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.