94 GRAVITATION. 



* Now, although the force of gravity might not be sen- 

 sibly less, at the tops of the highest mountains, than at 

 the ordinary level of the earth's surface, he conceived it 

 to be very possible, that at so great a distance as that of 

 the moon, it might be considerably different. To make 

 an estimate of what might be the degree of diminution, 

 he considered that, if the moon be retained in her orbit 

 by the force of gravity, no doubt the primary planets are 

 carried round the sun by a like power ; and by compar- 

 ing the periods of the several planets with their distances 

 from the sun, he found that if any power like gravity kept 

 them in their orbits, its strength must decrease inj>ro- 

 portion as the squares of the distances increase.' 



Supposing therefore, that the force of gravity decreas- 

 ed, i. c. so far as its operation upon particular bodies is 

 concerned, in the above named ratio, he proceeded with 

 the laborious calculation. The result disappointed him. 

 It did not correspond with the fact. The hopes which 

 he had cherished of throwing new light on this subject 

 were blasted, and he gave up the consideration of it. 



The reason of this failure was, however, an error gen- 

 erally prevalent at that time in regard to the size of the 

 earth. He supposed it smaller than it really was. This 

 affected the calculation so as to produce the wrong result 

 which had discouraged him ; and it was not until several 

 years afterwards that he discovered this cause of error. 

 When he did discover it, he resumed the calculation. As 

 his work, on this second attempt drew towards the close, 

 he foresaw the successful result. The joy of attaining 

 success after a previous failure, the magnificence of 

 the expected discovery the changes, which he could 

 easily foresee would be produced in the opinions of man- 

 kind on this subject, the rapid advances which astron- 

 omy might now be expected to make, all burst upon his 

 mind, bringing with them so many agitating emotions, 

 that he could not complete his work. He was obliged 

 to call in the assistance of a friend by whom the result 

 was obtained. 



We cannot follow this great philosopher through the 

 remaining incidents of his life. He pursued with much 

 diligence and success his mathematical and philosophical 



