LESSON XIII. 



ON GRAVITY, WEIGHT, <$-c. 



This problem let philosophers resolve, 

 What makes the globes from east to west revolve? 

 What is the strong impulsive cause declare, 

 That rolls tUe pond'rous orbs so swift in air. 



BLACKMORE. 



VY HILST you are reflecting upon tbfe planetary 

 bodies, and considering the regularity of their re- 

 turning periods, these inquiries will very naturally 

 be suggested to your mind. As a research of this 

 kind cannot be unpleasing, the present Lesson will 

 be appropriated to it. Sir Isaac Newton has left 

 us no room to doubt that the bodies in the solar 

 system are 



All combin'd 



And rul'd unerring, by that simple power 

 Which draws the stone projected to the ground. 



THOMSON. 



The sagacious Kepler had determined from ob- 

 servations, that the planetary motions agree with 

 this regular law, viz. The squares of their perio- 

 dical times are as the cubes of their mean distances 

 from the Sun or focus of their orbits; and this law 

 has been since demonstrated by Newton, from the 

 principles of attraction and projectile motion. If 

 we carry back our researches to the creation of the 

 D 4 worldj 



