dSo 



INDUCTION. 



management of three of the four 

 meth^s of experimental inquiry, as 

 to supersede the necessity of any 

 further exemplification of them. The 

 remaining method, that of Residues, 

 not having found a place in any of 

 the preceding investigations, I shall 

 quote 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 phe- 

 nomena which Nature presents are 

 very complicated ; and when the 

 effects of all known causes are esti- 

 mated with exactness, and subducted, 

 the residual facts are constantly ap- 

 pearing 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 agreement of its 

 calculated with its observed place dur- 

 ing any one of its periods of visibility, 

 would lead us to say that its gravita- 

 tion 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 observed motion, there is found to 

 remain behind a residual phenomenon, 

 which would never have been other- 

 wise ascertained to exist, which is a 

 small anticipation of the time of its 

 reappearance, or a diminution of its 

 periodic time, which cannot be ac- 

 counted for by gravity, and whose 

 cause is therefore to be inquired into. 

 Such an anticipation would be caused 

 by the resistance of a medium dissemi- 

 nated 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.* 



• In his subsequent work, Outlinet of 

 Attronomy (J 570), Sir John Herschel sug- 

 geets another possible explanation of the 

 acceleration of the revolution of a comet. 



"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 verce causae" (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 knovn by 

 the observation made in the absence 

 of the copper, and being thus albwed 

 for and subducted, a residual pheno- 

 menon appeared, in the fact that a 

 retarding influence was exerted by 

 the copper itself ; and this fact, once 

 ascertained, speedily led to the know- 

 ledge of an entirely new and unex- 

 pected class of relations." This ex- 

 ample belongs, however, not to the 

 Method of Residues but to the Method 

 of Difference, the law being ascer- 

 tained 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 

 of the air and that of the rigidity of 

 the silk should have been calculated 

 d priori from the laws obtained by 

 separate and foregone experiments. 



" Unexpected and peculiarly strik- 

 ing confirmations of inductive laws 

 frequently occur in the form of re- 

 sidual 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 of sound. The 

 inquiry into the cause of sound had 

 led to conclusions respecting its mode 

 of propagation, from which its velocity 

 in the air could be precisely calculated. 

 The calcvdations were performed, but, 



