Under the Pharaohs and under the Romans the whole of Lower as well as Upper Egypt was under basin irrigation, and the whole country was cultivated. In those days Egypt was the granary of the Mediterranean, and at the time of the Arab conquest, A.D. 700, her population was estimated to number 12,000,000. Under Arab rule began that period of deterioration which lasted for 1,100 years, and which, had the system of irrigation been less natural to the physical conditions of the country and less simple, would have resulted in an absolute abandonment of cultivation, and reduced Egypt to the state in which Mesopotamia, once the garden of the world, finds itself to-day. Even as it was, by A.D. 1800 the population was brought down to about 2,000,000, and all the northernmost and greater half of the Delta had become a neglected and uncultivated swamp. War, famine, and pestilence, in turn, had played their part; but the fundamental cause of all was the misgovernment, which had neglected the irrigation. For in basin irrigation, as, indeed, with all irrigation, two things are of the utmost importance: the first, to get the water on to the land; and the second, to drain it off again. Salt is the great enemy to be fought. Not only do the Nile waters contain a large quantity of salts, in solution, but the strata underlying the alluvial deposits, being of marine origin, are also rich in salts. If the water is allowed to stand on the land, evaporation takes place, until nothing but a salt efflorescence is left. While if the land be so water-logged that the level at which water can be obtained by digging is brought near to the surface, the water containing salts from below is drawn upwards by means of capillary attraction, and once more evaporation takes place, leaving the salts in the soil. It is clear that as the natural level of the land approaches sea-level it becomes more and more troublesome to provide proper means of carrying off the water. Accordingly, the northernmost parts of the Delta were the first to suffer, and gradually the line of cultivation receded.