EXAMPLES OF THE FOUR METHODS. 247 



Here, too, therefore, the causation is directly proved. We can, it is 

 true, accomplish this only on a small scale ; hut we have ample I'eason 

 to conclude that the same operation, if conducted iu JSTature's great 

 laboratory, would equally produce the elVect. 



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 shown them to be) in 

 which Nature works the experiment for us in the same manner in 

 which we ourselves perform it; introducing ji^to the. previous state of 

 things a single and pcrfec;tly definite new circumstance, and manifest- 

 ing the ett'ect so rapidly that there is not time for any other material 

 change in the preexisthig circumstances. Let us quote again Sir John 

 Horschel : — " 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 \f the clouds ivithdraiv even for afow viiinitea, and have a 

 clear opening, a deposition of dew presently begins, and goes on increas- 

 ing 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 communi- 

 cation with the sky causes the deposition 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 iluid, tliat 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 con- 

 sequent 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 fullness of assurance which the 

 inductive evidence of laws of causation may attain, in cases in which 

 the invariable sequence is by no means obvious to a superficial view. 

 It is unnecessary to subjoin Sir John Herschel's summary of the result, 

 as it does not contain all the ])roofs which I have given, and our more 

 detailed analysis of each step of the process renders such a recapitula- 

 tion unnecessary. 



§ 5. This admirable example will have conveyed to any one by 

 whom it has been duly followed, so clear a conception of the use and 

 practical management of three of the four methods of experimental 



* I must, however, remark, that this example, which seems to militate against the asser- 

 tion we made of the comparative inapplicabihty of the Method of Difference to cases of 

 pure observation, is really one of those exceptions which, according to a proverbial expres- 

 sion, 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 jihenomcnon, lie does so by employing com- 

 plex means, which he is unable (lerfectly to analyze, and can form, therefore, no sutlicient 

 judgment what portion of the ctlects 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 experi- 

 ment 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 inde- 

 pendently of any thermometric effect at the earth's surface. Fhen, therefore, in a case so 

 fkvorable as this to Nature's experimental talents, her experiment is of little value except 

 ia corroboration of a conclusion already attained through other means. 



