©N THE BTSTORT OF ASTRONOMY. 599 



of '' general congregation", and attributes the tiJcs to the attraction of the 

 moon. Kepler mentions also the perfect reciprocality of the action of gravita- 

 tion, and considers the lunar irregularities as produced by the attraction of the 

 sun. But our most ingenious countryman, Dr. Hooke, was still more decided 

 in attributing the revolutions of the planets to the combination of a pro- 

 jectile motion with a centripetal force ; he expresses his sentiments on thd 

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

 lished in J 674, 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 Cott- 

 fined his genius to speculations purely mathematical; had he beeii 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 I676, 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 respect- 

 ing the periods of the different planets, some time before 1(571, as he asserts to 

 Dr. Halley, and, to the best of his recollection, about 1668, although in his 

 Principiahe 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 Principia, 

 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 gra- 

 vity: 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 ? 



