8l6 THE AMERICAN JOURNAL OF SOCIOLOGY
The settling-room bears a similar relation to a sewer, that a lake does to a river. It is a large chamber whose bottom is much below the sewer, but whose outlet is at the same level as that of the sewer. The stream of sewage, flowing into a quiet body of water, loses its current, and consequently its carrying power, dropping all heavy materials. Chain buckets collect this deposit, raising it to the street level, whence it may be carted away. Hamburg, Magdeburg, and Frankfurt a. M. exhibited notably good models of settling-rooms and chain buckets.
Skimmers are large iron sieves or gratings which catch and hold back all larger floating objects, as the sewage passes through them. They are placed at the outlet of the settling- rooms. Self-cleaning skimmers of two kinds were shown. The Frankfurt model is a wheel-like arrangement. An automatic brush- work brushes the material caught in the large arms onto an end- less belt. The Hamburg model is an endless belt of iron grating, with buckets at intervals, placed at right angles to the outflowing stream. The buckets empty themselves at the top.
Sewage usually receives this much treatment, whether or not any further purification follows. Hamburg, after such treatment, simply discharges its sewage, through three six-foot Duker, into the Elbe. Coin dilutes its sewage two and one-half fold. Bacterio - logical examinations of the water, above and below the city, prove that such dilution is sufficient, as the swiftly flowing Rhine puri- fies itself rapidly. Dresden insists upon a quintuple dilution by means of an intercepting canal before allowing the sew- age to flow into the Elbe, and is planning for a purification plant to replace this treatment.
The so-called mechanical precipitation consists in allowing the sewage to stand, undisturbed for some time, in subsidence basins. Suspended material of positive specific gravity sinks to the bot- tom. Here it is collected at points of maximal depth, whence suction pumps remove it. Among other exhibits were : the draw- ings for a mechanical precipitation plant to be built by Hannover at a cost of 1,500,000 marks ; and a model of the plant at Frank- furt a. M. After having studied the various systems of sewage purification, to see if they were adapted to the needs and condi-
