Personal Narrative Of Travels To The Equinoctial Regions Of America During The Years 1799-1804 - Volume 2 - By Alexander Von Humboldt And Aime Bonpland.
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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 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
different heights above the surface of the ocean.
A great number of local circumstances cause the produce of evaporation
to vary; it changes in proportion as more or less shade covers the
basin of the waters, with their state of motion or repose, with their
depth, and the nature and colour of their bottom; but in general
evaporation depends only on three circumstances, the temperature, the
tension of the vapours contained in the atmosphere, and the resistance
which the air, more or less dense, more or less agitated, opposes to
the diffusion of vapour. The quantity of water that evaporates in a
given spot, everything else being equal, is proportionate to the
difference between the quantity of vapour which the ambient air can
contain when saturated, and the quantity which it actually contains.
Hence it follows that the evaporation is not so great in the torrid
zone as might be expected from the enormous augmentation of
temperature; because, in those ardent climates, the air is habitually
very humid.
Since the increase of agricultural industry in the valleys of Aragua,
the little rivers which run into the lake of Valencia can no longer be
regarded as positive supplies during the six months succeeding
December. They remain dried up in the lower part of their course,
because the planters of indigo, coffee, and sugar-canes, have made
frequent drainings (azequias), in order to water the ground by
trenches. We may observe also, that a pretty considerable river, the
Rio Pao, which rises at the entrance of the Llanos, at the foot of the
range of hills called La Galera, heretofore mingled its waters with
those of the lake, by uniting with the Cano de Cambury, on the road
from the town of Nueva Valencia to Guigue. The course of this river
was from south to north. At the end of the seventeenth century, the
proprietor of a neighbouring plantation dug at the back of the hill a
new bed for the Rio Pao. He turned the river; and, after having
employed part of the water for the irrigation of his fields, he caused
the rest to flow at a venture southward, following the declivity of
the Llanos. In this new southern direction the Rio Pao, mingled with
three other rivers, the Tinaco, the Guanarito, and the Chilua, falls
into the Portuguesa, which is a branch of the Apure. It is a
remarkable phenomenon, that by a particular position of the ground,
and the lowering of the ridge of division to south-west, the Rio Pao
separates itself from the little system of interior rivers to which it
originally belonged, and for a century past has communicated, through
the channel of the Apure and the Orinoco, with the ocean.
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