November 10, 1905.] 



SCIENCE. 



585 



was no sooner finished than it was con- 

 demned as useless. 



EGYPTIAN IRRIGATION SINCE THE ENGLISH 

 OCCUPATION. 



With the English occupation in 1883 

 came some English engineers from India, 

 who, supported by the strong arm of Lord 

 Cromer, soon changed the situation. The 

 first object of their attention was the 

 Ijarrage at the head of the delta, which was 

 made thoroughly sound in six years and 

 capable of holding up fifteen feet of water. 

 Three great canals were taken from above 

 it, from which a network of branches are 

 taken, irrigating the province to the left 

 of the western, or Rosetta branch of the 

 river, the two . provinces between the 

 branches, and the two to the right of the 

 eastern, or Damietta brslnch. 



In upper Egypt, with one very impor- 

 tant exception (the Ibrahimieh Canal, 

 which is a perennial one), the early flood 

 system of irrigation, yielding one crop a 

 year, prevailed until very recently, but it 

 was immensely improved after the British 

 occupation by the addition of a great num- 

 ber of masonry head sluices, aqueducts, es- 

 cape weirs, etc., on which some £800,000 

 was spent. With the completion of these 

 works, and of a complete system of drain- 

 age, to be alluded to further on, it may be 

 considered that the irrigation system of 

 Egypt was put on a very satisfactory basis. 

 There was not much more left to do, unless 

 the volume of water at disposal could be 

 increased. 



Probably no large river in the world 

 is so regular as the Nile in its periods of 

 low supply and of flood. It rises steadily 

 in June, July and August. Then it be- 

 gins to go down, at first rapidly, then 

 slowly, till the following June. It is never 

 a month before its time, never a month 

 behind. It is subject to no exceptional 

 floods from June to June. Where it enters 



Egypt the difference between maximum 

 and minimum Nile is about twenty -five feet. 

 If it rises three and one half feet higher 

 the > country is in danger of serious flooding. 

 If its rise is six feet short of the average 

 there existed in former days a great risk 

 that the floods would never cover the great 

 flats of upper Egypt, and thus the ground 

 would remain as hard as stone, and sowing 

 in November would be impossible. Fortu- 

 nately the good work of the last twenty 

 years very much diminishes this danger. 



THE ASSUAN DAM AND RESERVOIR. 



In average years the volume of water 

 flowing past Cairo in September is from 

 thirty-five to forty times the volume in 

 June. Far the greater part of this flood 

 flows out to the sea useless. How to catch 

 and store this supply for use the following 

 May and June was a problem early pressed 

 on the English engineers in Egypt. 



During the time of the highest flood the 

 Nile carries along with it an immense 

 amount of alluvial matter, and when it 

 was first proposed to store the flood-water 

 the danger seemed to be that the reservoir 

 would in a few years be filled with deposit, 

 as those I have described in India. Fortu- 

 nately it was found that after November 

 the water was fairly clear, and that if a 

 commencement were made even as late as 

 that there would still be water enough 

 capable of being stored to do enormous 

 benefit to the irrigation. 



A site for a great dam was discovered at 

 Assuan, 600 miles south of Cairo, where a 

 dyke of granite rock crosses the valley of 

 the river, occasioning what is known as the 

 First Cataract. On this ridge of granite 

 a stupendous work has now been created. 

 A great wall of granite 6,400 feet long has 

 been thrown across the valley, 23 feet thick 

 at the crest, 82 feet at the base. Its height 

 above the rock-bed of the river is 130 feet. 

 This great wall or dam holds up a depth 



