INTRODUCTION. 



WE enter now upon a new region of the human 

 mind. In passing from Astronomy to Me- 

 chanics we make a transition from the formal to 

 the physical sciences ; from time and space to 

 force and matter ; from phenomena to causes. 

 Hitherto we have been concerned only with the 

 paths and orbits, the periods and cycles, the angles 

 and distances, of the objects to which our sciences 

 applied, namely, the heavenly bodies. How these 

 motions are produced; by what agencies, impulses, 

 powers, they are determined to be what they are ; 

 of what nature are the objects themselves; 

 are speculations which we have hitherto not dwelt 

 upon. The history of such speculations now comes 

 before us ; but, in the first place, we must consider 

 the history of speculations concerning motion in 

 general, terrestrial as well as celestial. We must 

 first attend to Mechanics, and afterwards return to 

 Physical Astronomy. 



In the same way in which the developement of 

 Pure Mathematics, which began with the Greeks, 

 was a necessary condition of the progress of Formal 

 Astronomy, the creation of the science of Mechanics 

 now became necessary to the formation and pro- 

 gress of Physical Astronomy. Geometry and Me- 



