OF NATURAL PHILOSOPHY. 169 



be, that the gravity of every material body is in the 

 direct proportion of its mass, which is only another 

 mode of expressing Galileo's law above mentioned . 

 The time of falling from any moderate height cannot 

 be measured with precision enough for our purpose : 

 but if it can be repeated a very great multitude of 

 times without any loss or gain in the intervals, and the 

 whole amount of the times of fall so repeated mea- 

 sured by a clock; and if at the same time the resist- 

 ance of the air can be rendered exactly alike for all the 

 bodies tried, we have here Galileo's trial in a much 

 more refined state ; and it is evident that almost un- 

 limited exactness may be obtained. Now, all this 

 Newton accomplished by the simple and elegant 

 contrivance of enclosing in a hollow pendulum the 

 same weights of a great number of substances the 

 most different that could be found in all respects, as 

 gold, glass, wood, water, wheat, &c.*, and ascertain- 

 ing the time required for the pendulum so charged 

 to make a great number of oscillations ; in each of 

 which it is clear the weights had to fall, and be 

 raised again successively, without loss of time, 

 through the same identical spaces. Thus any differ- 

 ence, however inconsiderable, that might exist in the 

 time of one such fall and rise would be multiplied and 

 accumulated till they became sensible. And none 

 having been discovered by so delicate a process in 

 any case, the law was considered verified both in re- 

 spect of generality and exactness. This, however, is 

 nothing to the verifications afforded by astronomical 

 phenomena, where the deviations, if any, accumu- 

 late for thousands of years instead of a few hours. 



* Principia, book iii. prop. 6 



