Page:Philosophical Transactions - Volume 145.djvu/128

112 was formed in 1846, under the patronage of the Viceroy of Egypt,, for the express purpose of making a careful survey of the isthmus, in order to determine the question of the level of the two seas. The association consisted of Messieurs and  on the part of the French, Mr. on the part of the English, M. on the part of the Austrians, and M.  (Linant Bey) on the part of the Pacha. Mr. undertook to observe the levels of the tides at Suez, M. those at Tineh on the Mediterranean, near the ancient Pelusium, and the survey of the land was undertaken by M.  and M.  with several assistants. A report of their operations, accompanied by maps on a large scale, and detailed tables, was printed at Nismes in 1847, under the title of "Société d'Etudes de l'Isthme de Suez, Travaux de la Brigade Française, Rapport de l'Ingénieur," but has not been published. The results obtained were as follows:—

That the low-water mark of ordinary tides in the two seas, at Suez and at Tineh, is very nearly on the same level, the difference being, that at Suez it is three centimetres lower, that is, rather more than 1 inch;

That the mean rise of ordinary tides in the Red Sea is somewhat higher than in the Mediterranean, but that the maximum difference is not more than 80 centimetres, or 31 inches;

That the rise of the equinoctial spring tides at Suez over the low-water mark in the Mediterranean at Tineh at the same period is 2⋅38 metres, or 7 feet 9 inches; and

That the deepest low-water mark at the same period at Suez is 0⋅45 metre, or 17⋅7inches under the deepest low-water mark at Tineh.

The highest point of the isthmus between Pelusium and Suez is 12⋅74 metres, or 41 feet above the Mediterranean, the distance between the two places being 12 myriametres, or about 78 English miles.

The north-westernly prolongation of the Libyan range of hills, which form the western boundary of Upper Egypt, is composed of the same nummulite limestone, covered by the upper sandstone. The sandy desert at the foot of the range, like that on the eastern or Arabian side, contains pebbles of agate and flint, and masses of fossil wood, stems of which have been found 40 feet in length changed into hornstone. Parallel to the direction of the Libyan hills, and on their eastern side, are two depressions of great extent, one of which there is every reason to consider as a former channel of the Nile, and goes by the name of the Valley of the Waterless River; the other is the Valley of the Natron Lakes.

Lower Egypt, geologically considered, is formed of the low and almost level land included between the Mediterranean and the hilly regions which form what may be termed the natural boundary of Upper Egypt. The central portion formed by the divergence of the Nile about sixteen Miles below Cairo, into two branches that fall into the sea at Rosetta and Damietta, constitutes the present Delta. The distance