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 degrees; my hygrometrical observations of the month of
February, taking the mean of day and night, gave 71.4 degrees of the
hair hygrometer. As the words great drought and great humidity have no
determinate signification, 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 degrees; corresponding to the mean temperature of 27.7 degrees.
Taking into account 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 obtained, for the valleys of Aragua,
at farthest of 74 degrees, the temperature being 25.5 degrees. 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