THE LAW OF EVOLUTION CONTINUED. 387 



the same general truth was exemplified in the progress 

 from the " method of exhaustion " and the " method of 

 indivisibles" to the "method of limits;" which is the 

 central idea of the infinitesimal calculus. In early 



mechanics, too, may be traced a dim perception that action 

 and re-action are equal and opposite ; though, for ages after, 

 this truth remained unformulated. And similarly, the prop- 

 erty of inertia, though not distinctly comprehended until 

 Kepler lived, was vaguely recognized long previously. 

 " The conception of statical force," " was never presented 

 in a distinct form till the works of Archimedes appeared; " 

 and " the conception of accelerating force was confused, in 

 the mind of Kepler and his contemporaries, and did not 

 become clear enough for purposes of sound scientific reason- 

 ing before the succeeding century." To which specific as- 

 sertions may be added the general remark, that " terms 

 which originally, and before the laws of motion were fully 

 known, were used in a very vague and fluctuating sense, were 

 afterwards limited and rendered precise." When 



we turn from abstract scientific conceptions to the con- 

 crete provisions of science, of which astronomy furnishes 

 numerous examples, a like contrast is visible. The times 

 at which celestial phenomena will occur, have been pre- 

 dicted with ever-increasing accuracy. Errors once amount- 

 ing to days are now diminished to seconds. The corre- 

 spondence between the real and supposed forms of orbits, 

 has been gradually rendered more precise. Originally 

 thought circular, then epicyclical, then elliptical, orbits are 

 now ascertained to be curves which always deviate from 

 perfect ellipses, and are ever undergoing changes. 



But the general advance of Science in definiteness, is 

 best shown by the contrast between its qualitative stage, 

 and its quantitative stage. At first the facts ascertained 

 were, that between such and such phenomena some connex- 

 ion existed— that the appearances a and b always occurred 

 together or in succession ; but it was known neither what was 



