Page:Physical Geography of the Sea and its Meteorology.djvu/131

Rh required to express its equivalent in horse-power. Yet what is the horse-power of the Niagara, falling a few steps, in comparison with the horse-power that is required to lift up as high as the clouds and let down again all the water that is discharged into the sea, not only by this river, but by all the other rivers and all the rain in the world? The calculation has been made by engineers, and, according to it, the force for making and lifting vapour from each area of one acre that is included on the surface of the earth is equal to the power of 30 horses.



270. Rivers considered as rain-gauges—the ten largest.—Rivers are the rain-gauges of nature. The volume of water annually discharged by any river into the sea expresses the total amount by which the precipitation upon the valley drained by such river exceeds the evaporation from the same valley during the year. There are but ten rivers that we shall treat as rain-gauges; and there are only ten in the world whose valleys include an area of more than 500,000 square miles. They are:

These areas are stated in round numbers, and according to the best authorities. The basin of the Amazon is usually computed at 1,512,000 square miles; but such computation excludes the Tocantines, 204,000 square miles, which joins the Amazon near its mouth, and the Orinoco, with a hydrographic area of 252,000 square miles, which, by means of the Casiquiare, is connected also with the Amazon. We think that these three rivers should