INTRODUCTION. 3 



making discoveries, and that which is requisite for understanding 

 what others have done. 



Our knowledge of external objects is founded upon experience, 

 which furnishes facts ; the comparison of these facts establishes 

 relations, from which the belief that like causes will produce like 

 effects leads to general laws. Thus, experience teaches that 

 bodies fall at the surface of the earth with an accelerated velocity, 

 and with a force proportional to their masses. By comparison, 

 Newton proved that the force which occasions the fall of bodies 

 at the earth's surface is identical with that which retains the 

 moon in her orbit ; and he concluded, that, as the moon is kept 

 in her orbit by the attraction of the earth, so the planets might 

 be retained in their orbits by the attraction of the sun. By such 

 steps he was led to the discovery of one of those powers with 

 which the Creator has ordained that matter should reciprocally 

 act upon matter. 



Physical astronomy is the science which compares and identifies 

 the laws of motion observed on earth with the motions that take 

 place in the heavens: and which traces, by an uninterrupted 

 chain of deduction from the great principle that governs the 

 universe, the revolutions and rotations of the planets, and the 

 oscillations (N. 4) of the fluids at their surfaces ; and which 

 estimates the changes the system has hitherto undergone, or may 

 hereafter experience changes which require millions of years for 

 their accomplishment. 



The accumulated efforts of astronomers, from the earliest dawn 

 of civilization, have been necessary to establish the mechanical 

 theory of astronomy. The courses of the planets have been 

 observed for ages, with a degree of perseverance that is astonish- 

 ing, if we consider the imperfection and even the want of instru- 

 ments. The real motions of the earth have been separated from 

 the apparent motions of the planets j 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. 



B 2 



