110 THE HISTORY OF SCIENCE 



Willughby, F., Historia Piscium. 1686. 



Ray, John, Historia Plantarum. 2 vols., 1686-88. 



Flamsteed, John, Tide-Table for 1687. 



Newton, Isaac, Philosophies Naturalis Principia Mathe- 

 matica. Autore Is. Newton. Imprimatur: S. Pepys, 

 Reg. Soc. Prases. 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 Hal- 

 ley 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 Fel- 

 lows 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 vary- 

 ing 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 unset- 

 tled points, to publish his results. 



He writes: "I began (1666) to think of gravity 

 extending to the orb of the moon, . . . and thereby 



