FLOW OF STREAMS IN CORNWALL. 305 



with other districts, evaporation and absorption vary between 

 the limits of 1 and 1 8 Ins ; the former amount appljdng to steep 

 precipitous mountains of non-absorbent rock, whilst the latter 

 takes place on flat spongy moorland or cultivated ground. At 

 Eivington Pike, Lancashire, a steep moorland district, it is 15 

 Ins., at Glencoose 17 Ins.,f at the Manchester Waterworks 

 Reservoir 17 Ins.j and at Paisley Waterworks 15 Ins.§ 



From these observations it appears that the mean Summer 

 flow of a stream in Cornwall from the Devonian strata, may be 

 roughly taken at one tenth of its average winter flow, exclusive 

 of exceptional floods, for in these cases the discharge of water 

 is enormous, the average summer flow often being not gJoth of 

 what a flood will bring down in a day. Taking the heavy fall 

 of 2-30 Ins. at Truro, on May 18th, 1852, || and assuming such 

 a fall to have occurred after a period of continuous rain, it is 

 probable that at least 1"50 Ins. would run off the ground in 24 

 hoiu's, giving a flood of 2420 cubic feet per square mile per 

 minute, or about 240 times the average July flow. 



As cultivation increases, and more attention is paid to field 

 drainage, and river courses are straightened and improved, these 

 floods are brought down more quickly and in greater volume into 

 the lowlands ; and it then becomes necessary that additional pre- 

 cautions should be taken to guard against the consecjuences that 

 may ensue therefrom. 



t Beardmore's Manual of Hydrology, p 137. 

 Ditto ditto. p. 310. 



X Bateman. 



§ Leslie Proc. Inst. C.E. Vol. 31 p 33. 

 II Beardmore's Manual of. Hydrology, p 320. 



