10 
RATE OF EVAPORATION. 
situated in opposite hemispheres, as, for example, Lombards 
bordered by the Alps, and Lower Peru inclosed between the 
Pacific and the Cordillera of the Andes, afford striking proofs 
of the justness of this assertion. 
Till the middle of the last century, the mountains round 
the valleys of Aragua were covered with forests. Great 
trees of the families of mimosa, eeiba, and the fig-tree, 
shaded and spread coolness along the banks of the lake. 
The plain, then thinly inhabited, was filled with brushwood, 
interspersed with trunks of scattered trees and parasite 
plants, enveloped with a thick sward, less capable of emitting 
radiant caloric than the soil that is cultivated and conse- 
quently not sheltered from the rays of the sun. With the 
destruction of the trees, and the increase of the cultivation 
of sugar, indigo, and cotton, the springs, and all the natural 
supplies of the lake of Yalencia, have diminished from year 
to year. It is difficult to form a just idea of the enormous 
quantity of evaporation which takes place under the torrid 
zone, in a valley surrounded with steep declivities, where 
a regular breeze and descending currents of air are felt 
towards evening, and the bottom of which is flat, and looks 
as if levelled by the waters. It has been remarked, that 
the heat which prevails throughout the year at Cura, 
Guacara, Nueva Yalencia, and on the borders of the lake, 
is the same as that felt at midsummer in Naples and 
Sicily. The mean annual temperature of the valleys of 
Aragua is nearly 255°; my hygrometrieal observations of 
the month of February, taking the mean of day and night, 
gave 71'4° of the hair hygrometer. As the words great 
drought and great humidity have no determinate significa- 
tion, and air that would be called very dry in the lower 
regions of the tropics would be regarded as humid in 
Europe, we can judge of these relations between climates 
only by comparing spots situated in the same zone. Now 
at Cumana, where it sometimes does not rain during a 
whole year, and where I had the means of collecting a 
great number of hygrometric observations made at different 
hours of the day and night, the mean humidity of the air 
is 86°; corresponding to the mean temperature of 27'7°. 
Taking into account the influence of the rainy months, that 
is to say, estimating the difference observed in other parts 
