In the above table, as in the previous tables, in indicating the height to which the dividing wall between the settling tank and siphon chamber should be carried, allowance is made for a draught upon the contents of the settling tank at each discharge of a siphon of from four to eight inches. In the discussion relating to siphon chambers in connection with the description of intermittent sand filters will be found the necessary details as to the discharging depths of siphons of different diameters and the necessary depths of the siphon chambers in which such siphons are to be placed. The construction of contact beds will naturally be approached with hesitancy by property owners and others not familiar with such work, and it is strongly recommended that where it is possible the services of a sanitary engineer be engaged to design and supervise the construction of a plant involving any considerable outlay, unless it is felt that the descriptions and directions given above have afforded a clear understanding of the design and construction of this type of sewage-disposal works. SPRINKLING FILTERS One of the more recently developed methods of sewage disposal,—the sprinkling- or trickling-filter system,—has for its principal feature the thorough aëration of the settling-tank effluent before its passage through the filter. This filter, like the contact filter, is of the rapid, coarse-grained type, but in its operation resembles the process of intermittent sand filtration in that the sewage effluent passes through the filter continuously without being held in contact with the filtering material as in the contact bed. The aëration of the sewage effluent, which very greatly aids the final process of nitrification or oxidation in the filter, is accomplished by spraying the sewage effluent over the surface of the beds through a series of riser pipes with nozzles, or allowing it to fall in fine streams on dash plates which cause it to sprinkle over the beds and thus to absorb oxygen from the air. Sprinkling filters produce an effluent with a considerably less degree of purification than sand filters, but may in general be said to produce a more stable effluent, that is, one less liable to subsequent putrefaction than the effluent from contact beds. Furthermore, with the usual depths of contact beds and sprinkling filters, an area approximately four times greater is required to treat the same amount of sewage on contact beds than is necessary if sprinkling filters are constructed. However, since the effluent from sprinkling filters is much more turbid, necessitating, in most cases, subsequent sedimentation before discharge; since considerably greater operating head or fall is necessary; and since their operation requires much more supervision, the construction of sprinkling filters is not generally as advisable as contact beds for small installations, especially in cold climates.