THE WINDS. 273 



and polar currents of the sea, there are no unduly heated surfaces, 

 no mountain ranges, or other obstructions to the circulation of the 

 atmosphere — nothing to disturb it in its natural courses. The sea, 

 therefore, is the field for observing the operations of the general 

 laws which govern the movements of the great aerial ocean. Ob- 

 servations on the land will enable us to discover the exceptions. 

 But from the sea we shall get the rule. Each valley, every mount- 

 ain range and local district, may be said to have its own peculiar 

 system of calms, winds, rains, and droughts. But not so the sur- 

 face of the broad ocean ; over it the agents which are at work are 

 of a uniform character. 



779. Rain-winds are the winds which convey the vapor from 

 the sea, where it is taken up, to other parts of the earth, where it 

 is let down either as snow, hail, or rain. As a general rule, the 

 trade- winds (§ 179) may be regarded as the evaporating winds; 

 and when, in the course of their circuit, they are converted into 

 monsoons, or the variables of either hemisphere, they then gener- 

 ally become also the rain-winds — especially the monsoons — for cer- 

 tain localities. Thus the southwest monsoons of the Indian Ocean 

 are the rain-winds for the west coast of Hindostan (§ 202). In like 

 manner, the African monsoons of the Atlantic are the winds which 

 feed the springs of the Niger and the Senegal with rains. 



780. 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 wa- 

 ter which runs off through the river into the sea. In this view, all 

 rivers may be regarded as immense rain-gauges, and the volume 

 of water annually discharged by any one, as an expression of the 

 quantity which is annually evaporated from the sea, carried back 

 by the winds, and precipitated throughout 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 season 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 were evaporated. And thus, notwithstanding all the eddies 

 caused by mountain chains, and other uneven surfaces, we might 

 detect the general course of the atmospherical circulation over the 



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