INTROD.] INTRODUCTION. 3 



means the most extraordinary conclusions have been arrived 

 at, is within the reach of many who shrink from the task, 

 appalled by difficulties, not more formidable than those inci- 

 dent to the study of the elements of every branch of know- 

 ledge. There is a wide distinction between the degree of 

 mathematical acquirement necessary for making discoveries, 

 and that which is requisite for understanding what others 

 have done. 



Our knowledge of external objects is founded upon expe- 

 rience, 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, expe- 

 rience 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 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 w T hich 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 mo- 

 tions 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 sur- 

 faces ; and which estimates the changes the system has hither- 

 to 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 me- 

 chanical theory of astronomy. The courses of the planets 

 have been observed for ages, with a degree of perseverance 

 that is astonishing, if we consider the imperfection and even 



B 2 



