90 The Drainage of Fens and Low Lands. 



may be taken that these pumps can be run at from twenty to 

 forty revolutions a minute. They have been used in Holland 

 to lift the water 15 feet Mr. Korevaer, a Dutch engineer, 

 who has investigated the matter, places the limit of height at 

 14 feet, and the limit of discharge at 3500 (pSi tons) cubic 

 feet per minute. The ten screws erected at Katatbeh, in 

 Egypt, discharged 137*5 tons a minute each, making five to 

 six revolutions a minute. The screws were enclosed in iron 

 cases, but were found unequal to the weight of water they had 

 to carry, and were consequently removed Where the amount 

 of water to be lifted exceeds the capacity of one pump it is 

 customary to couple two or more together, all worked by the 

 same engine. 



The useful effect of these pumps is about the same as scoop 

 wheels, and varies according to construction, from 50 to 80 

 per cent, of the power applied. 



The Dutch screw pumps are constructed to work without 

 an external casing, the wheel revolving in a semi-cylindrical 

 trough of masonry. The weight of the water is thus borne 

 on the masonry, and the screw is relieved of the strain. An 

 example of one of these pumps is given in Plate 4. 



Mr. W. Airy, in a paper contributed to the Transactions 

 of the Institution of Civil Engineers in 1871 (vol. xxxii.), gives 

 the results of experiments carried out by him to test the rela- 

 tive merits of screw pumps of different construction. The 

 results he arrived at were : — (i) That the smaller the spiral 

 angle, i.e. the quicker the spiral, the flatter must the machine 

 be laid to the horizon to produce its best effect. (2) That in 

 an equal number of revolutions the quicker spirals will lift 

 much more water than the slower ones. (3) That there is a 

 great difference in the effect of the machines according as one 

 end or the other is upwards. The advantage is greatly in favour 

 of the machine when placed so that the acute angle which the 

 thread makes with the core is downwards. With regard to the 

 number of threads, Mr. Airy is of opinion that every machine 



