82 Beport of the Bilharzia Mission in Egypt, 1915 



given time, will succeed in traversing several inches of sand if there 

 is a continuous flow of water through it. Unlike the ankylostome 

 larvae, they are unable to traverse ordinary filter-paper. 



Temperaturk of Nile Water at Hawamdia, near 



Cairo. 



January 



February 



March 



April 



IMay 



June 



July 



August 



September 



October 



November 



December 



(From " The Physiography of the Nile," by G. H. Lyons.) 



In the spring of 1916 further experiments were 

 Water made at the request of Surgeon-General Sir W. 



i ILTRATION 



Babtie to determine the degree of protection afforded 

 by a modified Jewell system of filtration which it was proposed to 

 instal at various points on the Sweet-water Canal. This system 

 consists in the addition of alum in a settling tank prior to filtration 

 through sand about a metre in depth, and provides for six possible 

 traps for the Bilharzia cercaricB during the passage from the source 

 to the consumer, viz. : — 



(r() In the settling tank .. (1) Time factor. 



(2) Exposure to oxygen. 



(3) Chemical action of alum. 



(4) Arrest from entanglement iu the tloccu- 



lent precipitate. 

 {b) In the filter .. .. (5) Arrest on the surface by the " vital layer." 



(6) Arrest due to depth of sand. 



A working model was kindly supplied by Mr. McCroquidale, 

 manager of the Cairo Waterworks, and the rate of flow, head of 

 water and depth of sand were identical with those adopted for the 

 field systems, the only difference being in the superficial area of 

 filter and settling tank. 



The following conclusions were formulated : — 



(1) The B. cercarice survive and remain actively swimming for 

 a much longer period than the time (five to eight hours) that the 

 water takes to pass through the settling tank. 



- (2) Oxygen has a stimulating effect on the cercarite and is a 

 necessity for their continued activity. 



(3) Alum in the dilutions used for sedimentation of canal 

 water has no effect on the B. cercarice. 



