— 94 — 
Tables 65 and 67 of Appendix L contain the Assuan and Cairo 
gauges for typical years in a period of twenty years from 1873 to 
1892, and the mean gauges of these twenty years. Finding it impos- 
sible to understand the Nile without first referring every gauge to 
some uniform standard, I have had to choose the line of reference. 
The mean high water level and the mean low water level are both 
available. In Egypt the mean high water level varies very consider- 
ably whether we take it in August and the early part of September 
when the basin canals are running full supply, or in the latter half of 
September whan the canals are running only half supply, or in October 
when the basins are discharging back into the Nile. Early and quick 
rising floods have a different series of levels from slow and late floods; 
while again the recent works carried out in Upper Egypt by Col. Ross 
have so increased the discharging capacity of the canals that the flood 
gauges have been appreciably affected. All this points to the conclu- 
sion that the mean high flood is no satisfactory standard. The mean 
low flood on the other hand is much less liable to change and is very 
fairly constant from year to year. High floods are certainly followed 
by scouring out of the bed, and low floods by a silting up of the 
channel, but the changes are very moderate compared to those in 
hiffh flood. I have chosen the mean low water level as the line of 
reference, and referred all gauges to it. From the mean of twenty 
years’ observations, this level at Assuan is R. L. 85 metres. By 
observations along the Nile generally, and by calculations at Cairo, 
I have fixed it at all important places north of Assuan. Table 46 
of Appendix K gives the Reduced Levels at different places, while 
it is also drawn on the longitudinal section of the Nile in Plate XII. 
It was on this system that the ancient Egyptian engineers worked 
the Nile. They however chose the mean high water level during 
the early part of the flood as their standard of reference and conse- 
quently made the so called cubits in the flood reaches of the Cairo 
gauge half cubits. This means a discharge of 1600 cubic metres 
per second and fairly represents the discharges of the basin canals in 
flood. When it is considered that the level of the Nile valley is 
raised by about 10 centimetres per 100 years it will be seen that 
the old Cairo gauge, which was a living record 1500 years ago, is 
to-day a meaningless anachronism. It has also to be compared with 
the Assuan gauge which was erected in Ismail Pasha’s time with 
an arbitrary zero some 90 centimetres below mean low water level, 
