Page:Popular Science Monthly Volume 23.djvu/342

328 just mentioned is more than two thousand English miles, the mean inclination of the river, if it ran straight, would somewhat exceed a foot per mile. Taking the actual bends into consideration, however, it must be considerably less than this amount.

Without a knowledge of the facts thus briefly sketched, the periodical inundation of the valley of Egypt by the Nile is unintelligible. And, since no one till long after Herodotus's time possessed such knowledge, we may proceed to consider this singular phenomenon without troubling ourselves about his curious speculations as to its causes.

In the month of May and the beginning of June, the Egyptian Nile is little better than a great sluggish ditch, the surface of which, in Upper Egypt, lies many feet below that of its steep banks of irregularly stratified mud and sand. A short distance north of Cairo, it divides into two main branches, which take a northerly course through the delta and finally debouch, the one at Rosetta and the other at Damietta. Innumerable artificial canals connect these arms of the Nile with one another, and branch off east and west for purposes of irrigation; while, in the north, the complex system of water-courses communicates with the series of lakes and marshes, from Mariout, on the west, to Menzaleh on the east, which, as I have already said, occupy a large portion of the area of the delta southward of the sea-coast.

In the latter part of June, about the time of the summer solstice, the motion of the torpid waters of the Nile seaward is quickened, and their level rises, while at the same time they take on a green color. The rise and the flow quicken, and the green color is succeeded by a reddish brown; the water becomes turbid and opaque, and is found to be laden with sediment, varying in consistency from moderately coarse sand, which falls to the bottom at once when the water is still, to mud of impalpable fineness which takes a long time to subside. In fact, when the sun approaches the northernmost limit of his course, as the snows of Abyssinia begin to melt, and the heavy intertropical rains set in, a prodigious volume of water is poured into the White and Blue Niles, and drives before it the accumulated living and dead particles of organic matter which have sweltered in the half-stagnant pools and marshes of the Soudan during the preceding six months. Hence, apparently, the preliminary flow of green water. The Blue Nile and the Atbara must sweep down a vast quantity of river-gravel from the Abyssinian uplands, but it may be doubted whether any of this gets beyond the middle cataracts, except in the condition of fine sand. And I suspect that the chief part, if not the whole, of the coarse sediment of the waters of the high Nile must be derived from Nubia, from the weathering of the rocks, by the heating and cooling process already described, and the action of the winds in blowing the sand thus produced into the stream. The Nile continues to rise for three