166 DISCOURSE ON THE STUDY 



tied by all the circumstances of each case ; we must 

 estimate its effects, and show that nothing unex- 

 plained remains behind ; at least, in so far as the 

 presence of unknown modifying causes is not con- 

 cerned. 



(174.) Now, this is precisely the sort of process in 

 which residual phenomena (such as spoken of in art. 

 158.) may be expected to occur. If our induction 

 be really a valid and a comprehensive one, whatever 

 remains unexplained in the comparison of its con- 

 clusion with particular cases, under all their circum- 

 stances, is such a phenomenon, and comes in its 

 turn to be a subject of inductive reasoning to dis- 

 cover its cause or laws. It is thus that we may be 

 said to witness facts with the eyes of reason ; and it 

 is thus that we are continually attaining a know- 

 ledge of new phenomena and new laws which lie 

 beneath the surface of things, and give rise to the 

 creation of fresh branches of science more and 

 more remote from common observation. 



(175.) Physical astronomy affords numerous and 

 splendid instances of this. The law, for example, 

 which asserts that the planets are retained in their 

 orbits about the sun, and satellites about their pri- 

 maries, by an attractive force, decreasing as the 

 square of the distances increases, comes to be veri- 

 fied in each particular case by deducing from it 

 the exact motions which, under the circumstances, 

 ought to take place, and comparing them with fact. 

 This comparison, while it verifies in general the 

 existence of the law of gravitation as supposed, and 

 its adequacy to explain all the principal motions 

 of every body in the system, yet leaves some 



