INDUCTIVE EPOCH OF NEWTON. 405 



inversely as the square of the distance, in 1679, upon occasion of his 

 correspondence with Hooke. In 1684, at Halley's request, he returned 

 to the subject, and in February, 1685, there was inserted in the Regis- 

 ter of the Royal Society a paper of Newton's (Isaaci Newtoni Proposi- 

 tiones de Motu) which contained some of the principal Propositions of 

 the first two Books of the Principia. This paper, however, does not 

 contain the Proposition " Lunani gravitare in terrain," nor any of the 

 other propositions of the third Book. The Principia was printed in 

 1686 and 7, apparently at the expense of Halley. On the 6th of April, 

 1687, the third Book was presented to the Royal Society.] 



It does not appear, I think, that before Newton, philosophers in gen- 

 eral had supposed that terrestrial gravity was the very force by which 

 the moon's motions are produced. Men had, as we have seen, taken 

 up the conception of such forces, and had probably called them grav- 

 ity : but this was done only to explain, by analogy, what kind of forces 

 they were, just as at other times they compared them with magnetism ; 

 and it did not imply that terrestrial gravity was a force which acted in 

 the celestial spaces. After Newton had discovered that this was so, the 

 application of the term " gravity" did undoubtedly convey such a sug- 

 gestion : but we should err if we inferred from this coincidence of ex- 

 pression that the notion was commonly entertained before him. Thus 

 Huyghens appears to use language which may be mistaken, when he 

 says, 6 that Borelli was of opinion that the primary planets were urged 

 by " gravity" towards the sun, and the satellites towards the primaries. 

 The notion of terrestrial gravity, as being actually a cosmical force, is 

 foreign to all Borelli's speculations. 7 But Horrox, as early as 1635, 

 appears to have entertained the true view on this subject, although vi- 

 tiated by Keplerian errors concerning the connection between the 

 rotation of the central body and its effect on the body which revolves 

 about it. Thus he says, 8 that the emanation of the earth carries a pro- 

 jected stone along with the motion of the earth, just in the same way 

 as it carries the moon in her orbit ; and that this force is greater on 

 the stone than on the moon, because the distance is less. 



The Proposition in which Newton has stated the discovery of which 

 we are now speaking, is the fourth of his third Book : " That the moon 

 gravitates to the earth, and by the force of gravity is perpetually de- 



9 Cosmotheros, 1. 2. p. 720. 



7 I have found no instance in which the word is so used by him. 



8 Astronomia Kepleriana defensa et promota, cap. 2. See further on this subject in 

 the Additions to this volume. 



