Drainage by Gravitatmi, 9 



for the engines erected for the drainage of the East Fen in 

 Lincolnshire, and by Sir John Coode as the maximum quan- 

 tity to be lifted by the engines proposed to be erected for the 

 drainage of the North Level. 



Taking the rainfall in the Fen district as a guide, it may be 

 estimated that provision should be made for a daily rainfall 

 equal to about '0076 of the average annual rainfall of wet 

 seasons. 



In Holland, the quantity generally calculated as having to 

 be lifted off the Polders varies from a quarter to three-eighths 

 of an inch of rain in twenty-four hours. The rainfall at Lake 

 Haarlem averaged 3 1 ' 27 inches for the ten years ending 

 1872, and 32 inches for the ten years ending 1886, the great- 

 est fall being 39*13 inches in 1877, ^^ engines running 

 8056 hours that year, against 6823, the average for the ten 

 years. 



In Ireland, for the Rathdowney drainage on the river 

 Erkina, provision was made in the channel for a discharge of 

 600 cubic feet per square mile per minute, equal to a continu- 

 ous rainfall of about three-eighths of an inch in twenty-four 

 hours. The soil was a deep alluvium, the drainage area 

 containing S^'7 square miles of low-lying land on the lower 

 limestone.* At the "V\%xford Harbour Reclamation Works, 

 where the rainfall of wet years amounts to over 50 inches, 

 and the mean from 45 to 48 inches, it was estimated that 

 three-fourths of this quantity, or 34' 2 inches, would have 

 to be pumped ; but the machinery was made of sufficient 

 capacity to lift nearly an inch of rainfall in twenty-four hours. 



Tidal Outfalls. — When the outfall of a main drain is into 

 a tidal stream, it has not only to be capable of discharging all 

 the water due to the rainfall, but also must be able to dis- 

 charge this water within a limited time, the doors of the 

 sluice being closed by the tide for a certain period twice a 

 day. This element has to be taken into consideration m 



* Trans. Inst. C.E., vol. lix. p. 265 



