The long series of patient, careful, and sagacious observations, which have beencontinued now for two thousand years, bring us results, by means of which, through our powers of mental conception, we may take a comprehensive survey ofthe whole scene, analogous, in some respects, to that which direct and actual visionwould 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 longcontinued philosophical investigations and learned scientific research are, in such acase as this, after all, in some sense, only a sort of substitute for wings. A humanmind connected with a pair of eagle's wings would have solved the mystery ofEgypt in a week; whereas science, philosophy, and research, confined to the surfaceof the ground, have been occupied for twenty centuries in accomplishing theundertaking.It is found at last that both the existence of Egypt itself, and its strange insulation inthe midst of boundless tracts of dry and barren sand, depend upon certainremarkable results of the general laws of rain. The water which is taken up by theatmosphere 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 ofwhich vary very much in different portions of the earth. As a general principle, rainsare 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 havebeen expected; for, under the burning sun of the equator, the evaporation of watermust 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 evaporationtakes 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 beentaken up by evaporation mainly depends, is the cooling of the atmospheric stratumwhich contains it; and this effect is produced in very various ways, and many 5different causes operate to modify it. Sometimes the stratum is cooled by beingwafted over ranges of mountains, sometimes by encountering and becomingmingled with cooler currents of air; and sometimes, again, by being driven in windstoward a higher, and, consequently, cooler latitude. If, on the other hand, air movesfrom cold mountains toward warm and sunny plains, or from higher latitudes tolower, or if, among the various currents into which it falls, it becomes mixed withair warmer than itself, its capacity for containing vapor in solution is increased, and, consequently, instead of releasing its hold upon the waters which it hasalready in possession, it becomes thirsty for more. It moves over a country, underthese circumstances, as a warm and drying wind. Under a reverse of circumstancesit would have formed drifting mists, or, perhaps, even copious showers of