BATE OF EVAPORATION. 
li 
of South America between the mean humidity of tin dry 
months and that of the whole year ; an annual mean humi- 
dity is obtained, for the valleys of Aragua, at farthest of 7 4?, 
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, 0'3G mill., or 3'S lines 
in twenty-four hours. Assuming for the temperate zone, 
for instance at Paris, the mean temperature to he 10'6°, and 
the mean humidity 82°, we find, according to the same 
formulas, 0T0 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 Prance, Messrs. Clausade and 
Pin, found, that in subtracting the effects of filtrations, the 
waters of the canal of Languedoc, and the basin of Saint 
1'erreol lose every year from 0758 met. to 0'812 met., or 
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 TV est India Islands for instance, the 
ellect of evaporation is three times as much, according to 
Le Gaux, and double according to Cassau. 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, S'S 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 have stated will suffice to 
demonstrate how great must he 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 
