The most important characteristic of the Nile is its annual flooding or =Inundation=. By the end of May, in Egypt, the river is at its lowest level. During the month of June the Nile, between Cairo and Aswân, begins to rise, and a quantity of “green water” appears at this time. The cause of the colour is said to be myriads of minute algae, which subsequently putrefy and disappear. During August the river rises rapidly, and its waters assume a red, muddy colour, which is due to the presence of the rich red earth which is brought into the Nile by the Blue Nile and the Atbara. The rising of the waters continues until the middle of September, when they remain stationary for about a fortnight or three weeks. In October a further slight rise occurs, and then they begin to fall; the fall continues gradually until, in the May following, they are at their lowest level once more. The cause of the Inundation is, as Aristotle (who lived in the fourth century B.C.) first showed, the spring and early summer rains in the mountains of Ethiopia and the Southern Sûdân; these are brought down in torrents by the great tributaries of the Nile, viz., the Gazelle River, the Sobat (the Astasobas of Strabo), the Giraffe River, the Blue Nile, and the Atbara. The Sobat rises about April 15, the Gazelle River and the Giraffe River about the 15th of May, the Blue Nile at the end of May, and the Atbara a little later. The united waters of these tributaries, with the water of the Upper Nile, reach Egypt about the end of August, and cause the Inundation to reach its highest level. The Nile rises from 21 feet to 28 feet, and deposits a thin layer of fertilizing mud over every part of the country reached by its waters. Formerly, when the rise was about 26 feet, there was sufficient water to cover the whole country; when it was less, scarcity prevailed; and when it was more, ruin and misery appeared through over-flooding. In recent years, the British irrigation engineers in Egypt have regulated, by means of the Aswân Dam, the Barrage at Asyûṭ, and the Barrage near Al-Manâshî, a little to the north of Cairo,[2] the supply of water during the winter, or dry season, with such success, that, in spite of “low” Niles, the principal crops have been saved, and the people protected from want.