$169,170] First Discoveries : Gravity 213 



was in the two plague years of 1665 and 1666, for in those 

 days I was in the prime of my age for invention, and minded 

 Mathematicks and Philosophy more than at any time since." * 



170. He spent a considerable part of this time (1665- 

 1666) at Woolsthorpe, on account of the prevalence of 

 the plague. 



The well-known story, that he was set meditating on 

 gravity by the fall of an apple in the orchard, is based 

 on good authority, and is perfectly credible in the sense 

 that the apple may have reminded him at that particular 

 time of certain problems connected with gravity. That 

 the apple seriously suggested to him the existence of the 

 problems or any key to their solution is wildly improbable. 



Several astronomers had already speculated on the 

 " cause " of the known motions of the planets and satellites ; 

 that is they had attempted to exhibit these motions as 

 consequences of some more fundamental and more general 

 laws. Kepler, as we have seen (chapter VIL, 150), had 

 pointed out that the motions in question should not be 

 considered as due to the influence of mere geometrical 

 points, such as the centres of the old epicycles, but to 

 that of other bodies ; and in particular made some attempt 

 to explain the motion of the planets as due to a special 

 kind of influence emanating from the sun. He went, 

 however, entirely wrong by looking for a force to keep 

 up the motion of the planets and as it were push them 

 along. Galilei's discovery that the motion of a body 

 goes on indefinitely unless there is some cause at work 

 to alter or stop it, at once put a new aspect on this as 

 on other mechanical problems ; but he himself did not 

 develop his idea in this particular direction. Giovanni 

 Alfonso Borelli (1608-1679), in a book on Jupiter's satellites 

 published in 1666, and therefore about the time of Newton's 

 first work on the subject, pointed out that a body revolving 

 in a circle . (or similar curve) had a tendency to recede 

 from the centre, and that in the case of the planets this 

 might be supposed to be counteracted by some kind of 

 attraction towards the sun. We have then here the idea 



* From a MS. among the Portsmouth Papers, quoted in the Preface 

 to the Catalogue of the Portsmouth Papers. 



