226 COEFFICIENTS OF FRICTION. 



which in (3) gives 



/* = 0.03026^, (7) 



a 9g 



which agrees with (5) of Art. 104 and (1) of Art 105. 



EXAMPLE. 



How much fall must a canal, whose length is 2600 feet, 

 whose lower width is 3 feet, whose upper width is 7 feet, 

 and whose depth is 3 feet, have in order to carry 40 cubic 

 feet of water per second ? Here we have 



p 3 + 2\/22~+~3 2 ~ = 10.211, 

 = (7 + 3)3 = 18> 



40 8 



and 



Substituting in (5), we have 



15 



0.305422 x 10.211 x 6.4 

 15x9 



= 1.48 feet. 



122. Coefficients of Friction. The coefficient of 

 friction / varies greatly with the degree of roughness of the 

 channel sides, and somewhat also with the velocity, as in 

 the case of pipes, increasing slightly when the velocity 

 diminishes, and decreasing when the velocity increases. A 

 common mean value assumed for / is 0.007565, which we 

 used in the last Art., though it has quite a range of values. 

 Weisbach, from 255 experiments, obtained for / the follow- 

 ing values at different velocities : 



