156 DISCOURSE ON THE STUDY 



(158.) 9th, Complicated phenomena, in which 

 several causes concurring, opposing, or quite in- 

 dependent of each other, operate at once, so as 

 to produce a compound effect, may be simplified by 

 subducting the effect of all the known causes, as 

 well as the nature of the case permits, either by 

 deductive reasoning or by appeal to experience, and 

 thus leaving, as it were, a residual phenomenon to be 

 explained. It is by this process, in fact, that 

 science, in its present advanced state, is chiefly pro- 

 moted. Most of the phenomena which nature pre- 

 sents are very complicated ; and when the effects of 

 all known causes are estimated with exactness, and 

 subducted, the residual facts are constantly appear- 

 ing in the form of phenomena altogether new, and 

 leading to the most important conclusions. 



(159.) For example: the return of the comet pre- 

 dicted by professor Encke, a great many times in 

 succession, and the general good agreement of its 

 calculated with its observed place during any one 

 of its periods of visibility, would lead us to say that its 

 gravitation towards the sun and planets is the sole 

 and sufficient cause of all the phenomena of its 

 orbitual motion ; but when the effect of this cause 

 is strictly calculated and subducted from the ob- 

 served motion, there is found to remain behind a 

 residual phenomenon, which would never have been 

 otherwise ascertained to exist, which is a small 

 anticipation of the time of its reappearances or a 

 diminution of its periodic time, which cannot be 

 accounted for by gravity, and whose cause is there- 

 fore to be enquired into. Such an anticipation 

 would be caused by the resistance of a medium dis- 



