10 *ATE OF EVAPORATION. 



situated in opposite hemispheres, as, for example, Lombard/ 

 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, ceiba, 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- 1 ., and the increase of the cultivation 

 of sugar, indigo, and cotton, the springs, and all the natural 

 supplies of the lake of Valencia, 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 Valencia, 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 25 - 5; my hygrometrical 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 part* 



