EXAMPLES OF THE FOUR METHODS. 457 



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 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 happily-selected exemplifications of induc- 

 tive 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 characterized 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 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 authorities. The pas- 

 sages in inverted commas are extracted verbatim from the 

 Discourse.f 



" Suppose dew were the phenomenon proposed, whose cause 

 we would know. In the first place" we must determine pre- 

 cisely what we mean by dew : what the fact really is, whose 



* This view of the necessary coexistence 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 existence 

 of the negative charge on the rubber was equally dependent on a contrary state 

 of the surfaces 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 machine. 



f Pp. 159-162.. 



