8 1 4 THE AMERICAN JO URNAL OF SOCIOLOGY
buckets are used the escape of gases from the sewer is prevented by odor traps (cf. illustration 1 ). For ventilation of the network of sewers, Frankfurt has adapted four old watchtowers. Among the various interesting drawings and models of sewer accessories exhibited by Wiesbaden were several types of -manholes and shafts. One of the exhibiting firms presented an automatic trap for use with single-conduit sewers to prevent sewage from "back- ing up" and flooding cellars in time of excessive rainfall. Dresden exhibited sewer-gates for closing the mouth of the sewer in times of high-water level in the Elbe.
3. Cleaning of sewers. The sewers are generally cleaned by flushing, the main differences being in the apparatus which
wer Back Row
causes the expulsive rush of water which washes the sewer. The best exhibits of such appliances were full-sized objects in opera- tion, shown by the industrial firms which supply the municipalities therewith. The simplest of these appliances, especially adapted to the single-pipe sewer, is a gate either controllable or auto- matic across the lower half, or two-thirds, of the sewer pipe. This holds back the contents of the sewer until a great volume has accumulated. The sudden release, caused by the opening of the gate, permits a rush of the sewage, which carries with it even the heavier materials along the bottom. A second form is a movable dam, for large, walled sewers exhibited by Berlin which is almost the size of the cross-section of the sewer. At the bottom is a comparatively small opening. The sewage, dammed back except at that point, rushes through, carrying with it the sand and similar deposits along the bottom. A third kind of flushing apparatus is a reservoir with automatic siphon discharge, at regular intervals. The chamber of the reservoir is filled by means of a stream of water (from the ordinary water pipes). When the reservoir is full, discharge through the siphon takes "Out
