EXAMPLES 243 



As / increases, this approximates to the value c or 



HI a" 

 By substitution in (9") the value of -~ at any instant and therefore of 



P may be obtained, and knowing this the pressure behind the valve may 



M r 



be readily obtained. 



EXAMPLES. 



(1) Experiments by Dr. Lampe on the 16 -inch asphalted pipes 

 of the Dantzic main gave results which for velocites between T6 

 and 3'0 feet per second agree with the value as deduced from the 



7, 



formula T = '000371 j^. Determine the corresponding values of / in 



I Cl 



f j o 



the formulae h =4- when v = 1*6 and 3*0 feet per second. 



Ar -00485. 



(2) Darcy's experiments on a cast-iron pipe '617 feet diameter give the 



h v 1 ' 95 



result = '000380-^5 with fair accuracy. Determine the velocity of 

 I cl 



flow and the discharge per minute from a similar pipe 1,000 feet long 

 under a head of 50 feet. 



Answer, j Velocity = 2'893 feet per second. 



I Volume = 51*9 cubic feet per minute. 



(3) The following is a convenient mnemonic for flow in clean cast-iron 

 pipes of diameters between 2 and 6 inches with a velocity of 3 feet 



per second; h = -, where h is in feet, d in inches. Taking Unwin's 



exponential formula as being correct, determine the percentage error for 

 pipe diameters 3, 6, 9, and 12 inches. 



/ 3 in. diameter. Error '62 per cent. 



*-* 9 6 ;; ;; ;; J 9 " ;; 



l 12 + 16-4 



(4) W. Cox (Engineering (1892), p. 613) gives the following formula. 



B 2 



