600 LECTURE XLVIII. 



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 Ave 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. Tliis he concluded by supposing them to move 

 in perfect circles concentrical to the sun, from which the orbits of the great- 



• est 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 geographers and our seamen, before Norwood had 

 measured the earth, that 60 English miles were contained in one degree of 

 latitude on the surface of the earth. But as this is a very faulty supposition, 

 each degree containing about 69^ of our miles, his computation did not 

 answer expectation; whence he concluded tliat some other cause must at least 

 join with the action of the power of gravity on the moon. On this account 

 he laid aside for that time any further thoughts upon this matter. But some 

 years after, a letter, which he received from Dr. Hooke, put him on inquiring 

 what was the real figure, in which a body let fall from any high place 

 descends, taking the motion of the earth I'ound its axis into consideration. 

 Such a body, having the same motion, which by the revolution of the earth 

 the place has from whence it falls, is to be considered as projected forwards, 

 and at the same time drawn down to the centre of the earth. This gave 

 occasion to his resuming his former thoughts concerning the moon; and 

 Picart, iu France, having lately measured the earth, by using his measures, the 

 moon appeared to be kept in her orbit purely by the power of gravity ; and 

 consequently, that this power decreases as you recede from the centre of the 

 tarth, in the manner our author had formerly conjectured. Upon this prin- 

 ciple he found the line described by a falling body to be an ellipsis, the centre 



