§ 270. KAINS AND RIVERS. 99 



CHAPTER y. 



§ 270-803. — RAINS AND RIVERS. 



270. Rivers are the rain-gauges of nature. The volume of 

 Rivers considered as water annuallv discharsjed by any river into the 



rain-gauges— the ten "^ '-' ^ ^ . -, -, 



largest. sea expresses the total amount by which the precip- 



itation upon the valley drained by such river exceeds the evap- 

 oration 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 : 



Square miles. 



The Amazon, including the Tocantines and Orinoco 2,048,000 



Mississippi 982,000 



La Plata 886,000 



Yenisei 785,000 



Obi 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 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 all 

 be regarded as belonging to one Lydrographic basin, for a canoe 

 may pass inland from any one to either of the others without 

 portage. Of these hydrographic basins, three, including an area 

 of 8,916,000 square miles, are American ; six, which contain an 

 area of 3,772,000 square miles, belong to Asia, one to Africa, and 

 none to Europe. The three largest rivers of Asia, the Yenisei, 

 Obi, and Lena (2,101,000 square miles), discharge their waters 

 into the Arctic Ocean; their outlets are beyond the reach of the 



