SEWER DIAGRAMS 187 



It will be noticed that for values of # = .5, i.e.; for 24-inch 

 pipe, the velocities are identical, but that for smaller sizes, 

 Weisbach's formula gives velocities that are too large, and for 

 larger sizes, velocities that are too small. The difference is 

 more marked in a comparison of quantities, especially for the 

 larger sizes. For example, the difference for a 6o-inch sewer 

 on a grade of i : 500 is that between a discharge of 100.2 

 and 119.8 cubic feet per second, or nearly 20 cubic feet per 

 second, which is about the caparcity of a 24-inch pipe. 



A recent book (1910) on water supply and sewerage, in spite 

 of the discrepancies between the values of the old formulae 

 and the more modern ones, has included in its text a plate 

 giving values of v and Q for varying values of d and s, basing 

 the diagram on Latham's old tables. From the diagram, the 

 values given in Table XXV in the columns marked Weis- 

 bach can be checked and the differences shown there would 

 be found also in any designs based on the diagram referred to. 



All the diagrams consider the four quantities, size of pipe, 

 velocity of flow, grade, and discharge, while the element of 

 roughness is left out, considering that it is the same for all the 

 quantities included in the diagram; and if, as when the sewer 

 changes from pipe to brick, it is necessary to change n, 

 another diagram has to be made. It is possible to construct a 

 diagram having any two of the above quantities as ordinates 

 and abscissae, while the other two quantities appear as curves 

 crossing the axial lines, and each other at various angles. 

 Further variations can be made by constructing the dia- 

 grams in parts, each to a different scale; by using a logarithmic 

 scale for one or both axes; by laying off, instead of the diam- 

 eters or the corresponding values of R, the values of the square 

 root of R. The grade may be expressed and drawn in per cent, 

 in feet per mile, or in number of feet for a fall of one foot. 

 Separate diagrams have to be prepared for brick sewers and for 

 pipe, for circular sewers and for egg-shapes, so that for com- 

 pleteness three diagrams, however made, should be provided. 



The diagrams in Plates 3, 4, and 5 are given with the idea 



