— 49 — 
1869 and has been recorded daily since then -(a cubit = 54 centimetres). 
The accidental zero of the gauge is R. L. 84.16. The mean low-water 
level or true zero is R. L. 85.00. 
22. The Nile from Assuan to the Barrage. — From Assu&n 
to the Barrage, the length of the river is 973 kilometres in summer 
and 923 in flood. The slope in summer is 1 3 -q Q ~q and in flood tWWo~o ' ■ 
The mean fall of the valley is xoiro'o* The slopes vary in the different 
mean reaches, the least being x ci o o i n Rena Mudiria and the 
greatest xriow i n Beni Suef. In a high flood with a rise of 9 metres 
at Assu&n, the rise in Kena will be 9.5 metres and only 8.2 in Beni 
Suef. Table 42 gives the mean areas of cross sections of the Nile, 
while table 44 gives the mean widths. Neglecting spill channels, we 
may state that in a high flood the mean area of the section of the Nile 
is 7,500 square metres and the mean width 900 metres. In the Kena 
Mudiria, the area is 7,000 square metres and the width 800 metres, 
while in Beni Suef the mean area is 8,000 square metres and the 
mean width 1,000 metres. Speaking generally it may be stated that 
where the Nile valley is narrow the slope of the river is small, its 
depth great and width contracted ; while where the valley is broad 
the slope is great, the depth small and the width enlarged. The mean 
velocity in flood ranges betwen 2.0 metres and 1.0 metre per second, 
while the velocity in summer varies from 0.5 to 0.9 metre per second. 
We may say that the Nile in soil has a natural section whose width 
in flood is 110 times its depth, while its mean velocity is 1.50 metres 
per second. 
The natural canals, which take off the river and which never silt, 
have a mean velocity of some 65 centimetres per second, while the 
proportion of width to depth is about 12 to 1. Artificial canals of 
this section do not silt if their velocities are 80 centimetres per second, 
while silting takes place as readily when the velocity is greater as 
when it is less than the above. In muddy streams, like the Nile in 
flood, certain velocities demand certain proportions of width to depth, 
and if these are not given to it, they will make it for themselves by 
eating away the sides if they can, or, if they cannot eat away the sides, 
by silting up and raising the bed. 
To the north of Assiout is situated the Assiout weir or barrage 
across the Nile with 111 openings of 5 metres and 10 metres depth 
of water in high flood. It was regulated on for the first time in 
August 1902. 
4 
