292 TURBINES. 



COR. When F 2 = 2gJi, we have, from (3), 



efficiency = 0.5. 

 When F 3 = tyh, we have, from (3), 



efficiency = -. 7=-. = .85. 



2(2- V%) 



When F 8 = 6gh, we have, from (3), 

 efficiency = 0.9. 



Hence, theoretically, the centrifugal pump has a con- 

 siderable efficiency when the velocity of rotation 

 exceeds the velocity due to twice the height of the col- 

 umn of water raised. 



Sen. Centrifugal pumps work to the best advantage 

 only at the particular lift for which they are designed. 

 When employed for variable lifts, as is constantly the case 

 in practice, their efficiency is much reduced, and does not 

 exceed .5, and is often much less. 



The earliest idea of a centrifugal pump was to employ an 

 inverted Barker's Mill, consisting of a central pipe dipping 

 into water, connected with rotating arms placed at the level 

 at which water is to be delivered. The first pump of this 

 kind which attracted notice was one exhibited by Mr. Ap- 

 pold in 1851, and the special features of this pump have 

 been retained in the best pumps since constructed. The 

 experiments conducted at the Great Exhibition on Appold's 

 Centrifugal Pump with curved arms, gave the maximum 

 efficiency 0.68. But when the arms were straight and ra- 

 dial, the efficiency was as low as .24, showing the great 

 advantage of having the curved form of the arms, which 

 causes the water to be projected in a tangential direction. 



160. Turbines. A reaction wheel is defective in prin- 

 ciple, because the water after delivery has a rotatory veloc- 

 ity, in consequence of which a large part of the head is 



