170 HISTORY OF PHYSICAL ASTRONOMY. 



philosophers in general had supposed that terres- 

 trial 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 gravity : 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 suggestion ; but we 

 should err if we inferred from this coincidence of 

 expression that the notion was commonly enter- 

 tained before him. Thus Huyghens appears to use 

 language which may be mistaken, when he says 7 , 

 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 ter- 

 restrial gravity, as being actually a cosmical force, 

 is foreign to all Borelli's speculations 8 . But Horrox, 

 as early as 1635, appears to have entertained the 

 true view on this subject, although vitiated by Kep- 

 lerian errours concerning the connexion between 

 the rotation of the central body and its effect on 

 the body which revolves about it. Thus he says 9 , 



7 Cosmotheoros, 1. 2. p. 720. 



8 I have found no instance in which the word is so used by 

 him. 



9 Astronomia Kepler iana defensa et promoia^ cap. 2. 



