EXAMPLES OF THE FOUR METHODS. 297 



course on the Study of Natural Philosophy, a work replete with happily- 

 selected exemplifications of inductive jsrocesses from almost every depart- 

 ment of physical science, and in which alone, of all books which I have met 

 with, the four methods of induction are distinctly recognized, 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 com- 

 pass ;" the theory of dew, first promulgated by the late Dr. "Wells, and now 

 universally adopted by scientific authorities. The passages in inverted 

 commas are extracted verbatim from the Discourse.* 



" Suppose deio were the phenomenon proposed, whose cause we would 

 know. In the first place " we must determine precisely what we mean by 

 dew : what the fact really is whose cause we desire to investigate. " We 

 must separate dew from rain, and the moisture of fogs, and limit the ap- 

 plication of the term to what is really meant, which is the spontaneous ap- 

 pearance of moisture on substances exposed in the open air when no rain 

 or visible wet is falling." This answers to a preliminary operation which 

 will be characterized in the ensuing book, treating of operations subsidiary 

 to induction.f 



"Now, here we have analogous phenomena in the moisture which be- 

 dews a cold metal or stone when we breathe upon it ; that which appears 

 on a glass of water fresh from the well in hot weather ; that which appears 

 on the inside of windows when sudden rain or hail chills the external air ; 

 that which runs down our walls when, after a long frost, a warm, moist 

 thaw comes on." Comparing these cases, we find that they all contain the 

 phenomenon which was proposed as the subject of investigation. Now 

 " all these instances agree in one point, the coldness of the object dewed, 

 in comparison with the air in contact with it." But there still remains the 

 most important case of all, that of nocturnal dew: does the same circum- 

 stance exist in this case? "Is it a fact that the object dewed is colder 

 than the air? Certainly not, one would at first be iiicliiiod to say; for 

 what is to make it so ? But .... the experiment is easy : we have only to 

 lay a thermometer in contact with the dewed substance, and hang one at a 

 little distance above it, out of reach of its influence. The experiment has 

 been therefore made, the question has been asked, and the answer has been 

 invariably in the affirmative. Whenever an object contracts dew, it is 

 colder than the air." 



Here, then, is a complete application of the Method of Agreement, estab- 

 lishing the fact of an invariable connection between the deposition of dew 

 on a surface, and the coldness of that surface compared with the external 

 air. But which of these is cause, and which effect ? or are they both ef- 

 fects of something else? On this subject the Method of Agreement can 

 afford us no light : we must call in a more potent method. " We must 

 collect more facts, or, which comes to the same thing, vary the circum- 

 stances; since every instance in which the circumstances differ is a fresh 

 fact : and especially, we must note the contrary or negative cases, i. e., 

 where no dew is produced :" a comparison between instances of dew and 

 instances of no dew, being the condition necessary to bring the Method of 

 Difference into play. 



" Now, first, no dew is produced on the surface of polished metals, but 



* Pp. 110, 111. t Infra, book iv., chap, ii., On Abstraction. 



