180 



EGYPTIAN AGRICULTURE. 



would cease to .act when its speed was reduced below a 

 certain point, and if ran too fast would not turn out a 

 correspondingly increased volume of water. 



With an archimedean screw, the height of water on the 

 down-stream side may vary without interfering with its 

 work, but the discharge channel should maintain an 

 uniform level. The limit of lift is about one third of the 



. , ^^pr^ HE 



Fir,. 57 li. CONSTRUCTION OF TIU: AHCIII.MKIU.YX S<T,K\\ . 



length, and length is limited by the heavy weight of water 

 and tube which at Khatatbeh amounted to 70 tons and 

 proved too much for safe working. The angle of the tube 

 with the horizon is usually 33 and the angle of the spiral 

 45, but variation of either of those angles involves alter- 

 ing the other. 



Scoop Wheel. Otherwise called Dutch wheel. Except 

 at Atfeh on the Mahmoudieh Canal these wheels are not in 

 use in Egypt. In Holland, where they originated and 

 where they are often used in connection with windmills, 

 they have been largely displaced by centrifugal pumps. 



