CHAP. XIII. 



OF UNIVERSAL GRAVITY, AND CENTRIPETAL AND 

 CENTRIFUGAL FORCE. 



Laws of the Movement of the Planets, according to 

 their Distance from the common 



1. I'f is here the moderns flatter themselves they 

 have a remarkable advantage, imagining, that they 

 were the first who discovered the principle of unu 

 f ersal gravitation, which they look upon as a truth 

 unknown to the ancients. It is however easy to 

 make it appear, that they have done nothing but 

 trod in the paths of those ancients. It is true, the 

 moderns have demonstrated the laws of this universal 

 gravitation, and explained them with clearness and 

 precision : but this is all they have done in this re. 

 spcct, and have added nothing. 



2. With the least attention to the knowledge of 

 the ancients, we find that they were not unacquainted 

 with universal gravitation ; and knew besides, that 

 the circular motion by which the planets describe their 

 course, is the result of the combination of two 

 moving forces, a rectilinear and a peqsciHiicuJ&r. 

 which united together form a curve. They knew 

 the reason why the*e two movement*, or contrary 

 forces retain the planets in their orbs j and have ex- 



