Graunt, John, _Natural and Political Observations ... made upon the Bills of Mortality, with reference to the Government, Religion, Trade, Growth, Air, Diseases, and the several changes of the City_. 3d edition, 1665. Sprat, Thomas, _The History of the Royal Society of London, for the Improving of Natural Knowledge_. 1667. Malpighi, Marcello, _Dissertatio epistolica de Bombyce; Societati Regiæ Londini dicata_. 1669. (On the silkworm.) Evelyn, John, _Sylva, or a Discourse of Forest Trees_. 1670. Horrocks, Jeremiah, _Opera [Astronomica] postuma_. 1673. Malpighi, Marcello, _Anatome Plantarum_. 1675. Willughby, Francis, _Ornithology_ (revised by John Ray). 1676. Evelyn, John, _A Philosophical Discourse of Earth, relating to the Culture and Improvement of it for Vegetation_. 1676. Grew, Nehemiah, _The Anatomy of Plants_. 1682. Willughby, F., _Historia Piscium_. 1686. Ray, John, _Historia Plantarum_. 2 vols., 1686-88. Flamsteed, John, _Tide-Table for 1687_. Newton, Isaac, _Philosophiæ Naturalis Principia Mathematica_. Autore Is. Newton. Imprimatur: S. Pepys, Reg. Soc. Præses. Julii 5, 1686. 4to. Londini, 1687. After the Society had ordered that Newton's _Mathematical Principles of Natural Philosophy_ should be printed, it was found that the funds had been exhausted by the publication of Willughby's book on fishes. It was accordingly agreed that Halley should undertake the business of looking after it, and printing it at his own charge, which he had engaged to do. Shortly after, the President of the Royal Society, Mr. Samuel Pepys, was desired to license Mr. Newton's book. It was not merely by defraying the expense of publication that Halley contributed to the success of the _Principia_. He, Wren, Hooke, and other Fellows of the Royal Society, concluded in 1684 that if Kepler's third law were true, then the attraction exerted on the different planets would vary inversely as the square of the distance. What, then, would be the orbit of a planet under a central attraction varying as the inverse square of the distance? Halley found that Newton had already determined that the form of the orbit would be an ellipse. Newton had been occupied with the problem of gravitation for about eighteen years, but until Halley induced him to do so, had hesitated, on account of certain unsettled points, to publish his results. He writes: "I began (1666) to think of gravity extending to the orb of the moon, ... and thereby compared the force requisite to keep the moon in her orb with the force of gravity at the surface of the earth, and found them answer pretty nearly." As early as March of that same year Hooke had communicated to the Society an account of experiments in reference to the force of gravity at different distances from the surface of the earth, either upwards or downwards. At this and at every point in Newton's discovery the records of co-workers are to be found.