The Beds Of Rivers Are Heated As Far As The Depth To Which The Solar Rays Can Penetrate Without Undergoing Too Great An Extinction In Their Passage Through The Superincumbent Strata Of Water.

Besides, filtration extends in a lateral direction far beyond the bed of the river.

The shore, which appears dry to us, imbibes water as far up as to the level of the surface of the river. We saw water gush out at the distance of fifty toises from the shore, every time that the Indians struck their oars into the ground. Now these sands, wet below, but dry above, and exposed to the solar rays, act like sponges, and lose the infiltrated water every instant by evaporation. The vapour that is emitted, traverses the upper stratum of sand strongly heated, and becomes sensible to the eye when the air cools towards evening. As the beach dries, it draws from the river new portions of water; and it may be easily conceived that this continual alternation of vaporization and lateral absorption must cause an immense loss, difficult to submit to exact calculation. The increase of these losses would be in proportion to the length of the course of the rivers, if from their source to their mouth they were equally surrounded by a flat shore; but these shores being formed by deposits from the water, and the water having less velocity in proportion as it is more remote from its source, throwing down more sediment in the lower than in the upper part of its course, many rivers in hot climates undergo a diminution in the quantity of their water, as they approach their outlets.

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