Page:Philosophical Transactions - Volume 145.djvu/130

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above Cairo, and the distance between the two places by the Nile being 556 miles, the average fall of the river is little more than half a foot in a mile, viz. 0⋅54 ft.; and Assouan being 365 feet above the Mediterranean, and 696 miles distant from it, the average fall of the Nile from the foot of the First Cataract to the sea is 0⋅524 ft. in a mile[1]. Russegger does not give the place in Cairo from which he measured the above 300 feet, but by the careful measurements of the French Brigade in 1847, before referred to, M. Talabot states the lowest water of the Nile in the Nilometer of Rhoda near Cairo in that year to have been 14⋅08 metres, or 46 feet 2 inches above low-water mark of the Mediterranean at Tineh; and the distance to the mouth of the Damietta branch, following the course of the river, being 149 miles, the average fall is thus only 33/4 inches in a mile.


The Inundations of the Nile.

The commencement of the rise of the Nile, immediately below Assouan, is about the summer solstice. The first rise at Cairo, indicated by an increasing motion in the stream, is usually in the first week of July. The rise is scarcely perceptible for six or eight days, and it then becomes more rapid. About the middle of August it has obtained two-thirds of the height between the lowest ebb and highest rise. At this period the water enters the great side branch on the left bank, the Bahr el Jusef, Joseph's Canal, called also the Magrour, and now is the time when the artificial branches or canals are opened, the commencement of the inundation over the parched plains. The rise attains its maximum between the 20th and 30th of September, and this state of the inundation is called the Salibe. The water remains pretty stationary for fourteen days; it then begins to fall, at first at a more rapid rate than that with which it rose, but after it has fallen one-half, the decrease is very gradual. About the 10th of November it has usually fallen one-half, and it goes sinking slowly until somewhat beyond the following May. The rise of the river continues therefore about ninety days (from 1st of July to 28th of Sept.), but it continues falling about 230 days (12th of October to end of May). The changes of level are well illustrated by the annexed diagram, given to M. Talabot in 1847 by Mougel Bey, the engineer for the Barrage of the Nile at the apex of the Delta.

  1. Russegger, Reisen, ii. 271. The fall of the Thames from Chertsey to Teddington Lock, a distance of 131/2 miles, is nearly 171/2 inches in a mile. See Rennie, Report to the British Association in 1834, p. 487.

    "Colonel Cautley, the projector of the Ganges Canal (recently constructed), decided after careful thought and due regard to the experience gained on canals previously opened, that a fall of fifteen inches in every mile of length would best secure the desired ends." -Short Account of the Ganges Canal, p. 7.