PROPORTION REACHING THE SEWERS 65 



and 38 cubic feet per second, or 52, 32, and 78 per cent. These 

 districts are 22 square miles, 8.3 square miles and 0.15 square 

 mile in area, and the densities of population are given as 

 J 9> 5-3> an d 35 P er acre, the last being in the centre of the 

 business district of South Chicago. This verification of the 

 work of the engineers of 1860 is a tribute to the work of those 

 pioneers. 



Mr. Parmley, after much experience hi Cleveland, believes* 

 that, for safety in business districts, 100 per cent of the rainfall 

 should be expected to reach the sewers. For residence districts, 

 he suggests 20 to 50 per cent, although he says that where 

 the lots are not large and the district is well built up, the per- 

 centage of the rain entering the sewer may be 70 per cent. 



In Kansas City, approaching the problem from the other 

 side, Mr. Balcomb f has assigned values to the amount of 

 rain in inches per hour that may be expected to be absorbed 

 by surfaces of various soils. Thus he assumes that paved 

 streets absorb 0.50 inch per hour at the beginning of a storm, 

 decreasing to 0.25 inch at the end of fifteen minutes, and that 

 garden soils absorb i.oo inch at the beginning, decreasing to 

 o at the end of 120 minutes. Most engineers, however, prefer 

 the direct percentage, although absorption is undoubtedly 

 one of its important factors. 



Some experiments were carried on by students { of the 

 College of Civil Engineering (Cornell) in 1910, in which it was 

 found that from a residential area of 42 acres on a steep side- 

 hill, the maximum percentage of rainfall intensity shown in the 

 sewage flow was 34.8. The resident population, exclusive of 

 students, was 22 per acre, and was not different from other sub- 

 urban property where there is one house and a lawn to every 100- 

 foot lot with occasionally one not built on. In the thesis based 

 on these experiments, it was pointed out that the needs of 

 growing vegetation should be considered, since the amount 



* Jour. Assn. Eng. Soc., Vol. XX, p. 212. 

 t Jour. West. Soc. Engrs., Vol. XV, p. 707. 

 J Thesis by P. Z. Horton and R. Taylor, 1910. 



