the first book of the Principia. See also Ismaël Bullialdus De Motu (Berkeley's essay) Elements of the Philosophy of Newton Gauss–Newton algorithm History of calculus List of independent discoveries Newton's cannonball