170 SEWER DESIGN 



stant slope and constant roughness a series of experiments, 

 using formula in the form 



RS , 



^ = a+ F 



DO ... 



the ^-ordinates being values of or , and the #-ordinates, 

 values of . a then gives the distance of the origin from the 



A 



point where the F-axis is cut by the straight line, drawn as 

 nearly as may be through the points; and (i is the tangent of 

 the angle which the line makes with the X-axis. 



In this way four sets of coefficients were obtained, here 

 given in English measure. 



I. Cement and carefully planed wood : 



a = .000,046 ; ji = .000,0045 

 II. Smooth ashlar, brick, and wood: 



a = .000,058 ; & = .000,0133. 



III. Rubble masonry: 



a = .000,073 > = .000,0600. 



IV. Earth: 



a = .000,085; g = . 000,3500. 



V. Carrying detritus and coarse gravel: 



a = . 000,122; $ = . 000,7000. 



The last class was added later by Kutter. 



In his treatise M. Bazin says: " One must regret the sub- 

 stitution for a single simple formula of a new formula with 

 variable constants in the coefficients; but the indeterminate 



