To obtain information about the trough of the Nile, the area exposed to evaporation and the area of absorption, a longitudinal section of the Nile from Assuân to Cairo has been levelled, and cross sections taken at every 3 kilometres. The kilometrage on Plate XII counts from the Assuân gauge and is measured down the centre of discharge of the flood, since it is with flood discharges that we are principally dealing. As the Nile winds about considerably and is often broken into numerous channels, the areas of the cross sections vary very appreciably according as they are taken at right angles to the centre line of discharge or of the deep channel of the river. The former gives the more reliable results. I have taken 8 millimetres per day as the evaporation during flood in Upper Egypt. The absorption has been calculated from the water consumption during the floods of 1892 and 1893, and found to be about 300 cubic metres per second between Assuân and Assiout, where there is practically no perennial irrigation. Between Assiout and Cairo, where there is a considerable length of perennial irrigation on one bank and limestone rock on the other, the absorption is about 100 cubic metres per second. When perennial irrigation has once established itself in Upper Egypt, we may assume that the absorption during flood will be halved in quantity for the reasons given above, and become 150 cubic metres per second between Assuân and Assiout, and 50 cubic metres per second between Assiout and Cairo. The amount of water expended in irrigation will be about 700 cubic metres per second. The evaporation during flood will be approximately 120 cubic metres per second. The quantity of water needed to fill the trough of the Nile will depend on the gauges and may be calculated from table 43 of Appendix K. The last item will be the only variable one and the others may be approximately tabulated as follows:-- _Expenditure of water in flood in cubic metres per second_: BETWEEN ASSUÂN BETWEEN ASSIOUT TOTAL AND ASSIOUT. AND CAIRO. Perennial irrigation 350 350 700 Evaporation 65 55 120 Absorption 150 50 200 --- --- ---- Total of above 565 455 1020 === === ==== Taking these quantities and calculating directly for the filling of the trough from the gauges themselves, I have collected in Tables 50 to 52, the Cairo gauges corresponding to the Assuân gauges for the high years 1874 and 1878 and the minimum year 1877. As far as the more important results are concerned, I tabulate them here:-- =Gauges at Assuân and Cairo.=