350 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



Senegal witli rains. Upon every water-slied wliicli is drainecl 

 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 annually 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 through- 

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

 we knew the ram winds from the dry for each locality and season 

 generally throughout such a basin, we should be enabled to deter- 

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

 ocean from which such rains were evaporated. And thus, not- 

 withstandmg all the eddies caused by mountain chains and other 

 uneven surfaces, we might detect the general course of the atmo- 

 spherical 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 cm^rent of the Mississippi, or of any 

 other great river, to his senses. The greatest move that can 



NOW BE made for THE ADVANCEMENT OF METEOROLOGY IS TO EXTEND 

 THIS SYSTEM OF CO-OPERATION AND RESEARCH FROM THE SEA TO THE 

 LAND, AND TO BRING THE MAGNETIC TELEGRAPH REGULARLY INTO 

 THE SERVICE OF METEOROLOGY. 



CHAPTEK XYI. 



§ 681-711. — MONSOONS. 



681. Monsoons are, for the most part, trade-winds deflected^, 

 The cause of. When, at stated seasons of the year, a trade-wind 

 is turned out of its regular course, as from one quadrant to an- 

 other, it is regarded as a monsoon. The African monsoons of 

 the Atlantic (Plate YIII.), the monsoons of the gulf of Mexico, 

 and the Central American monsoons of the Pacific are, for the 

 most part, formed of the trade-winds which are turned back or 

 deflected to restore the equilibrium which the overheated plains of 

 Africa, Utah, Texas, and New Mexico have disturbed ; these winds, 

 carrying their fuel (§ 254?) with them in vapour, have their equi- 

 librium still further disturbed by the heat which is liberated when 

 that vapour is condensed. Thus, with regard to the N.W. and 



