212 On the Cold Caves of the Monte Testaccio at Rome. 



air at 80° to reduce the temperature to 59S as it did in that 

 experiment. This is solved by the formula above quoted, which 



in this case becomes SOf-^ |-- — — ' whence we have 



1.0014 — .6195 = S819, and as 10014 : 6239 : : 3819 : 2379, 

 or .002379 grains in a cubic inch, and §yj = .381, the ratio 

 to saturation or dryness the air would have required to have 

 had, instead of .411, as we found above that it really had, 

 which shows the smaller degree of dryness that was necessary 

 in this case than in the simple one ; and the difference would 

 undoubtedly have been greater had the formulae been appli- 

 cable to both cases. 



What the value of this reduction of temperature would be, 

 owing to the coolness of the humid medium, when the result- 

 ant temperature is only 44°, as in Saussure's case, I do not 

 feel myself able to pronounce ; but my experiments show that 

 it must have an effect probably of several degrees, suppose of 

 four. In that case, the means of reducing the temperature to 

 48° would be all that was required by ordinary evaporation. 



Having attained this point, it is still evident that we can 

 hardly admit quite so great a stretch of evaporating force as 

 this requires ; but nature, I must observe, has extended me- 

 thods of operation, like a great series, of which our experi- 

 mental knowledge reaches only to a few terms. Though we may 

 understand, in the main, the plan of any particular operation, 

 yet all the combining minutiae, the effects alternately augment- 

 ing and augmented, have only their proper scope in her great 

 scale of action. 



'' Mazes intricate. 

 Eccentric, intervolvecl, yet regular 

 Then most, when most irregular they seem." 



One circumstance, the temperature of the moistening water 

 is liable to be cooled to a much greater degree than we have 

 yet considered, and in some respects, I may say, to an almost 

 indefinite degree. Let us review the steps of the process. 

 Air highly dried, and at the temperature of 78°, enters an 

 enormous mass of aggregated materials, — a mass so large, that 

 not only is it retained throughout at the mean temperature of 

 the area of soil on which it is placed, 60°, but, being completely 



