490 INDUCTION. 



It thus appears by the combined evidence of the Me- 

 thod of Agreement, the Method of Concomitant Vari- 

 ations, 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 con- 

 dition of the possibility of the other, and the quan- 

 tity of the one an impassable limit to the quantity 

 of the other. A scientific result of considerable 

 interest in itself, and illustrating those three me- 

 thods in a manner both characteristic and easily 

 intelligible. 



4. Our third example shall be extracted from 

 Sir John Herschel's Discourse on the Study of Natural 

 Philosophy, a work replete with admirably-selected 

 exemplifications of inductive 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 recognised, although not 

 characterised 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 promulgated by the late Dr. Wells, and now 

 universally adopted by scientific men. 



The passages in inverted commas are extracted 

 verbatim from Sir John Herschel *, but to those who 

 possess his work I would strongly recommend to read 

 the entire passage in the original, and fully possess 



* Discourse, pp. 15916*2. 



