e, but the
generalizations of science furnish us with a sort of substitute for
them.

The long series of patient, careful, and sagacious observations, which
have been continued now for two thousand years, bring us results, by
means of which, through our powers of mental conception, we may take a
comprehensive survey of the whole scene, analogous, in some respects, to
that which direct and actual vision would afford us, if we could look
down upon it from the eagle's point of view. It is, however, somewhat
humiliating to our pride of intellect to reflect that long-continued
philosophical investigations and learned scientific research are, in
such a case as this, after all, in some sense, only a sort of substitute
for wings. A human mind connected with a pair of eagle's wings would
have solved the mystery of Egypt in a week; whereas science, philosophy,
and research, confined to the surface of the ground, have been occupied
for twenty centuries in accomplishing the undertaking.

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 wi