100 The Drainage of Fens and Low Laiids. 



always charged and ready for starting; being also covered 

 by a considerable depth of water it is free from the action 

 of frost, which is liable to freeze the water if left in a pump 

 exposed to its action, and burst the case. The friction of the 

 water along the suction and delivery pipes necessary in the 

 other form of pump is also avoided. On the other hand, 

 the gain in efficiency due to a properly formed pump case is 

 sacrijficed. 



The action of a centrifugal pump is as follows :— As soon 

 as the fan begins to revolve the blades carry the water with 

 them, which is then pushed forward and drawn into the case 

 partly by the mechanical action of the blades propelling the 

 water forward, and partly by the centrifugal action created by 

 the rapid rotary motion created by the fan. The vacuum 

 created by the water driven out of the fan is immediately 

 filled by a fresh supply of water from the inlets. The water 

 driven out by the fan is propelled along the discharge passage, 

 and having no other means of escape, rises up the pump well 

 till the outlet is reached. A constant and continuous stream 

 without check or shock, as in bucket pumps, is thus created, 

 and the water is kept in motion along its whole passage. 



The best velocity for the water to flow through the passages 

 of the fan is from 6 feet to 8 feet per second. The discharge 

 increases with the increase of velocity, a small increase in the 

 number of revolutions producing a large increase in the 

 delivery. Mr. Parsons* states that he found an increase of 

 14 revolutions — 392 to 406, or about 3J per cent — increased 

 the discharge from 1012 gallons to 1753, or 42 per cent. 

 Up to a certain speed the pump does not act, and the fan 

 revolves without lifting the water over the overflow. Unless 

 the speed for which the pump is intended to be run at is 

 attained, the machine does not work at its best, and fuel is 

 wasted. It is important therefore that the engineman in 

 charge should know the velocity for which his pump is 



* * Trans.' lust. C.E., vol. xlvii. 



