304 



FLO"^ OF STREAMS IN CORNWALL. 



TABLE No. V. 



Details of Flow, Truthan Basin, 1865-6. 



Month. 



January 



February 



March 



April 



May 



June 



July 



August 



September 



October 



November 



December 



1866 



1864 

 1865 



Rain 

 Fall. 



5-72 

 513 

 4-65 



3-80 

 1-43 

 1-45 

 419 

 4-88 

 0-74 

 8-83 

 4-65 

 350 



Discharge at Truthan. 



Max. 



Min. Mean. 



Cubic feet per minute. 



37000 



407-00 



18200 



180-0 



28-8 



26-4 



1600 



32 00 



39-00 



191-56 



211-50 



190-00 



The geological formation is Devonian, which, imaecompaniecl 

 by flat boggy land, is very favourable to the rapid flow of water. 



It will be seen by an examination of the table, that the maxi- 

 mum and minimum flows occiu' in February and July, and are 

 respectively 260^50 and 10^25 cubic feet per square mile per 

 minute. 



Comparing this with other observations made for me at 

 "Wrafton Weir, North Devon, in the same years, with a basin of 

 an area of 6-8 square miles, the Weir being 20 feet above the 

 Ordnance Datum ; the hills around rising to 300 to 600 feet above 

 that level, and of the same geological formation ; the maximum 

 and minimum flows gauged were, in January and July, 231 and 

 29 cubic feet per minute per square mile. 



The ordinary summer flow (not the minimum) of the Plym at 

 Sheepstor, with a drainage area of 7^609 miles, at an elevation 

 of 800 feet, with hills around rising to 1500 feet, is 71.4 cubic 

 feet ]Der minute per square mile.*' 



The amount of evaporation and absorption shewn as taking 

 place in the Truthan basin, is 1 6 Ins. per annum. The remain- 

 ing rainfall flowing off the ground by streams. Comparing this 



* Peardmore's Manual of Hydrology, p 138. 



