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.
It will be evident, from these considerations, that the frequency of the showers, and the quantity of the rain which will fall, in the various regions respectively which the surface of the earth presents, must depend on the combined influence of many causes, such as the warmth of the climate, the proximity and the direction of mountains and of seas, the character of the prevailing winds, and the reflecting qualities of the soil. These and other similar causes, it is found, do, in fact, produce a vast difference in the quantity of rain which falls in different regions. In the northern part of South America, where the land is bordered on every hand by vast tropical seas, which load the hot and thirsty air with vapor, and where the mighty Cordillera of the Andes rears its icy summits to chill and precipitate the vapors again, a quantity of rain amounting to more than ten feet in perpendicular height falls in a year. At St. Petersburg, on the other hand, the quantity thus falling in a year is but little more than one foot. The immense deluge which pours down from the clouds in South America would, if the water were to remain where it fell, wholly submerge and inundate the country. As it is, in flowing off through the valleys to the sea, the united torrents form the greatest river on the globe—the Amazon; and the vegetation, stimulated by the heat, and nourished by the abundant and incessant supplies of moisture, becomes so rank, and loads the earth with such an entangled and matted mass of trunks, and stems, and twining wreaths and vines, that man is almost excluded from the scene. The boundless forests become a vast and almost impenetrable jungle, abandoned to wild beasts, noxious reptiles, and huge and ferocious birds of prey.
Of course, the district of St. Petersburg, with its icy winter, its low and powerless sun, and its twelve inches of annual rain, must necessarily present, in all its phenomena of vegetable and animal life, a striking contrast to the exuberant prolificness of New Grenada. It is, however, after all, not absolutely the opposite extreme. There are certain regions on the surface of the earth that are actually rainless; and it is these which present us with the true and real contrast to the luxuriant vegetation and teeming life of the country of the Amazon. In these rainless regions all is necessarily silence, desolation, and death. No plant can grow; no animal can live. Man, too, is forever and hopelessly excluded. If the exuberant abundance of animal and vegetable life shut him out, in some measure, from regions which an excess of heat and moisture render too prolific, the total absence of them still more effectually forbids him a home in these. They become, therefore, vast wastes of dry and barren sands in which no root can find nourishment, and of dreary rocks to which not even a lichen can cling.
The most extensive and remarkable rainless region on the earth is a vast tract extending through the interior and northern part of Africa, and the southwestern part of Asia. The Red Sea penetrates into this tract from the south, and thus breaks the outline and continuity of its form, without, however, altering, or essentially modifying its character. It divides it, however, and to the different portions which this division forms, different names have been given. The Asiatic portion is called Arabia Deserta; the African tract has received the name of Sahara; while between these two, in the neighborhood of Egypt, the barren region is called simply the desert. The whole tract is marked, however, throughout, with one all-pervading character: the absence of vegetable, and, consequently, of animal life, on account of the absence of rain. The rising of a range of lofty mountains in the center of it, to produce a precipitation of moisture from the air, would probably transform the whole of the vast waste into as verdant, and fertile, and populous a region as any on the globe.
[Illustration: VALLEY OF THE NILE]
As it is, there are no such mountains. The whole tract is nearly level, and so little elevated above the sea, that, at the distance of many hundred miles in the interior, the land rises only to the height of a few hundred feet above the surface of the Mediterranean; whereas in New Grenada, at less than one hundred miles from the sea, the chain of the Andes rises to elevations of from ten to fifteen thousand feet. Such an ascent as that of a few hundred feet in hundreds of miles would be wholly imperceptible to any ordinary mode of observation; and the great rainless region, accordingly, of Africa and Asia is, as it appears to the traveler, one vast plain, a thousand miles wide and five thousand miles long, with only one considerable interruption to the dead monotony which reigns, with that exception, every where over the immense expanse of silence and solitude. The single interval of fruitfulness and life is the valley of the Nile.
There are, however, in fact, three interruptions to the continuity of this plain, though only one of them constitutes any considerable interruption to its barrenness. They are all of them valleys, extending from north to south, and lying side by side. The most easterly of these valleys is so deep that the waters of the ocean flow into it from the south, forming a long and narrow inlet called the Red Sea. As this inlet communicates freely with the ocean, it is always nearly of the same level, and as the evaporation from it is not sufficient to produce rain, it does not even fertilize its own shores. Its presence varies the dreary scenery of the landscape, it is true, by giving us surging waters to look upon instead of driving sands; but this is all. With the exception of the spectacle of an English steamer passing, at weary intervals, over its dreary expanse, and some moldering remains of ancient cities on its eastern shore, it affords scarcely any indications of life. It does very little, therefore, to relieve the monotonous aspect of solitude and desolation which reigns over the region into which it has intruded.
The most westerly of the three valleys to which we have alluded is only a slight depression of the surface of the land marked by a line of oases. The depression is not sufficient to admit the waters of the Mediterranean, nor are there any rains over any portion of the valley which it forms sufficient to make it the bed of a stream. Springs issue, however, here and there, in several places, from the ground, and, percolating through the sands along the valley, give fertility to little dells, long and narrow, which, by the contrast that they form with the surrounding desolation, seem to the traveler to possess the verdure and beauty of Paradise. There is a line of these oases extending along this westerly depression, and some of them are of considerable extent. The oasis of Siweh, on which stood the far-famed temple of Jupiter Ammon, was many miles in extent, and was said to have contained in ancient times a population of eight thousand souls. Thus, while the most easterly of the three valleys which we have named was sunk so low as to admit the ocean to flow freely into it, the most westerly was so slightly depressed that it gained only a circumscribed and limited fertility through the springs, which, in the lowest portions of it, oozed from the ground. The third valley—the central one—remains now to be described.
The reader will observe, by referring once more to the map, that south of the great rainless region of which we are speaking, there lie groups and ranges of mountains in Abyssinia, called the Mountains of the Moon. These mountains are near the equator, and the relation which they sustain to the surrounding seas, and to currents of wind which blow in
