DEVELOPMENT OF FORMULAE FOR FLOW 



167 



It was noticed, however, that while this formula agreed 

 with the experiments for certain conditions of slope and 

 velocity, it would not hold for others; so that, as an improve- 

 ment, Ruhlman and Weisbach deduced from the same experi- 

 ments varying values of the constant to correspond with the 

 varying values of velocity. The following table gives the 

 values of c varying with v as given by Weisbach. 



TABLE XXII 



The table shows the values of c, for velocities common to 

 sewers, to lie between 89 and 91, which are undoubtedly cor- 

 rect for certain kinds of channels. But, as will be seen later, 

 the physical conditions of the channel also affect the values 

 of c, so that, without knowing the conditions of the experi- 

 mental channels on which these values of c are based, the 

 results are uncertain for general use. 



Baldwin Latham gives very elaborate tables based on these 

 values of c, giving grades necessary to produce velocities of 

 from 2 to 10 feet per second in pipes flowing f , |, f , and full, 

 the pipes being both circular and egg-shaped. Similar tables 

 are given for discharge. Except that diagrams are now so 

 largely used, a reproduction of these tables with better values 

 of v might well be made, for their convenience and general 

 adaptability are remarkable.* 



According to Dubuat, de Prony, and all hydraulicians 

 up to their time, differences in roughness in the wet perimeter, 

 or irregularities in the direction of the stream, had no effect 

 on the value of the coefficient. It was assumed by Dubuat 

 that a layer of water adheres to the walls of the pipe or chan- 

 nel, and is therefore to be regarded as the wall proper sur- 



*The elaborate treatise on Sanitary Engineering, by Col. E. C. S. Moore, 

 published early in 1899, contains such tables as are here suggested. 



