=21. Density of Population.=—So far the study of population has been confined to the entire city. It is frequently necessary to predict the population of a district or small section of a city. A direct census may be taken, or more frequently its population is determined by estimating its density based on a comparison with similar districts of known density, and multiplying this density by the area of the district. In determining the density, statistics of the population of the entire city will be helpful but are insufficient for such a problem. A special census of the area involved would be conclusive but is generally considered too expensive. A count of the number of buildings in the district can be made quickly, and the density determined by approximating the number of persons per building. Statistics of the population of various districts together with a description of the character of the district are given in Table 5. [Illustration: FIG. 9.—Density, Area, and Population, Cincinnati, Ohio. 1850 to 1950. ] TABLE 5 DENSITIES OF POPULATION ────────────┬────────────────────────────────────────┬────────┬──────── City │ Character of District │ Area, │Density │ │ Acres │per Acre ────────────┼────────────────────────────────────────┼────────┼──────── Philadelphia│Thomas Run. Residential. Mostly pairs of│ │ │ two and three-story houses. 1204 acres│ │ │ settled. │ 1,840│ 59 │Pine Street. Residential. Mostly solid │ │ │ four to six-story houses. 156 acres │ │ │ settled. │ 160│ 97 │Shunk Street. Residential. Mostly pairs │ │ │ of two and three-story houses. 539 │ │ │ acres settled. │ 539│ 119 │Lombard Street. Tenements and hotels, │ │ │ 145 acres settled. │ 147│ 113 │York Street. Residential and │ │ │ manufacturing. 354 acres settled. │ 358│ 94 │ │ │ New York │Residential. Three-story dwellings with │ │ City │ 18–foot frontage, and four-story flats│ │ │ with 20–foot frontage. │ │ 100 │Residential. Five-story flats. │ │520–670 │Residential. Six-story flats. │ │800–1000 │Residential. Six-story apartments. High │ │ │ class. │ │ 300 │ │ │ Chicago │1st Ward. Retail and commercial. The │ │ │ “Loop”.