60 Beport of the Bilharzia Mission in Efjypt, 1915 



still be infective when passing the city. Minia is a little more than 

 this distance from Cairo, so that all the drainage water which, as 

 we have seen, discharges into the Nile between Minia and El Ayat 

 would still be infective at Cairo during the time of high Xile. 

 During the summer months when the Nile is at low stage the 

 maximum velocity at Wasta (see fig. 25) varies between thirty and 

 thirty-three kilometres per diem, according to Craig. Farther down 

 stream this will be less as the Delta Barrage is reached, owing to 

 decrease of slope. About thn-ty kilometres per day would be a fair 

 average over this reach. According to this, active cercaria would 

 only travel thirty miles in a day and a half, so that infected water 

 entering the Nile more than thirty miles up-stream, i.e., above El 

 Ayat, should have become innocuous by the time it reached Cairo 

 except in so far as occasional infected molluscs may be carried down 

 by the current. 



If the facts upon which these conclusions are based are approxi- 

 mately correct, the Nile at Cairo, and therefore the unfiltered water 

 supply, should be infective chiefly during the autumn ; the source 

 of infection during the rest of the year being apparently limited to 

 the escapes between El Ayat and Cairo. 

 Velocities of Down stream from Cairo, water only re-enters 

 THE Canals ii<i the Nile and the main canals by seepage. From 

 THE Delta. September to December water takes I'B to 1*9 days 

 to travel from the Delta Barrage to the sea. From January to 

 April the period gradually lessens from 2'1 to 2'6 days ; in May, 

 June and July 28 days are occupied. From this we conclude 

 that even during Nile flood the branches of the Nile and the main 

 canals in the northern half of the Delta are less liable than the 

 Nile above Cairo to be infective. The bulk of infections in the 

 Delta must therefore originate directly from the small tertiary 

 canals, the agricultural drains and the large drains which carry 

 the discharge from these to the sea. 



Infection in the Maeitime Canal Zone. 

 This brings us to a consideration of the Ismailia or sweet water 

 canal carrying fresh water from the Nile north of Cairo to Ismailia, 

 Suez and Port Said. 



The Ismailia Canal from Cairo to Ismailia is 



wIte/cTnIl -^^^ kilometres, i.e., 80 miles. From Ismailia to Suez 



90 kilometres, i.e., 56 miles, from Ismailia to Qantara 



it is approximately 34 kilometres, and thence to Port Said about 



43 kilometres The maximum velocity on the Ismailia Canal is near 



