

MATE OF EVA.POHATIOir, ]\ 



of South America between the mean humidity of tin dry 

 months and that of the whole year ; an annual mean hami- 

 dity is obtained, for the valleys of Aragua, at farthest rjf 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 3'8 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 

 formula, 0*10 mill, an hour, and 1 line for twenty-four 

 hours. If we prefer substituting for the uncertainty of 

 these theoretical deductions the direct results of observa- 

 tion, we may recollect that in Paris, and at Montmorency, 

 the mean annual evaporation was found by Sedileau and 

 Cotte, to be from 32 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 

 waters of the canal of Languedoc, and the basin of Saint 

 Ferreol lose every year from 0'758 met. to 0*812 met., OP 

 from 336 to 360 lines. M. de Prony found nearly similar 

 results in the Pontine marshes. The whole of these experi- 

 ments, made in the latitudes of 41 and 49, and at 10'5 

 and 16 of mean temperature, indicate a mean evaporation 

 of one line, or one and three-tenths a day. In the 

 torrid zone, in the West India Islands for instance, the 

 effect of evaporation is three times as much, according to 

 Le Gaux, and double according to Cassan. At Cumana, in 

 a place where the atmosphere is far more loaded with humi- 

 dity than in the valley of Aragua, I have often seen evapo- 

 rate during twelve hours, in the sun, 8*8 mill., in the shade 

 3*4 mill.; and 1 believe, that the annual produce of evapo- 

 ration in the rivers near Cumana is not less than one 

 hundred and thirty inches. Experiments of this kind are 

 extremely delicate, but what I nave stated will suffice to 

 demonstrate how great must be the quantity of vapour that 

 rises from the lake of Valencia, and from the surrounding 

 country, the waters of which flow into the lake. I shall 

 have occasion elsewhere to resume this subject; for, in a 

 work which displays the great laws of nature in different 



