64 The Drainage of Fens and Low Lands, 



The quantity of water due to rainfall has already been dealt 

 with in the chapter on ' Drainage by Gravitation.' Assuming 

 that the quantity to be raised to be that due to a quarter of 

 an inch in 24 hours, this is equal to about 2^\ tons per acre, 

 which, multiplied by the number of acres and the height to be 

 lifted, gives the work to be done from which the actual horse- 

 power of the engine required can be calculated. Thus, for 

 example, taking a district of 1000 acres, with a lift of 5 feet, 

 and daily rainfall of \ inch, this is equal to 39,388 lb. lifted 

 5 feet every minute, or 196,940 foot-pounds, which, divided by 

 33,000, the unit of i-horse power, is equal to 6-horse power. 

 Allowing that 50 per cent, is absorbed in overcoming the 

 friction of the machinery and leakage of the pumps, 12 

 I.H.P. would be required. This is on the supposition 

 that the engine during extreme floods is running night and 

 day, which in cases of emergency is generally done. If the 

 work is required to be done in less time, the power required 

 will be proportionately larger. While it is desirable to 

 provide adequate power, any unnecessary expenditure should 

 be avoided as adding to the dead weight of capital on which 

 interest must be paid. On the other hand, it is never desirable 

 to put too much strain on an engine ; a machine that is well 

 master of the work will run more economically than one that 

 is much pressed. 



The best advantage is obtained when the machinery runs 

 continuously night and day. If the drains are capable of 

 delivering a full supply, a pump works at its best, and the 

 scoops of a wheel being fully charged deliver their full 

 quantity with less head than if the water-level is lowered and 

 gathers again during the night. The coals used in getting 

 up steam and restarting are saved, and usually a better result 

 is obtained. If the machinery does not run at night, it will 

 have to be so much larger as to render it capable of dis- 

 charging the rainfall of 24 hours in 14 or 16 as the case may 

 be, and the drains must be proportionately large to supply 



