248 INDUCTION. 



inquiry, as to supersede the necessity of any further exemphfication 

 of them. The remaining method, that of Residues, not having found 

 any place either in this or in the two preceding investigations, I shall 

 extract from Sir John Herschel some examples of that method, with 

 the remarks by which they are introduced. 



" It is by this process, in fact, that science, in its present advanced 

 state, is chiefly promoted. Most of the phenomena which Nature 

 presents are very complicated; and when the eflects 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. 



"For example: the return of the comet predicted by Professor 

 Encke, a great many times in succession, and the general good agree- 

 ment of its calculated with its obsei-ved 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 suflficient cause of all the phenom- 

 ena of its orbitual motion : but when the effect of this cause is strictly 

 calculated and subducted from the obsei'ved 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 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 anticipation would be caused by the resistance 

 of a medium disseminated through the celestial regions; and as there 

 are other good reasons for believing this to be a vera causa,"" (an 

 actiially 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 Terce causm''^ 

 (antecedents known to exist) " why it sJiould 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 

 fiilk thread. But the effect of these causes being exactly known by 

 the observation made in the absence of the copper, and behig 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 unexpected class of relations." This example belongs, how- 

 ever, 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 coj^per. To have made it exemplify the Method of Res- 

 idues, the effect of the resistance of the air and that of the rigidii^, of 

 the silk should have been calculated d jjriori, fi-om the laws obtained 

 by separate and fcu'egone experiments. 



" Unexpected and peculiarly striking confirmations of inductive 

 laws frequently occm- in the form of residual phenomena, in the course 

 of investigations of a widely different nature from those which gave rise 

 to the inductions themselves. A very elegant example may be cited 

 in the unexpected confirmation of the law of the development of heat 

 in elastic fluids by compression, which is afforded by the phenomena 



