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 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 (§ 1 26) may be regarded as the evaporating winds ; 
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 — especially 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 rains. 
472. Upon every water-shed which is drained into the sea, the 
precipitation may be considered as greater than the evaporation, 
for the whole 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 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 
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- 
