INDUCTIVE EPOCH OF NEWTON. 167 



place close to him ; considered them as of the 

 same kind, and applied the same rules to each, 

 without hesitation or obscurity. But so far, this 

 thought was merely a guess : its occurrence showed 

 the activity of the thinker; but to give it any value, 

 it required much more than a "why not?" a "per- 

 haps." Accordingly, Newton's " why not ?" was im- 

 mediately succeeded by his " if so, what then ?" His 

 reasoning was, that if gravity reach to the moon, it 

 is probably of the same kind as the central force of 

 the sun, and follows the same rule with respect to 

 the distance. What is this rule ? We have already 

 seen that, by calculating from Kepler's laws, and 

 supposing the orbits to be circles, the rule of the 

 force appears to be the inverse duplicate proportion 

 of the distance : and this, which had been current 

 as a conjecture among the previous generation of 

 mathematicians, Newton had already proved by 

 indisputable reasonings, and was thus prepared to 

 proceed in his train of inquiry. If then, he went 

 on, pursuing his train of thought, the earth's gravity 

 extend to the moon, diminishing according to the 

 inverse square of the distance, will it, at the moon's 

 orbit, be of the proper magnitude for retaining her 

 in her path ? Here again came in calculation, and a 

 calculation of extreme interest ; for how important 

 and how critical was the decision which depended 

 on the resulting numbers ? According to Newton's 

 calculations, made at this time, the moon, by her 

 motion in her orbit, was deflected from the tangent 



