458 INDUCTION. 



cause we desire to investigate. " We must separate dew from 

 rain, and the moisture of fogs, and limit the application of the 

 term to what is really meant, which is the spontaneous appear- 

 ance of moisture on substances exposed in the open air when 

 no rain or visible wet is falling." This answers to a prelimi- 

 nary operation which will be characterized in the en suing book, 

 treating of operations subsidiary to induction.* 



" Now, here we have analogous phenomena in the mois- 

 ture which bedews 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 circumstance exist in this case ? " Is it a fact that the 

 object dewed is colder than the air ? Certainly not, one 

 would at first be inclined 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 affir- 

 mative. Whenever an object contracts dew, it is colder than 

 the air." 



Here then is a complete application of the Method of 

 Agreement, establishing the fact of an invariable connexion 

 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 effects of 

 something else ? On this subject the Method of Agreement 

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



* Infra, book iv. ch. ii. On Abstraction. 



