328 THE POPULAR SCIENCE MONTHLY. 



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 hends into consideration, however, 

 it must be considerably less than this amount. 



"Without a knowledge of the facts thus briefly sketched, the peri- 

 odical 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 irreg- 

 ularly 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 Da- 

 mietta. Innumerable artificial canals connect these arms of the Nile 

 with one another, and branch off east and west for purposes of irriga- 

 tion ; while, in the north, the complex system of water-courses com- 

 municates 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 par- 

 ticles 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 



