EXAMPLES OF THE FOUR METHODS. 



275 



with a portion of its water, which ac- 

 cordingly will, by the ordinary laws 

 of gravitation or cohesion, attach it- 

 self to the surface of the body, thereby 

 constituting dew. This deductive 

 proof, it will have been seen, has the 

 advantage of at once proving causa- 

 tion as well as co-existence ; and it has 

 the additional advantage that it also 

 accounts for the exceptions to the oc- 

 currence of the phenomenon, the cases 

 in which, although the body is colder 

 than the air, yet no dew is deposited ; 

 by showing that this will necessarily 

 be the case when the air is so under- 

 supplied with aqueous vapour, com- 

 paratively to its temperature, that 

 even when somewhat cooled by the 

 contact of the colder body it can still 

 continue to hold in suspension all the 

 vapour which was previously sus- 

 pended in it : thus in a very dry 

 summer there are no dews, in a very 

 dry winter no hoar-frost. Here, there- 

 fore, is an additional condition of the 

 production of dew, which the methods 

 we previously made use of failed to 

 detect, and which might have re- 

 mained still undetected if recourse 

 had not been had to the plan of de- 

 ducing the effect from the ascertained 

 properties of the agents known to be 

 present. 



The second corroboration of the 

 theory is by direct experiment, ac- 

 cording to the canon of the Method 

 of Difference. We can, by cooling 

 the surface of any body, find in all 

 cases some temperature (more or less 

 inferior to that of the surrounding air, 

 according to its hygrometic condition) 

 at which dew will begin to be de- 

 posited. Here, too, therefore, the 

 causation is directly proved. We 

 can, it is true, accomplish this only 

 on a small scale ; but we have ample 

 reason to conclude that the same 

 operation, if conducted in Nature's 

 great laboratory, would equally pro- 

 duce the effect. 



And, finally, even on that great 

 scale we are able to verify the result. 

 The case is one of those rare cases, as 

 we have sho\vn them to be, in which 



Nature works the experiment for us 

 in the same manner in which we our- 

 selves perform it, introducing into the 

 previous state of things a single and 

 perfectly definite new circumstance, 

 and manifesting the effect so rapidly 

 that there is not time for any other 

 material change in the pre-existing 

 circumstances. " It is observed that 

 dew is never copiously deposited in 

 situations much screened from the 

 open sky, and not at all in a cloudy 

 night ; but if the clouds withdraw 

 even for a few minutes, and leave a 

 clear opening, a deposition of dew pre- 

 sently begins, and goes on increasing. 

 . . . Dew formed in clear intervals 

 will often even evaporate again when 

 the sky becomes thickly overcast." 

 The proof, therefore, is complete that 

 the presence or absence of an unin- 

 terrupted communication with the 

 sky causes the deposition or non-depo- 

 sition of dew. Now, since a clear 

 sky is nothing but the absence of 

 clouds, and it is a known property of 

 clouds, as of all other bodies between 

 which and any given object nothing 

 intervenes but an elastic fluid, that 

 they tend to raise or keep up the 

 superficial temperature of the object 

 by radiating heat to it, we see at 

 once that the disappearance of clouds 

 will cause the surface to cool ; so that 

 Nature, in this case, produces a 

 change in the antecedent by definite 

 and known means, and the conse- 

 quent follows accordingly : a natural 

 experiment which satisfies the re- 

 quisitions of the Method of Differ- 

 ence.* 



* I must, however, remark, that this 

 example, which seems to militate against 

 the assertion we made of the comparative 

 inapplicability of the Metht.d of Difference 

 to cases of pure observation, is really one 

 of those exceptions which, according to a 

 proverbial expression, prove the general 

 rule. For in this case, in which Nature, 

 in her experiment, seems to have imitated 

 the type of the experiments made by man, 

 slie has only succeer.ed in producing the 

 hkeness of man's most imperfect experi- 

 ments, namely, those in which, though ho 

 succeeds in producing the phenomenon, 

 he does so by employing complex means, 

 which he ia unable perfectly to analyse, 



