EXAMPLES OF THE FOUR METHODS. 499 



had to the plan of deducing the effect from the ascer- 

 tained properties of the agents known to be present. 



The second corroboration of the theory is by direct 

 experiment, according 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 hygrometric condition,) at which dew will begin 

 to be deposited. 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 produce 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 shown them to be,) in which Nature 

 works the experiment for us in the same manner in 

 which we ourselves 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. Let us quote again 

 Sir John Herschel: " 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 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-deposition of dew. Now, since a 

 clear sky is nothing but the absence of clouds, and 



2 K 2 



