364 rnvsiCAL geography of the sea, and its meteorology. 



or other obstructions to the circulation of the atmosphere — 

 nothing to disturb it in its normal courses. The sea, therefore, 

 is the field for observing the operations of the general laws which 

 govern the movements of the great aerial ocean. Observations 

 on the land will enable us to discover the exceptions, but from 

 the sea we shall get the rule. Each valley, every mountain 

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

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

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

 are of a more uniform character. 



680. Bain-winds. — Eain-winds are the winds which convey the 

 vapour 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 (§ 293) 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 hemi- 

 sphere, they then generally become also the rain-winds — 

 especially the monsoons — for certain localities. Thus the south- 

 west monsoons of the Indian Ocean are the rain-winds for the 

 west coast of Hindostan (§ 298). In like manner, the African 

 monsoons of the Atlantic 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 rivers into the sea. In this view% all rivers may be regarded 

 as immense rain-gauges, and the volume of water annualh' 

 discharged by any one, may be taken 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 dr}^ 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, not- 

 withstanding 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 in the current of the Mississippi, 



