VOLUME AND PERIODICAL EISING OF THE NILE. 71 



The two zones of alluvial land skirting the Nile are intersected by numerous 

 irrigation canals, which distribute the fecundating waters far and wide. Like 

 those of other streams flowing through alluvial plains, the banks of the river are 

 higher than the surrounding country. A cutting running transversely to the 

 direction of the valley would show that from cliff to cliff the plain presents the 

 form of a convex curve, so that at high water the stream occupies the most 

 elevated level between the ranges of hills bordering both sides of its valley. From 

 this central elevation the surface of the current inclines right and left, and the 

 slope is continued in both directions across the riverain plains. This disposition of 

 the groimd is due to the greater quantity of sedimentary matter deposited along 

 the banks of the stream. The waters have thus a double incline, that is, according 

 to the general direction of the river valley, and according to the lateral slope of its 

 banks. If they met with no obstacle in the irrigating canals, they would flow at 

 once to the lowest level on either side, and convert the whole depression into a 

 vast lake. Hence they have to be retained at the higher elevation by means of a 

 transverse dyke, which is opened only when the upper levels have been sufficiently 

 submerged. The overflow is then arrested in a second sectioii also confined by 

 embankment works, and in this way the water is distributed to every part of the 

 surrounding plains by a system of canals disposed at successive levels. 



Nevertheless the normal incline of the land has in many places been modified 

 by the local alluvial deposits, and by the action of opposing currents in the lateral 

 channels. The shifting sands brought by the winds from the neighbouring 

 escarpments have also here and there raised the low-lying plains to a level with, 

 and even higher than, the banks of the Nile, thus obliging the cultivators to change 

 the whole plan of their irrigation works. Formerly, when the Nile was inhabited 

 by five different species of the crocodile, the rising flood was preceded and heralded 

 by the siik, a small and harmless variety, which was accordingly Welcomed with 

 much ceremony by the villagers, and even honoured with divine worship in many 

 towns far removed from the Nile. Temples were dedicated to them, where they 

 were kept alive, decked with armlets and pendants, and fed on the flesh of victims. 

 But none of these saurians are now seen in the Egyptian Nile, even as high up as 

 Thebes, although the canals intersecting Cairo were still infested by them at the 

 beginning of the present century. None appear to be met below Ombos, south of 

 Asuan, and this region of the cataracts is also inhabited by electric fish. But the 

 hippopotamus has retreated still higher up to the neighbourhood of the Atbara 

 confluence. 



When the flood begins to subside, the water in the higher canals would at once 

 flow back to the main stream were it not retained by sluices, and thus stored to 

 meet the requirements of the following spring and summer. During the sub- 

 sidence the level of the overflow is still maintained in the plains some 18 or 20 

 feet above the bed of the main stream. The peasantry also utilise the waters 

 which filtrate laterallj^ into the ground to a distance of some miles, but so slowly 

 that the effect of the inundations is not felt for weeks and even months after the 

 normal period of the rising. Even within 300 or 400 feet of the Nile the water in 



