§ GSO. SEA ROUTES, CALM BELTS, ETC. 368 



earth, where it is let down either as snow, hail, or rain. As a gen- 

 eral rule, the trade- winds (§ 293) may be regarded as the evapo- 

 rating winds; and when, in the course of their circuit, they are 

 converted into monsoons, or the variables of either hemisphere, 

 they then generally become also the rain-winds — especially the 

 monsoons — for certain localities. Thus the southwest monsoons 

 of the Indian Ocean are the rain-winds for the west coast of Hin- 

 dostan (§ 298). In like manner, the African monsoons of the At- 

 lantic are the winds which feed the springs of the Niger and the 

 Senegal with rains. Upon every water-shed which is drained 

 into the sea, the precipitation, for the whole extent of the shed so 

 drained, may be considered as greater than the evaporation, by the 

 amount of water which runs off through the river into the sea. 

 In tills view, all rivers may be regarded as immense rain-ganges, 

 and the volume of water annually discharged by any one as an 

 expression of the quantity which is annually evaporated from 

 the SGa, carried back by the winds, and precipitated through- 

 out the whole extent of the valley that is drained by it. Now, if 

 we knew the rain-winds from the dry for each locality and sea- 

 son generally throughout such a basin, we should be enabled to 

 determine, with some degree of probability at least, as to the part 

 of the ocean from which such rains w^ere evaporated. And thus, 

 nowithstanding all the eddies caused by mountain chains and 

 other uneven surfaces, we might detect the general course of the 

 atmospherical circulation over the land as well as the sea, and 

 make the general courses of circulation in each valley as obvious 

 to the mind of the philosopher as is the current of the Mississippi, 

 or of any other great river, to his senses. 



