9^ The Drainage of Fens and Low Lands. 



and nuts, and in case of any substance finding its way into 

 the pump sufficiently large to cause a stoppage, the taking to 

 pieces and putting together is a work occupying only a very 

 short time. The bearing of the driving shaft should be taken 

 off the pump case by a sliding iron seat fixed close up to the 

 case, which can be pushed away when the glands of the pump 

 require packing. 



Pumps of this description require charging with water before 

 they can be started. This can be done by a small donkey 

 pump, or by exhausting the air by a steam jet. In the latter 

 case the outlet pipe and flap or non-return valve, which in all 

 cases is necessary to prevent the back-flow of the water when 

 pumping is stopped, are made sufficiently air-tight to allow 

 the water to flow up and fill the pump. In the former case a 

 valve opening inwards is placed on the inlet, which has also a 

 perforated rose at its termination to prevent, as far as possible, 

 the entrance of foreign substances. In large drainage pumps 

 a special air pump is provided for chai'ging the pump. 



In order to avoid unnecessary lifting of the water the dis- 

 charge pipe is carried down below the lowest level of the 

 water on the outer side. The lift is then not greater than the 

 difference of level of the inner and outer channels, the suction 

 and delivery pipe acting as a syphon. 



The turbine form of pump has a vertical spindle, and must 

 be placed below the water, at the lowest level from which the- 

 water has to be pumped. The earlier forms were made with 

 the fan divided into two parts with a centre diaphragm, but 

 pumps of more recent construction have only a single fan with 

 one inlet When the water enters on one side of the wheel 

 only, it causes a thrust in that direction. This thrust is made 

 use of to counterbalance the weight of the driving shaft and 

 pump vanes ; then after the pump is once in motion all weight 

 being thus taken off the bearings. For the same quantity of 

 water to be discharged the single pumps necessitate the fans 

 being made of larger diameter than those having double fans. 



