— 50 — 
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 20J 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 Assuan 
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 
55 
17.4 
55 
11.1 
55 
6.3 
55 
9th 
55 
17.5 
55 
11.2 
55 
6.3 
55 
10th 
55 
17.5 
55 
11.3 
55 
6.2 
55 
11th 
55 
17.5 
55 
11.4 
55 
6.1 
55 
12th 
55 
17.7 
55 
11.5 
55 
6.2 
55 
13th 
55 
17.7 
55 
11.6 
55 
6.1 
55 
14th 
55 
17.9 
55 
11.7 
55 
6.2 
55 
15th 
55 
18.2 
55 
11.8 
« 
55 
6.4 
55 
16th 
55 
18.4 
55 
11.9 
55 
6.5 
55 
17th 
55 
18.8 
55 
12.0 
55 
6.8 
55 
18th 
55 
19.1 
55 
12.1 
55 
7.1 
55 
19th 
55 
19.5 
55 
12.2 
55 
7.3 
55 
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 bifur- 
cation was gradually approaching Cairo, while since the 13th it has 
been receding. 
