242 INDUCTION. 



we have ascribed the effect. We found in the former instances that 

 whenever electricity of one kind was excited in one body, electricity 

 of the opposite kind must be excited in a neighboring body ; and the 

 interpretation of this, in the language of cause and effect, is, that all 

 causes which can excite the one kind of electricity, have the property 

 of simultaneously exciting an equal amount of the other. But in 

 Faraday's experiment this indispensable opposition exists within the 

 wire itself From the nature of a voltaic charge, the two opposite' 

 currents necessary to the existence of each other are both accommo- 

 dated in one wire ; and there is no need of another wire placed be- 

 side it to contain one of them, in the same way as the Leyden jar 

 must have a positive and a negative surface. The exciting cause can 

 and does produce all the effect which its laws require, independently 

 of any electric excitement of a neighboring body. Now the result 

 of Faraday's experiment with the second wire, was that no opposite 

 current was produced. There was an instantaneous effect at the 

 closing and breaking of the voltaic circuit; electric inductions ap- 

 peared when the two wires were moved to and from one another ; 

 but these are phenomena of a different class. There was no in- 

 duced electi-icity in the sense in which this is predicated of the 

 Leyden jar; there was no sustained current running up the one wire 

 while an opposite cuiTent ran down the neighboring wire; and this 

 alone would have been a true parallel case to the other. 



It thus appears by the combined evidence of the Method of Agree- 

 ment, the Method of Concomitant Variations, and the most rigorous 

 form of the Method of Difference, that neither of the two kinds of 

 electricity can be excited without an equal excitement of the other 

 and opposite kind : that both are effects of the same cause, that the 

 possibility of the one is a condition of the possibility of the other, and' 

 the quantity of the one an impassable limit to the quantity of the other. 

 A scientific result of considerable interest in itself, and illustrating 

 those three methods in a maimer both characteristic and easily in- 

 , telligible. 



§ 4. Our third example shall be extracted from Sir John Herschel's 

 Discourse on the Study of Natural Phil osopJiy, ViVfork Te^lete v/'ith. 

 admirably selected exemplifications of inductive processes fi'om almost 

 every department of physical science, and in which alone, of all books 

 which I have met with, the four methods of induction are recognized, 

 although not characterized and defined nor their correlation shown, so 

 distinctly as has appeared to me desirable. The present example is 

 justly described by Sir John Herschel as " one of the most beautiful 

 specimens" which can be cited ."of inductive experimental inquiry 

 lying within a moderate compass ;" the theory of dew, first promul- 

 gated by the late Dr. Wells, and now universally adopted by scien- 

 tific men. 



The passages in inverted commas are extracted verbatim fi-om Sir 

 John Herschel, * but to those who possess his work I would strongly 

 recommend to read the entire passage in the original, and fully pos- 

 sess themselves of the purport of the speculation as a whole, before 

 applying themselves, with me, to the logical analysis of the different 

 steps of the argument. 



* Discourse, pp. ISO — 162. 



