INDUCTIVE EPOCH OF NEWTON. 173 



we believe that the Medicean globes are not, like 

 other planets, impelled with a greater velocity when 

 they approach the sun: and thus they are acted 

 upon by two moving forces, one of which produces 

 their proper revolution about Jupiter, the other 

 regulates their motion round the sun." And in 

 another place, (cap. 20,) he attempts to show an 

 effect of this principle upon the inclination of the 

 orbit; though, as might be expected, without any 

 real result. 



The case which most obviously suggests the 

 notion that the sun exerts a power to disturb the 

 motions of secondary planets about primary ones, 

 might seem to be our own moon; for the great 

 inequalities which had hitherto been discovered, 

 had all, except the first, or elliptical anomaly, a 

 reference to the position of the sun. Nevertheless, 

 I do not know that any one had attempted thus to 

 explain the curiously irregular course of the earth's 

 attendant. To calculate, from the disturbing agency, 

 the amount of the irregularities, was a problem 

 which could not, at any former period, have been 

 dreamt of as likely to be at any time within the 

 verge of human power. 



Newton both made the step of inferring that 

 there were such forces, and, to a very great extent, 

 calculated the effects of them. The inference is 

 made on mechanical principles, in the sixth Theo- 

 rem of the third Book of the Principia ; that the 

 moon is attracted by the sun, as the earth is ; that 



