472 INDUCTION. 



which Nature presents are very complicated ; and when the 

 effects of all known causes are estimated with exactness, and 

 subducted, the residual facts are constantly appearing in the 

 form of phenomena altogether new, and leading to the most 

 important conclusions. 



&quot; For example : the return of the comet predicted Jby Pro 

 fessor 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 

 inotion ; but when the effect of this cause is strictly calculated 

 and subducted from the observed motion, there is found to 

 remain behind a residual phenomenon, which would never have 

 been otherwise ascertained to exist, which is a small anticipa 

 tion of the time of its reappearance, or a diminution of its 

 periodic time, which cannot be accounted for by gravity, and 

 whose cause is therefore to be inquired into. Such an antici 

 pation would be caused by the resistance of a medium dis 

 seminated through the celestial regions ; and as there are other 

 good reasons for believing this to be a vera causa,&quot; (an actually 

 existing antecedent,) &quot; it has therefore been ascribed to such a 

 resistance.* 



&quot; M. Arago, having suspended a magnetic needle by a silk 

 thread, and set it in vibration, observed, that it came much 

 sooner to a state of rest when suspended over a plate of copper, 

 than when no such plate was beneath it. Now, in both 

 eases there were two verce causce &quot; (antecedents known to 

 exist) &quot;why it should come at length to rest, viz. the resist 

 ance of the air, which opposes, and at length destroys, all 

 motions performed in it ; and the want of perfect mobility in 

 the silk thread. But the effect of these causes being exactly 

 known by the observation made in the absence of the copper, 

 and being thus allowed for and subducted, a residual pheno 

 menon appeared, in the fact that a retarding influence was 



* In his subsequent work, Outlines of Astronomy ( 570), Sir John 

 Herschel suggests another possible explanation of the acceleration of the revolu 

 tion of a comet. 



