500 INDUCTION. 



it is a known property of clouds, as of all other bodies 

 between which and any given object nothing inter- 

 venes 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 disappear- 

 ance of clouds will cause the surface to cool ; so that 

 Nature, in this case, produces a change in the ante- 

 cedent by definite and known means, and the conse- 

 quent follows accordingly: a natural experiment which 

 satisfies the requisitions of the Method of Difference*. 

 The accumulated proof of which the Theory of 

 Dew has been found susceptible, is a striking example 

 of the fulness of assurance which the inductive 

 evidence of laws of causation may attain, in cases in 

 which the invariable sequence is by no means obvious 



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

 militate against the assertion we made of the comparative inappli- 

 cability 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, be it observed, 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 likeness of man's most imperfect experiments, 

 namely those in which, though he succeeds in producing the pheno- 

 menon, he does so by employing complex means, which he is unable 

 perfectly to analyze, 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 

 upon what it depends, to be certain a priori that it might not 

 operate upon the deposition of dew independently of any thermo- 

 metric effect at the earth's surface. Even, therefore, in a case so 

 favourable as this to Nature's experimental talents, her experiment 

 is of little value except in corroboration of a conclusion already 

 attained through other means. 



