months until the autumnal equinox, by which time the level of its surface at Assouan is usually forty feet, at Thebes thirty-six feet, at Cairo twenty-four or twenty-five feet, and at Rosetta four feet higher than it is in May; and, before reaching the delta, it flows at the rate of three or four miles an hour.
Under these circumstances, the river overflows its banks on all sides. When it does so, the movement of the water is retarded or even arrested, and the suspended solid matters sooner or later fall to the bottom, and form a thin layer of sandy mud. When the Nile waters spread out over the great surface of the delta, the retardation is of course very marked. The coarse sediment is soon deposited, and only the very finest particles remain in suspension at the outflow into the Mediterranean. As the sun goes southward, his action on the Abyssinian snows diminishes, the dry season sets in over the catchment basin of the White Nile, and the water-supply of the Nile diminishes to its minimum. Hence, after the autumnal equinox, the Nile begins to fall and its flow to slacken, as rapidly as it rose. By the middle of November, it is half-way back to its summer level, and it continues to fall until the following May. In the dry air of Nubia and of Egypt, evaporation is incredibly rapid, and the Nile falls a prey to the sun. As the old Egyptian myth has it, Osiris is dismembered by Typhon.
Relatively to the bulk of water, the amount of solid matter transported annually by the Nile must be far less than that which is carried down by the rapid streams of mountainous countries in temperate climates—such, for example, as the upper Rhône. We have no very satisfactory estimate of what that amount may be, but I am disposed to think that the ordinary computation, according to which the average deposit over the delta amounts to not more than a layer one-twentieth of an inch thick annually, is, at any rate, not under the mark.
But this is a very interesting question, for it is obvious that, if we may assume that the deposit of the Nile has taken place uniformly at a known rate, it becomes possible, given the thickness of the alluvial deposit in the delta, to calculate the minimum time occupied in its formation. The borings made under the direction of the late Mr. Leonard Horner in the upper part of the delta, and those subsequently conducted by Figari Bey, favor the conclusion that the natural loose soil which fills the flat basin of the delta nowhere exceeds sixty feet in depth. Assuming it to have this thickness in any spot, it follows that, at one twentieth of an inch of deposit per annum, it must have taken at least fourteen thousand four hundred years to accumulate to that thickness at that place. And if so, Herodotus seems, at first, to have made a wonderfully good guess, when he said that the Arabian Gulf and, by implication, that of the delta, might have been filled up in "twenty thousand years, or even half the time."