EXAMPLES OF THE FOUR METHODS. 493 



the under side of a horizontal plate of glass is also 

 dewed *." Here is an instance in which the effect is 

 produced, and another instance in which it is not 

 produced ; but we cannot yet pronounce, as the canon 

 of the Method of Difference requires, that the latter 

 instance agrees with the former in all its circum- 

 stances except one ; for the differences between glass 

 and polished metals are manifold, and the only thing 

 we can as yet be sure of is, that the cause of dew will 

 be found among the circumstances by which the 

 former substance is distinguished from the latter. 

 But if we could be sure that glass, and the various 

 other substances on which dew is deposited, have only 

 one quality in common, and that polished metals and 

 the other substances on which dew is not deposited 

 have also nothing in common but the one circum- 

 stance, of not having the one quality which the others 

 have ; the requisitions of the Method of Difference 

 would be completely satisfied, and we should recog- 

 nise, in that quality of the substances, the cause of 

 dew. This, accordingly, is the path of inquiry which 

 is next to be pursued. 



"In the cases of polished metal and polished glass, 

 the contrast shows evidently that the substance has 

 much to do with the phenomenon ; therefore let the 

 substance alone be diversified as much as possible, by 

 exposing polished surfaces of various kinds. This 



* This last circumstance (adds Sir John Herschel) " excludes 

 the fall of moisture from the sky in an invisible form, which would 

 naturally suggest itself as a cause." I have omitted this passage in 

 the text, as not pertinent to the purpose in hand, the argument 

 which it contains being deductive and a priori. The fall of 

 moisture is rejected as a cause, because from its laws previously 

 known, we infer that it could not have produced the particular 

 phenomenon last mentioned. 



