290 THE CENTRIFUGAL PUMP. 



When F 2 = 2gh, we have, from (5), neglecting friction, 



(-v/2 l) F 2 

 efficiency = ^- y - = 0.828, (7) 



about 17 per cent o'f the energy of the fall being carried 

 away by the water discharged. The actual efficiency real- 

 ized of these machines appears to be about 60 per cent, so 

 that about 22 per cent of the whole head is spent in over- 

 coming frictional resistances, in addition to the energy 

 carried away by the water. 



SCH. The reaction wheel in its crudest form is a very 

 old machine known as " Barker's Mill" It has been em- 

 ployed to some extent in practice as an hydraulic motor, the 

 water being admitted below and the arms curved. In this 

 case the water is transmitted by a pipe which descends be- 

 neath the wheel and then turns vertically upwards. The 

 vertical axle is hollow, and fits on to the extremity of the 

 supply pipe with a stuffing box. In this construction the 

 upward pressure of the water may be made equal to the 

 weight of the wheel, so that the pressure upon the axis may 

 be nothing. These modifications do not in any way affect 

 the principle of the machine, but the frictional resistances 

 may probably be diminished. 



159. The Centrifugal Pump. When large quanti- 

 ties of water are to be raised on a low lift, no pump is so 

 suitable as a centrifugal pump. In this pump, water is 

 raised by means of the centrifugal force given to the water 

 in a curved vane or arm, proceeding from the vertical axis. 

 The dynamic principles of this machine are the same as 

 those of the reaction wheel (Art. 158) ; but they differ in 

 their objects. In the latter machine, a fall of water gives a 

 rotatory motion to a vertical axis, while in the former a 

 rotatory motion is given to a vertical axis in order to ele- 

 vate a column of water. 



