On Roda island, opposite Cairo, has stood a gauge from the earliest times. It has been frequently reconstructed. The present gauge is reputed to have been erected in A.D. 861 with its zero at the same level as a more ancient one whose readings have been preserved since A.D. 641. When the gauge was constructed, a reading of 16 cubits meant the lowest level at which flood irrigation could be insured everywhere. The level to-day is 20¹⁄₂ cubits on the gauge and the difference between them is 1.22 metres. As 1,026 years have elapsed since the construction of the gauge it means a rise of 12 centimetres per 100 years. This is slightly under the rise calculated at Assuân by the French savants. The following table gives the means of the maximum flood and low water levels per century:-- 7th century 17.5 R. L. flood 11.0 R. L. low water 6.5 Difference. 8th „ 17.4 „ 11.1 „ 6.3 „ 9th „ 17.5 „ 11.2 „ 6.3 „ 10th „ 17.5 „ 11.3 „ 6.2 „ 11th „ 17.5 „ 11.4 „ 6.1 „ 12th „ 17.7 „ 11.5 „ 6.2 „ 13th „ 17.7 „ 11.6 „ 6.1 „ 14th „ 17.9 „ 11.7 „ 6.2 „ 15th „ 18.2 „ 11.8 „ 6.4 „ 16th „ 18.4 „ 11.9 „ 6.5 „ 17th „ 18.8 „ 12.0 „ 6.8 „ 18th „ 19.1 „ 12.1 „ 7.1 „ 19th „ 19.5 „ 12.2 „ 7.3 „ It is evident from the above that the head of the Delta, or the bifurcation of the Nile, was much nearer to Cairo in early days than just now, and the last three centuries have seen great changes. The fall of watersurface is very considerable at every bifurcation, and the difference between mean high and low supply at the Barrage to-day is 6.0 metres against 7.2 metres at Cairo. Judging from the above figures, I should say that from the 7th to the 13th century the bifurcation was gradually approaching Cairo, while since the 13th it has been receding. [Illustration: PLATE XII. LONGITUDINAL SECTION OF THE NILE FROM ASSUAN TO CAIRO ALONG CENTRE LINE OF FLOOD Lith. Sur. Dep. Cairo. CROSS SECTIONS OF THE NILE VALLEY IN EGYPT] The following table gives the highest and lowest floods at Cairo in intervals of 25 years from A.D. 639 to A.D. 1904. The gauges are in pics and kirats and are referred to mean low water or R. L. 12·25 metres above mean sea.