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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On The Other Hand, The Famous
Pongos Of The River Amazon, So Dangerous To Go Up, The Falls Of
Rentema, Of Escurrebragas, And Of Mayasi, Are But A Few Feet In
Perpendicular Height.
Those who are engaged in hydraulic works know
the effect that a bar of eighteen or twenty inches' height produces in
a great river.
The whirling and tumultuous movement of the water does
not depend solely on the greatness of partial falls; what determines
the force and impetuosity is the nearness of these falls, the
steepness of the rocky ledges, the returning sheets of water which
strike against and surmount each other, the form of the islands and
shoals, the direction of the counter-currents, and the contraction and
sinuosity of the channels through which the waters force a passage
between two adjacent levels. In two rivers equally large, that of
which the falls have least height may sometimes present the greatest
dangers and the most impetuous movements.
It is probable that the river Orinoco loses part of its waters in the
cataracts, not only by increased evaporation, caused by the dispersion
of minute drops in the atmosphere, but still more by filtrations into
the subterraneous cavities. These losses, however, are not very
perceptible when we compare the mass of waters entering into the
raudal with that which issues out near the mouth of the Rio Anaveni.
It was by a similar comparison that the existence of subterraneous
cavities in the yellalas or rapids of the river Congo was discovered.
The pongo of Manseriche, which ought rather to be called a strait than
a fall, ingulfs, in a manner not yet sufficiently explored, a part of
the waters and all the floating wood of the Upper Maranon.
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