RAINS AND RIVERS. 105 



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. 



CHAPTER V. 



§ 270-303. — RAINS AND RIVERS. 



270. Bivers considered as rain-gauges — the ten largest. — Eivers 

 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 : 



Sfiuare miles. 



The Amazon, includrng the Tocantines and Orinoco , . 2, 048, 000 



Mississippi 982,000 



La Plata 886,000 



Yenisei 785,000 



Ohi 725,000 



Lena 594,000 



Amoor 583,000 



Yang-tse-kiang 548,000 



Hoang-ho 537,000 



Nile 520,000 



These areas are stated in round numbers, and accordino: 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. Wo think that these three rivers should 



