INTRODUCTION. 3 



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 gen- 

 eral 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 com- 

 parison, 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 con- 

 cluded, 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 oscilla- 

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

 estimates the changes the system has hitherto under- 

 gone, 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 theoiy of astronomy. The 

 courses of the planets have been observed for ages, with 

 a degree of perseverance that is astonishing, if we con- 

 sider the imperfection and even 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 mo- 

 tion in the solar system has been so completely explained, 

 that the laws of any astronomical phenomena that may 

 hereafter occur, are already determined. 



