EXAJIPLES OF TUK FOUR AIimiODS. 243 



■ " Suppose dew were tlie phenomenon proposed, w-liosc cause we 

 woulJ know. In the first phice" we must determine precisely what 

 we nicun 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 application of the term to what is really meant, 

 which is, the spontaneous appearance of moisture on substances 

 exposed in the open air when no rain or risible wet is falling." This 

 answers to a preliminary operation which will be characterized in the 

 ensuing book, treating of operations subsidiary to induction.* The 

 state of the question being iixed, we come to the solution. 



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

 bedews a cold metal or stone when wO breathe upon it ; that which 

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

 which appears oil tlie inside of windows when sudden rain or hail 

 chills the ext-ernal 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 1 " Is 

 it a fact that the object dewed is colder than the air 1 Certainly not, 

 one would at first be inclined to say; for what is to make it so 1 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 inva- 

 riably 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, 

 establishing the fact of an, invariable connexion between the deposition 

 of dew on a surface, and die 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 Agree- 

 ment can afford us no light: we must call in a moi'e potent method. 



" That dews are accompanied with a chill is a common remark; but 

 vulgar prejudice would make the cold the effect rather than the cause. 

 We must therefore collect more facts, or which come& to the same thing, 

 vary tlie circumstances ; since every instance in which the circum- 

 stances differ is a fresh fact; and especially, we must note the contrary 

 or negative cases, i. e., where no dew is produced :" for vyO are aware 

 that a comparison between instances of dew, and instances of no dew, 

 is the condition necessary to bring the Method of Difference into play. 



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

 but it is very copiously on glass, both exposed with their faces 

 upwards, and in some cases the under side of a horizontal plate of 

 glass is also dewed."t Here is an instance in which the effect is pro- 



* Vide infra, book iv., chap. ii. On Abstraction, 



+ This last circumstance (adds Sir John Herschel) " cxchides the fall of moisture from 

 the sivy 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 pilrpose in hand, the argument 

 which it contains being deductive and h priori. The fall of moisture is rejected as a cause, 

 because from its laws previously known, we inXer that it could not have produced the par- 

 ticular phenomenon last mentioned 



