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 they 
and the trade-winds, 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-winds ; it turns them back ; 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 ; 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 winds 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 v^hich 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 
