Equinoctial Regions of America, vol 2 | Page 6

for example, Lombardy 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 consequently 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 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 hours. Assuming for the temperate zone, for
instance at Paris, the mean temperature to be 10.6 degrees, and the
mean humidity 82 degrees, we find, according to the same formulae,
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 observation, 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., or from 336 to 360 lines. M. de Prony found nearly
similar results in the Pontine marshes. The whole of these experiments,
made in the latitudes of 41 and 49 degrees, and at 10.5 and 16 degrees
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 humidity than in the valley of
Aragua, I have often seen evaporate during twelve hours, in the sun, 8.8
mill., in the shade 3.4 mill.; and I believe, that the annual produce of
evaporation 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 be the quantity
of vapour that rises from the lake of Valencia, and from the

surrounding country, the waters of which flow into the