68 CLASSICS OF MODERN SCIENCE 



Although he later made some chemical investigations, his papers 

 were accidentally destroyed, and it is said that he never recovered 

 from the shock of losing them. In i6p^ he was made warden, and in 

 i6pp promoted to the mastership of the mint, which office he re- 

 tained at a munificent salary until his death on March 20, i"/-?/. 



THE THEORY OF GRAVITATION * 

 BOOK III. PROPOSITION V. THEOREM V. SCHOLIUM 



The force which retains the celestial bodies in their orbits has 

 been hitherto called centripetal force ; but it being now made plain 

 that it can be no other than a gravitating force, we shall hereafter call 

 it gravity. For the cause of that centripetal force which retains the 

 moon in its orbit will extend itself to all the planets. 



BOOK III. PROPOSITION VI. THEOREM VI. 



That all bodies gravitate tozvards every planet; and that the weights 

 of bodies towards any the same planet, at equal distances from the cen- 

 tre of the planet, are proportional to the quantities of matter which 

 they severally contain. 



It has been, now of a long time, observed by others, that all sorts 

 of heavy bodies (allowance being made for the inequality of re- 

 tardation which they suffer from a small power of resistance in the 

 air) descend to the earth from equal heights in equal times; and that 

 equality of times we may distinguish to a great accuracy, by the help 

 of pendulums. I tried the things 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 equal threads of 

 II feet made a couple of pendulums perfectly equal in weight and 

 figure, and equally receiving the resistance of the air. And, placing 

 the one by the other, I observed them to play together forwards and 

 backwards, for a long time, with equal vibrations. . . . and the like 

 happened in the other bodies. By these experiments, in bodies of 

 the same weight, I could manifestly have discovered a difference of 



* Translated from the Philosophice Naturalis Principia Malhcmatica. 



