Equinoctial Regions of America, vol 2 | Page 7

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. What has been here effected on a small
scale by the hand of man, nature often performs, either by progressively
elevating the level of the soil, or by those falls of the ground
occasioned by violent earthquakes. It is probable, that in the lapse of
ages, several rivers of Soudan, and of New Holland, which are now lost
in the sands, or in inland basins, will open for themselves a course to
the shores of the ocean. We cannot at least doubt, that in both
continents there are systems of interior rivers, which may be considered
as not entirely developed; and which communicate with each other,
either in the time of great risings, or by permanent bifurcations.
The Rio Pao has scooped itself out a bed so deep and broad, that in the
season of rains, when the Cano Grande de Cambury inundates all the
land to the north-west of Guigue, the waters of this Cano, and those of
the lake of Valencia, flow back into the Rio Pao itself; so that this river,
instead of adding water to the lake, tends rather to carry it away. We
see something similar in North America, where geographers have
represented on their maps an imaginary chain of mountains, between
the great lakes of Canada and the country of the Miamis. At the time of
floods, the waters flowing into the lakes communicate with those which
run into the Mississippi; and it is practicable to proceed by boats from
the sources of the river St. Mary to the Wabash, as well as from the
Chicago to the Illinois. These analogous facts appear to me well worthy
of the attention of hydrographers.
The land that surrounds the lake of Valencia being entirely flat and
even, a diminution of a few inches in the level