Page:Popular Science Monthly Volume 23.djvu/341

Rh Nubia, the granitic masses of the desert on the eastern or Arabian side spread suddenly to the westward, and come to the surface in place of the sandstones. In the course of the six or seven miles between Assouan and Philæ the bed of the river rises sixteen feet, forming a declivity, down which the stream rushes in a rapid, known as the First Cataract. The alluvial soil has almost vanished, and the river flows amid a confused heap of granite blocks, with black and polished surfaces. For some eight degrees of latitude farther south, the granite and sandstone plateau which rises so suddenly at Assouan extends through Nubia, increasing in elevation, until at the foot of the second cataract (Wady Haifa) the level of low Nile reaches 392 feet; at the third cataract, 659 feet; at the fourth, 745 feet. Where the White and Blue Niles join, just below Khartoum, in 16° north, the river is 1,212 feet above the sea, or more than 900 feet above its height at Assouan.

Throughout the whole of this course the Nile receives but one affluent, the Atbara, which carries the drainage of a part of Abyssinia into it in about 18° north. And, as this solitary tributary is wholly inadequate to make good the loss which the main stream suffers by evaporation and percolation, on its course through thirteen degrees of one of the hottest and driest climates in the world, the Nile presents the singular spectacle of a river the volume of water in which is conspicuously less than that poured into it by its feeders.

The Blue and the White Niles, which unite to form the main stream close to Khartoum, are in fact very large rivers, each of which drains an immense area abundantly supplied with water. The one receives the overflow of the great equatorial Nyanza lakes and the drainage of the vast swampy region of the Soudan to the north of them, in which the heavy intertropical summer rains accumulate. The other is fed not only by such rains, but by the snows among the mountain-tops of Abyssinia, which melt as the sun advances to the northern tropic.

The height of the water in the Nilometer at Cairo is contingent upon the meteorological conditions of a region more than a thousand miles off; and the question whether Egypt shall have a year of famine or a year of plenty hangs upon the rainfall in Abyssinia and equatorial Africa. It is as if the prosperity of the agricultural interest in Berkshire depended on the state of the weather in Morocco.

The general course of the Nile is so directly north and south that the thirtieth parallel of east longitude, which traverses the Albert Nyanza Lake on the equator, passes close to the Rosetta mouth at its outfall. The Albert Nyanza is 2,500 feet above the sea; and, since the length of the part of the great circle inclosed between the points