DRIVEN MACHINES 105 



pumps operate without air-chambers and supply water 

 in a continuous stream. They may be speeded up to 

 throw water under high pressure for fire fighting, but 

 for economy in ordinary use the speed is kept down to 

 200 revolutions, or thereabout. Rotary pumps are also 

 made with one single water chamber cylinder. The 

 pump head, or shaft, is placed a little off center. A 

 double end cam moves the water. Both ends of the 

 cam fit against the bore of the cylinder. It works 

 loosely back and forth through a slotted opening in the 

 pump head. As the shaft revolves the eccentric motion 

 of the double cam changes the sizes of the water-pock- 

 ets. The pockets are largest at the intake and smallest 

 at the discharge. Rotary pumps are comparatively 

 cheap, as regards first cost, but they are not ecbnomi- 

 cal of power. In places where the water-table is near 

 the surface of the ground they will throw water in a 

 very satisfactory manner. But they are more used in 

 refineries and factories for special work, such as pump- 

 ing oil and other heavy liquids. 



Centrifugal Pumps. The invention and improve- 

 ment of modern centrifugal pumps has made the lift- 

 ing of water in large quantities possible. These pumps 

 are constructed on the turbine principle. Water is 

 lifted in a continuous stream by a turbine wheel re- 

 volving under high speed. Water is admitted at the 

 center and discharged at the outside of the casing. 

 Centrifugal pumps work best at depths ranging from 

 twenty to sixty feet. Manufacturers claim that farm- 

 ers can afford to lift irrigation water sixty feet with a 

 centrifugal pump driven by a kerosene engine. 



The illustrations show the principle upon which the 

 pump works and the most approved way of setting 



