THE WINDS. 275 



787. Monsoons (§ 763) are, for the most part, formed of trade- 

 winds. When at stated seasons of the year a trade-wind is de- 

 flected in its regular course from one quadrant to another, or drawn 

 in by overheated districts, it is regarded as a monsoon. Thus the 

 African monsoons of the Atlantic (Plate VIIL), the mopsoons of 

 the Gulf of Mexico, and the Central American monsoons of the Pa- 

 cific, are, for the most part, formed of the trade-winds, which are 

 turned back or deflected to restore the equilibrium which the over- 

 heated plains of Africa, Utah, Texas, and 'New Mexico have dis- 

 turbed. When the monsoons prevail for five months at a time, 

 for it takes about a month for them to change and become settled, 

 then both they and the trade- winds, which they replace, are called 

 monsoons. 



788. The northeast and the southwest monsoons of the Indian 

 Ocean afford an example of this kind. A force is exerted upon 

 the northeast trade-winds of that sea by the disturbance which the 

 heat of summer creates in the atmosphere over the interior plains 

 of Asia, which is more than sufficient to neutralize the forces which 

 cause those winds to blow as trade-winds ; it arrests them ; and 

 were it not for the peculiar conditions of the land about that ocean, 

 what are now called the northeast monsoons would blow the year 

 round ; there would be no southwest monsoons there ; and the 

 northeast winds, being perpetual, would become all the year what 

 in reality for several months they are, viz., northeast trade-winds. 



789. As long ago as 1831, Dove* maintained that the south- 

 west monsoon was the southeast trade-wind rushing forward to fill 

 the vacant places over the Northern deserts. Dove admits the 

 proofs of this to be indirect, and acknowledges the difficulty of 

 finding out and demonstrating the problem. f 



790. I had been studying the wind in his circuits, and hund- 

 reds of sailors were watching the vane for me, and my good friend 

 Jansen encouraged me, by his reasoning and suggestions, to un- 

 dertake the task of proving this difficult proposition of Mr. Dove. 



791. The northeast and southeast trade-winds meet, we know 



* Vide PoGG, Ann. xxi. 



t Annalcn der Physik, No. 94. Translated by Dr. Rosengarten for the American 

 Journal of Science, vol. xx., No. 60. 



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