332 THE POPULAR SCIENCE MONTHLY 



rapidity. He restated the laws of motion in such clear and simple 

 terms that for two and a half centuries no word or line has been 

 changed. Bringing to bear his new calculus he readily solved the 

 problem of falling bodies, and in 1666 discovered the laws of circular 

 motion, seven years before they were published by Huygens in 1673. 



To such a wide reader and deep thinker as Newton, the problem 

 of " gravity " would appeal with keen interest. It would have been 

 but natural that he should have gathered together all that was avail- 

 able of the literature of the subject. Thus he was familiar with 

 Kepler's views expressed in the following citation : 



The true theory of gravity is founded on the following axioms. Gravity is 

 a mutual affection between cognate bodies toward union or conjunction, similar 

 to the magnetic virtue. ... If the moon and the earth were not retained in 

 their orbits by their animal force or some other equivalent, the earth would 

 mount to the moon by a fifty-fourth part of their distance from each other, and 

 the moon would fall toward the earth through the other fifty-three parts, that 

 is, assuming that the substance of the earth is of the same density. ... The 

 sphere of the attractive virtue which is in the moon extends to the earth and 

 entices up the waters, but as the moon flies rapidly across the zenith and the 

 waters can not follow so quickly, a flow of the ocean is occasioned toward the 

 westward. If the attractive virtue of the moon extends to the earth, it follows, 

 with greater reason, that the attractive virtue of the earth extends to the moon 

 and much farther, and, in short, nothing which consists of earthy substance, 

 however constituted, although thrown up to any height, can ever escape the 

 powerful operation of this attractive virtue. 



Borelli (1608-1679) also expresses views no less explicit, and, in his 

 work, " On the Satellites of Jupiter," distinctly attributes the revolu- 

 tions of the heavenly bodies to the force of gravity. So also Bullialdus 

 wrote " that all force respecting the sun as its center, and depending 

 upon matter, must be in a reciprocally duplicate ratio of the distance 

 from the center." This last sentence is quoted from one of Newton's 

 letters, and shows how carefully he had read on the subject. 



Of Newton's immediate contemporaries, Eobert Hooke and Edmund 

 Halley were actively working in this field. Hooke (1635-1702) espe- 

 cially is ambitious to secure the honor of the solution of the problem the 

 answer to which he reads almost exactly right, but the proof of which — 

 poor man — he can not give. Failing in the demonstration himself, he 

 talks on the subject, about the subject, and all over the subject, in the 

 meetings of the Boyal Society, in his papers and in his letters. So full 

 of it is he that he imagines that whatever any one else does is stolen from 

 him. Finally Sir Cristopber Wren offers him a prize if in two months 

 he will produce the boasted of solution. None is forthcoming and 

 history must write Hooke down as a most ardent worker and ingenious 

 man, but as totally unequal to the great task imposed upon Newton. 



Halley is more modest; he applies the laws of circular motion pub- 

 lished by Huygens in 1671, sees clearly that the law of inverse squares 



