NEWTON AND THE LAW OF GRAVITATION 



portional to the quantities of matter which they severally 

 contain. 



" It has been now a long time observed by others 

 that all sorts of heavy bodies (allowance being made 

 for the inability of retardation which they suffer from 

 a small power of resistance in the air) descend to the 

 earth from equal heights in equal times ; and that equal- 

 ity of times we may distinguish to a great accuracy 

 by help of pendulums. I tried the thing in gold, 

 silver, lead, glass, sand, common salt, wood, water, 

 and wheat. I provided two wooden boxes, round and 

 equal: I filled the one with wood, and suspended an 

 equal weight of gold (as exactly as I could) in the centre 

 of oscillation of the other. The boxes hanging by 

 eleven feet, made a couple of pendulums exactly equal 

 in weight and figure, and equally receiving the resist- 

 ance of the air. And, placing the one by the other, I 

 observed them to play together forward and backward, 

 for a long time, with equal vibrations. And there- 

 fore the quantity of matter in gold was to the quan- 

 tity of matter in the wood as the action of the mo- 

 tive force (or vis motrix) upon all the gold to the 

 action of the same upon all the wood that is, as the 

 weight of the one to the weight of the other: and the 

 like happened in the other bodies. By these experi- 

 ments, in bodies of the same weight, I could manifestly 

 have discovered a difference of matter less than the 

 thousandth part of the whole, had any such been. But, 

 without all doubt, the nature of gravity towards the 

 planets is the same as towards the earth. For, should 

 we imagine our terrestrial bodies removed to the orb of 

 VOL. a.! 7 245 



