│ │ │ General │The most crowded conditions with │ │ average │ five-story and higher, contiguous │ │ │ buildings in poor class districts. │ │750–1000 │Five and six-story contiguous flat │ │ │ buildings. │ │500–750 │Six-story high-class apartments. │ │300–500 │Three and four-story dwellings, business│ │ │ blocks and industrial establishments. │ │ │ Closely built up. │ │100–300 │Separate residences, 50 to 75–foot │ │ │ fronts, commercial districts, │ │ │ moderately well built up. │ │ 50–100 │Sparsely settled districts and scattered│ │ │ frame dwellings for individual │ │ │ families. │ │ 0–50 ────────────┴────────────────────────────────────────┴────────┴──────── The density of population in Cincinnati from 1850 to 1913 with predictions to 1950 is given in Fig. 9.[18] This shows the densities for the entire city and is illustrative of the manner in which future conditions were predicted for the design of an intercepting sewer. The data given in Table 5 are of value in estimating the densities of population in various districts. The Committee on City Plan of the Board of Estimate and Apportionment of New York City obtained some valuable information on this point, especially in Manhattan. Three-story dwellings with 18–foot frontage, or four-story flats with 20–foot frontage, presumably contiguous, were found to hold 100 persons to the acre. Five-story flats held 520 to 670 persons per acre. Six-story flats held 800 to 1,000 persons per acre, and high-class six-story apartments held less than 300 per acre. =22. Changes in Area.=—In order to determine the probable extent of a proposed sewerage system it is important to estimate the changes in the area of a city as well as the changes in the population. With the same population and an increased area the quantity of sewage will be increased because of the larger amount of ground water which will enter the sewers. Predictions of the area of a city are less accurate than predictions of population because the factors affecting changes cannot be so easily predicted. An area curve plotted against time would be helpful in guiding the judgment, but its extension into the future based on past occurrences would be futile. A knowledge of the city, its political tendencies, possibilities of extension, and other factors must be weighed and judged. The engineer, if he is ignorant of the city for which he is making provision, is dependent upon the testimony of real estate men, business men and others acquainted with the local situation.