EXAMPLE. 



227 



In using this table for the value of/ when v is not known, 

 we must proceed by approximation. Determine v approxi- 

 mately from (4) of Art. 121. Then from this value of v 

 find / by means of the table, and substitute the value of / 

 so found in (2), and determine a new value of v. 



EXAMPLE. 



What must be the fall of a canal 1500 feet long, whose 

 lower width is 2 feet, upper width 8 feet, and depth 4 feet, 

 when it is required to convey 70 cubic feet of water per 

 second ? 



Here we have 



p = 2 + 2\/16 + 9 = 12, 

 a = 5 x 4 = 20, 

 v = W = 3.5', 



hence, from the table, 



/= 0.00784. 



Substituting in (1) of Art. 121, we have 



h = 0.00784 

 86.436 



~~ 64. r 



1500 x 12 3.5 a 

 20 X 2g 



= 1.34 feet. 



