EXAMPLES OF THE FOUR METHODS. 



271 



current nnining along a wire would 

 induce an opposite current upon an- 

 other wire laid parallel to it at a short 

 distance. Now this case is similar 

 to the cases previously examined, in 

 every circumstance except the one to 

 which 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 

 neighbouring bod}'. But in Faraday's 

 experiment this indispensable opposi- 

 tion exists within the wire itself. From 

 the nature of a voltaic charge, the two 

 opposite currents necessary to the ex- 

 istence of each other are both accom- 

 modated in one wire ; and there is no 

 need of another wire placed beside 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 excite- 

 ment of a neighbouring body. Now 

 the result of the 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 appeared when the 

 two wires were moved to and from 

 one another ; but these are pheno- 

 mena of a different class. There was 

 no induced electricity in the sense in 

 which this is predicated of the Ley- 

 den jar ; there was no sustained cur- 

 rent running up the one wire while 

 an opposite current ran down the 

 neighbouring 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 Agreement, 

 the Method of Concomitant Varia- 

 tions, 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 

 pouditiuu uf the possibility of the 



other, and the quantity of the one an 

 impassable limit to the quantity of 

 the other. A scientific result of con- 

 siderable interest in itself and illus- 

 trating those three methods in a 

 manner both characteristic and easily 

 intelligible.* 



§ 3. Our third example shall be 

 extracted from Sir John Herschel's 

 Discourse on the Study of Natural 

 Philosophy, a work replete with hap- 

 pily selected exemplifications of in- 

 ductive processes from almost every 

 department of physical science, and 

 in which alone, of all books which I 

 have met with, the four methods of 

 induction are distinctly recognised, 

 though not so clearly characterised 

 and defined, nor their correlation so 

 fully shown, as has appeared to me 

 desirable. The present example is 

 described by Sir John Herschel as 

 "one of the most beautiful speci- 

 mens " which can be cited " of induc- 

 tive experimental inquiry lying within 

 a moderate compass;" the theory of 

 dew, first promiilgated by the late Dr. 

 Wells, and now universally adopted 

 by scientific authorities. The pas- 

 sages in inverted commas are ex- 

 tracted verbatim from the Discourse.f 



"Suppose dew wei-e the phenomenon 

 proposed whose cause we would know. 

 In the first place " we must determine 



* This view of the necessary co-existence 

 of opposite excitements involves a great 

 extension of the original doctrine of two 

 electricities. The early theorists assumed 

 that, when amber was rubbed, the amber 

 was made positive and the rubber negative 

 to the same degree ; but it never occurred 

 to them to suppose that the existence of 

 the amber charge was dependent on an 

 opposite charge in the bodies with which 

 the amber was contiguous, while the exist- 

 ence of the negative charge on the rubber 

 was equally dependent on a contrary state 

 of the surfjices that might accidentally be 

 confronted with it ; that, in fact, in a case 

 of electrical excitement by friction, four 

 charges were the minimum that could 

 exist. But this double electrical action 

 is essentially implied in the explanation 

 now universally adopted in regard to the 

 phenomena of the common electric mv 

 chine. 



