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 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.