d is turned back, or diverted, by over-heated districts,
from its regular course at stated seasons of the year, it is regarded
as a monsoon. Thus, the African monsoons of the Atlantic, the
monsoons of the Gulf of Mexico, and the Central American monsoons of
the Pacific, are, for the most part, formed of the north-east
trade-winds, which are turned back to restore the equilibrium which
the over-heated plains of 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
monsoons."

Again (Sec. 476-7):

"The agents which produce monsoons reside 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 which the rays of a cloudless sun produce, during the
summer time, upon the Desert of Cobi 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 equilibrium; 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 south-west
monsoons of the Indian Ocean, which blow from May to September
inclusive.

"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 monsoon, when
neither the trade wind nor the monsoon force has fairly lost or
gained the ascendency. This debatable period amounts to about a month
at each change. So that the monsoons of the Indian Ocean prevail
really for about five months each way, viz.: from May to September,
from the south-west, in obedience to the influen