THE WINDS, 221 



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. 



471. Rain-winds are the wind* 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 (§ 1 26) may be regarded as the evaporating w4nds ; 

 and when, in the course of their circuit, they become monsoons, 

 or the variables of either hemisphere, they then generally become 

 also the rain-winds — espeoially the monsoons — for certain locali- 

 ties. Thus the southwest monsoons of the Indian Ocean are the 

 rain-winds for the west coast of the Peninsula (^ 139). In like 

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

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



472. Upon every water-shed w^hich is drained into the sea, the 

 precipitation may be considered as greater than the evaporation, 

 for the w^hole extent of the shed so drained, 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 w^ater 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, w^e 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 

 land as well as the sea, and make the general courses of circula- 

 tion 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. 



473. These investigations as to the rain-winds at sea indicate 

 that the vapors which supply the sources of the Amazon with rain 

 are taken up from the Atlantic Ocean by the northeast and south- 



