The Archimedian Screw Pump, 89 



driven by a spur wheel at the top geared into a bevel wheel 

 and shaft 



The water level on the inlet side may vary without affecting 

 the efficiency of the pump, except so far as the increased 

 weight is concerned, due to the greater length required to 

 meet the variation. But any change in the level on the 

 delivery side immediately affects the efficiency. These 

 pumps are not therefore adapted for use where there is much 

 change in the level of the water into which they discharge. 



The angle which the pump forms with the horizon when 

 fixed varies according to the ideas of different constructors, 

 but generally it may be taken that the most efficient position 

 for the pump is when the angle of tilt is rather less than the 

 spiral angle. Thus, for a machine having a spiral angle of 

 40° the angle of tilt for the pump should be 30°. The spiral 

 angle is the form which the screw assumes with reference to 

 the core, and is the angle made by a tangent drawn to the 

 spiral on the cylindrical core, and a vertical line parallel to 

 the axis of the cylinder. This angle varies from 30° to 60°. 

 The Romans usually made it 45°. In the most effective 

 machines it varies between 30*^ and 40°. 



The discharging power of these pumps varies so much with 

 the different circumstances under which they are woi'ked, 

 depending on the number of threads, the angle at which they 

 are placed, the angle at which the pump works, and other 

 matters, that it is difficult to give any precise formula for the 

 quantity discharged. Upon pumps working under nearly 

 similar conditions the discharge is as the cube of the diameter, 

 and approximately it may be taken that, under favourable 

 conditions, a pump i foot in diameter will discharge 0*32 

 cubic feet of water for every revolution. The number of 

 revolutions varies according to the kind of power applied and 

 the size of the pump. Small pumps of about i foot in 

 diameter may be run at sixty revolutions a minute, the larger 

 not reaching more than twenty. For drainage purposes it 



