502 INDUCTION. 



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 reap- 

 pearance, 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 anticipation 

 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," (an actually existing antecedent,) " it has 

 therefore been ascribed to such a resistance. 



" 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 cases there were two vercs 

 causa" (antecedents known to exist) <( why it should 

 come at length to rest, viz., the resistance 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 phenomenon appeared, in the 

 fact that a retarding influence was exerted by the 

 copper itself; and this fact, once ascertained, speedily 

 led to the knowledge of an entirely new and unex- 

 pected class of relations." This example belongs, 

 however, not to the Method of Residues but to the 

 Method of Difference, the law being ascertained by a 

 direct comparison of the results of two experiments, 

 which differed in nothing but the presence or absence 

 of the plate of copper. To have made it exemplify 

 the Method of Residues, the effect of the resistance 



