THE WINDS. 223 



Africa, Utah, Texas, and New Mexico have disturbed. 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 thev 

 and the trade-M^inds, of which they are formed, are called mon- 

 soons. 



475. 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-w^inds ; it turns them back ; and 

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

 w^hat are now^ called the northeast monsoons would blow the year 

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

 winds, being perpetual, would become all the year, what in reality 

 for five months (§ 474) they are, viz., northeast trade-winds. 



476. The agents which produce monsoons reside (§ 475) on the 

 land. These wdnds are caused by the rarefaction of the air over 

 large districts of country situated on the polar edge, or near the 

 polar edge of the trade- winds. Thus the monsoons of the Indian 

 Ocean are caused by the intense heat which the rays of a cloud- 

 less sun produce during the summer time upon the Desert of Gobi 

 and the burning plains of Central Asia. When the sun is north 

 of the equator, the force of his rays, beating down upon these wide 

 and thirsty plains, is such as to cause the vast superincumbent 

 body of air to expand and ascend. There is, consequently, a rush 

 of air, especially from toward the equator, to restore the equilib- 

 rium ; and in this case, the force which tends to draw the north- 

 east trade-winds back becomes greater than the force which is 

 acting to propel them forward. Consequently, they obey the 

 stronger power, turn back, and become the famous southwest 

 monsoons of the Indian Ocean, which blow from May to Septem- 

 ber inclusive. 



477. Of course, the vast plains of Asia are not brought up to 

 monsoon heat per saltum, or in a day. They require time both to 

 be heated up to this point .and to be cooled down again. Hence 

 there is a conflict for a few weeks about the change of the mon- 

 soon, when neither the trade-wind nor the monsoon force has 

 fairly lost or gained the ascendency. This debatable period 



