464 INDUCTION. 



shown them to be, in which nature works the experiment for 

 us in the same manner in which we ourselves perform it ; in- 

 troducing into the previous state of things a single and per- 

 fectly 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 presently begins, and goes on in- 

 creasing 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 uninterrupted communication with the sky causes the de- 

 position or non-deposition 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 consequent follows accordingly : a 

 natural experiment which satisfies the requisitions of the 

 Method of Difference.* 



* I must, however, remark, that this example, which seems to militate 

 against the assertion we made of the comparative inapplicability of the Method 

 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, she has only succeeded in producing the like- 

 ness of man's most imperfect experiments ; namely, those in which, though he 

 succeeds in producing the phenomenon, he does so by employing complex 

 means, which he is unable perfectly to analyse, and can form therefore no 

 sufficient judgment what portion of the effects may be due, not to the supposed 

 cause, but to some unknown agency of the means by which that cause was 

 produced. In the natural experiment which we are speaking of, the means 

 used was the clearing off a canopy of clouds ; and we certainly do not know 

 sufficiently in what this process consists, or on what it depends, to be certain 

 a priori that it might not operate upon the deposition of dew independently of 



