ON THE HISTORY OF ASTRONOMY. 459 



on the subject very clearly in his Attempt to prove the motion of the earth, 

 published in 1674, and had his skill in mathematics been equal to his 

 practical sagacity, he would probably have completed, or at least have 

 published the discovery before his great cotemporary. 



It must be confessed that Newton's good fortune was equal to his talents 

 and his application ; for had he lived earlier, he might probably have con- 

 fined his genius to speculations purely mathematical ; had he been later, 

 his discoveries in natural philosophy might have been anticipated by 

 others ; and yet Newton would perhaps have improved still more on their 

 labours than they have done on his. It was in 1676, when he was 34 

 years old, that he first demonstrated the necessary connexion of the 

 planetary revolutions in elliptic orbits, with an attractive force varying 

 inversely as the square of the distance. But he had collected the law of 

 the force, from the discoveries of Kepler respecting the periods of the dif- 

 ferent planets, some time before 1671, as he asserts to Dr. Halley, and, to 

 the best of his recollection, about 1668, although in his Principia he allows, 

 with the most laudable candour, to Wren, Hooke, and Halley, the merit 

 of having made the same discovery, without any connexion with each 

 other's investigations, or with his own. The manner, in which Newton 

 was led to attend particularly to the subject, is thus related by Pemberton, 

 in the preface to his View of Sir Isaac Newton's philosophy. 



" The first thoughts," says Pemberton, * "which gave rise to his Prin- 

 cipia, he had, when he retired from Cambridge in 1666, on account of the 

 plague. As he sat alone in a garden, he fell into a speculation on the 

 power of gravity ; that as this power is not found sensibly diminished 

 at the remotest distance from the centre of the earth, to which we can rise, 

 neither at the tops of the loftiest buildings, nor even on the summits of 

 the highest mountains ; it appeared to him reasonable to conclude, that 

 this power must extend much further than was usually thought ; why not 

 as high as the moon ? said he to himself ; and if so, her motion must be 

 influenced by it ; perhaps she is retained in her orbit thereby. However, 

 though the power of gravity is not sensibly weakened in the little change 

 of distance, at which we can place ourselves from the centre of the earth ; 

 yet it is very possible that so high as the moon this power may differ 

 much in strength from what it is here. To make an estimate, what might 

 be the degree of this diminution, he considered with himself, 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 the like power. And by comparing 

 the periods of the several planets with their distances from the sun, he 

 found, that if any power like gravity held them in their courses, its strength 

 must decrease in the duplicate proportion of the increase of distance. This 

 he concluded by supposing them to move in perfect circles concentrical to 

 the sun, from which the orbits of the greatest part of them do not much 

 differ. Supposing, therefore, the power of gravity, when extended to the 

 moon, to decrease in the same manner, he computed whether that force 

 would be sufficient to keep the moon in her orbit. In this computation 

 being absent from books, he took the common estimate in use among geo- 

 * View of Newton's Philosophy, 1728, Preface. 



