90 SEWER DESIGN 



15 is given, taken from the report on the drainage of the city 

 of the surveys, gagings, and observations made by the city 

 of New Orleans, 1895. These curves are based upon the results 

 engineer's department under the advice of the advisory board 

 during the years 1893 an d 1894, and upon a comparison of 

 these results with those of similar observations in other cities 

 presenting like conditions. 



Fig. 1 6 shows an ingenious arrangement which converts the 

 solution of the McMath formula into a mechanical process. 

 The logarithms of the quantities involved are taken and plotted 

 to form the runner and scale of a slide-rule. The device is 

 the invention of Mr. A. S. Crane, formerly of the Department 

 of Sewers, Brooklyn, N. Y., and has recently been largely 

 used in determining the sizes of storm-water sewers for that 

 city. 



By either of the two ways just outlined, viz., by estimat- 

 ing the probable future population of each district of the city 

 and, by Table IX of Chapter V, noting the percentage of rain- 

 fall that may be expected to run off, the rainfall having been 

 determined by the diagrams explained in Chapter III; or else, 

 more quickly but less intelligently, by using one of the formulae 

 or diagrams of this chapter, the amount of storm-water to be 

 cared for by the sewer can be found. In the report already 

 alluded to, Mr. McMath shows that, according to the experience 

 at St. Louis, the Biirkli-Ziegler formula gives, except in the case 

 of small areas, insufficient amounts. Comparisons might be 

 made in a similar way for all the formulae and diagrams extant, 

 but as each has been made to accord with some special data, 

 a discrepancy only shows that the amount of run-off varies in 

 different cities and localities. From the method of construc- 

 tion, the formula of Mr. McMath must give the best possible 

 results for St. Louis, and similar formulae might be built up for 

 other cities having an equally long sewer experience. Except- 

 ing only the use of a formula made up in the manner of that 

 for St. Louis, no method can give as intelligent and reliable 

 results as that detailed in Chapter V. 



