146 



of the air is 8(i° ; corresponding to the mean 

 temperature of 27*7° cent. Taking into ac- 

 count the influence of the rainy months, that is 

 to say, estimating the difference observed in 

 other parts of South America between the mean 

 humidity of the dry months and that of the 

 whole year ; an annual mean humidity is ob- 

 tained, for the valleys of Aragua, at farthest of 

 74°, the temperature being 25*5°. In this air, so 

 hot, and at the same time so little humid, the 

 quantity of water evaporated is enormous. The 

 theory of Dalton estimates, under the conditions 

 just stated, for the thickness of the sheet of 

 water evaporated in an hour's time 0*36 mill, or 

 8*8 lines in twenty-four hours # . Assuming for 

 the temperate zone, for instance at Paris, the 

 mean temperature to be 10*6°, and the mean 

 humidity 82°, we find, according to the same 

 formulae, 0*10 mill, an hour, and 1 line for 

 twenty-four hours. If we prefer substituting 

 for the uncertainty of these theoretical deduc- 

 tions the direct results of observation, we may 

 recollect, that at Paris, and at Montmorenci, 

 the mean annual evaporation was found by 

 Sedileau and Cotte, to be from 82 in. 1 line to 

 38 in. 4 lines. Two able engineers in the South 

 of France, Messrs. Clausade and Pin, found, 

 that in subtracting the effects of filtrations, the 



* Compare, at the end of the first book, vol. ii, p. 90. 



