It is found at last that both the existence of Egypt itself, and its
strange insulation in the midst of boundless tracts of dry and barren
sand, depend upon certain remarkable results of the general laws of
rain. The water which is taken up by the atmosphere from the surface of
the sea and of the land by evaporation, falls again, under certain
circumstances, in showers of rain, the frequency and copiousness of
which vary very much in different portions of the earth. As a general
principle, rains are much more frequent and abundant near the equator
than in temperate climes, and they grow less and less so as we approach
the poles. This might naturally have been expected; for, under the
burning sun of the equator, the evaporation of water must necessarily go
on with immensely greater rapidity than in the colder zones, and all the
water which is taken up must, of course, again come down.

It is not, however, wholly by the latitude of the region in which the
evaporation takes place that the quantity of rain which falls from the
atmosphere is determined; for the condition on which the falling back,
in rain, of the water which has been taken up by evaporation mainly
depends, is the cooling of the atmospheric stratum which contains it;
and this effect is produced in very various ways, and many different
causes operate to modify it. Sometimes the stratum is cooled by being
wafted over ranges of mountains, sometimes by encountering and becoming
mingled with cooler currents of air; and sometimes, again, by being
driven in winds toward a higher, and, consequently, cooler latitude. If,
on the other hand, air moves from cold mountains toward warm and sunny
plains, or from higher latitudes to lower, or if, among the various
currents into which it falls, it becomes mixed with air warmer than
itself, its capacity for containing vapor in solution is increased, and,
consequently, instead of releasing its hold upon the waters which it has
already in possession, it becomes thirsty for more. It moves over a
country, under these circumstances, as a warm and drying wind. Under a
reverse of circumstances it would have formed drifting mists, or,
perhaps, even copious showers of rain.

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