CHAPTER VII 



ESTIMATING FUTURE POPULATION 



THE amount of storm-water reaching a sewer, and the con- 

 sequent size of the sewer, bear only an indirect relation to the 

 population on the area drained, but the number of people in a 

 given district is a direct function of the amount of domestic 

 sewage to be cared for. In order to determine, therefore, the 

 amount of house-sewage which a system of sewers must carry, 

 it is primarily essential to determine the population on the area 

 to be sewered. 



The number of persons on a given area may be approximately 

 determined at any time in several ways. The U. S. Census 

 reports, published every ten years, furnish a basis for an estimate 

 of the population for intermediate years, but as a sewer system 

 has always, to be designed for use during an indefinite number 

 of years in the future, some method of predicting the popula- 

 tion for that future time must be devised. It is usual to base 

 the prediction on two things. First, after noting the past 

 growth of the city in question, it is assumed that it will con- 

 tinue to increase regularly according to the law of its past. 

 Thus in Chicago, at the time of the first report of the Sanitary 

 Commission, the future population of the Sanitary District 

 was estimated in this way, as shown in Fig. 17. Curves were 

 drawn for other large cities and used as a guide, but they proved 

 of little value. Messrs. Hering and Gray used the same method 

 in their Baltimore report as shown in Fig. 18. They had recourse 

 to other sources of information besides the U. S. Census reports; 

 the police estimates of population, made every year, and obtained 

 by multiplying the voting population by a constant, were made 

 the basis of the prediction quite as much as the more authen- 

 ticated U. S. reports. 



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