172 HISTORY OF PHYSICAL ASTRONOMY. 



difficult to prove ; this was much more the case 

 with the part of which we have now to speak, the 

 attraction of other bodies, besides the central ones, 

 upon the planets and satellites. If the mathema- 

 tical calculation of the unmixed effect of a central 

 force required transcendent talents, how much must 

 the difficulty be increased, when other influences 

 prevented those first results from being accurately 

 verified, while the deviations from accuracy were 

 far more complex than the original action ! If it 

 had not been that these deviations, though surpris- 

 ingly numerous and complicated in their nature, 

 were very small in their quantity, it would have 

 been impossible for the intellect of man to deal 

 with the subject; as it was, the struggle with its 

 difficulties is even now a matter of wonder. 



The conjecture that there is some mutual action 

 of the planets, had been put forth by Hooke in his 

 Attempt to prove the Motion of the Earth, (1674.) 

 It followed, he said, from his doctrine, that not only 

 the sun and the moon act upon the course and 

 motion of the earth, but that Mercury, Venus, Mars, 

 Jupiter, and Saturn, have also, by their attractive 

 power, a considerable influence upon the motion of 

 the earth, and the earth in like manner powerfully 

 affects the motions of those bodies. And Borelli, in 

 attempting to form "theories" of the satellites of 

 Jupiter, had seen, though dimly and confusedly, the 

 probability that the sun would disturb the motions 

 of these bodies. Thus he says, (cap. 14,) "How can 



