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 lake. I shall have occasion elsewhere to resume this subject;
for, in a work which displays the great laws of nature in different
zones, we must endeavour to solve the problem of the mean tension of
the vapours contained in the atmosphere in different latitudes, and at