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 that quarter of the world, is such, that they bring down from the atmosphere, especially in certain seasons of the year, vast and continual torrents of rain. The water which thus falls drenches the mountain sides and deluges the valleys. There is a great portion of it which can not flow to the southward or eastward toward the sea, as the whole country consists, in those directions, of continuous tracts of elevated land. The rush of water thus turns to the northward, and, pressing on across the desert through the great central valley which we have referred to above, it finds an outlet, at last, in the Mediterranean, at a point two thousand miles distant from the place where the immense condenser drew it from the skies. The river thus created is the Nile. It is formed, in a word, by the surplus waters of a district inundated with rains, in their progress across a rainless desert, seeking the sea.