4 INTRODUCTION. [INTROD. 



the want of instruments. The real motions of the earth have 

 been separated from the apparent motions of the planets ; the 

 laws of the planetary revolutions have been discovered ; and 

 the discovery of these laws has led to the knowledge of the 

 gravitation (N. 5) of matter. On the other hand, descending 

 from the principle of gravitation, every motion in the solar 

 system has been so completely explained, that the laws of any 

 astronomical phenomena that may hereafter occur are already 

 determined. 



