THE ATMOSPHERE. 87 



horses, and for the whole area of the earth it is 800 times greater 

 than all the water-power in Europe. 



169. Where does the vapor that mahes the rains which feed 

 the rivers of the northern hemisphere come from f 



The proportion between the land and water in the northern 

 hemisphere is very different from the proportion that obtains be- 

 tween them in the southern. In the northern hemisphere, the land 

 and water are nearly equally divided. In the southern, there is 

 several times more water than land. All the great rivers in the 

 world are in the northern hemisphere, where there is less ocean to 

 supply them. Whence, then, are their sources replenished ? Those 

 of the Amazon are supplied with rains from the equatorial calms 

 and trade-winds of the Atlantic. That river runs east, its branch- 

 es come from the north and south ; it is always the rainy season 

 on one side or the other of it ; consequently, it is a river without 

 periodic stages of a very marked character. It is always near its 

 high-water mark. For one half of the year its northern tributa- 

 ries are flooded, and its southern for the other half It discharges 

 under the line, and as its tributaries come from both hemispheres, 

 it can not be said to belong exclusively to either. It is supplied 

 with water made of vapor that is taken up from the Atlantic 

 Ocean. Taking the Amazon, therefore, out of the count, the Rio 

 de la Plata is the only great river of the southern hemisphere. 

 There is no large river in New Holland. The South Sea Islands 

 give rise to none, nor is there one in South Africa entitled to be 

 called great that we know of. 



170. The great rivers of North America and North Africa, and 

 all the rivers of Europe and Asia, lie wholly within the northern 

 hemisphere. How is it, then, considering that the evaporating sur- 

 face lies mainly in the southern hemisphere — how is it, I say, that 

 we should have the evaporation to take place in one hemisphere 

 and the condensation in the other? The total amount of rain 

 which falls in the northern hemisphere is much greater, meteorol- 

 ogists tell us, than that which falls in the southern. The annual 

 amount of rain in the north temperate zone is half as much again 

 as that of the south temperate. 



171. How is it, then, that this vapor gets, as stated § 170, from 



