INTRODUCTION. 



E enter now upon a new region of the human mind. In passing 

 from Astronomy to Mechanics 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 agen- 

 cies, 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 ce- 

 lestial. We must first attend to Mechanics, and afterwards return to 

 Physical Astronomy. 



In the same way in which the development of Pure Mathematics, 

 which began with the Greeks, was a necessary condition of the pro- 

 gress of Formal Astronomy, the creation of the science of Mechanics 

 now became necessary to the formation and progress of Physical As- 

 tronomy. Geometry and Mechanics were studied for their own sakes ; 

 but they also supplied ideas, language, and reasoning to other sciences. 

 If the Greeks had not cultivated Conic Sections, Kepler could- not have 

 superseded Ptolemy ; if the Greeks had cultivated Dynamics, 1 Kepler 

 might have anticipated Newton. 



1 Dynamics is the science which treats of the Motions of Bodies ; Statics is the 

 science which treats of the Pressure of Bodies which are in equilibrium, and there- 

 fore at rest. 



