Sections of the Warren and Jewell filters used at Pittsburg are presented herewith. The filters here shown are practically identical with those used at Lorain and Louisville, and nearly all the exact information regarding mechanical filters relates to filters of these types. These sections show clearly the constructions used at Pittsburg and Louisville, but there are some points in connection with the designs of these filters which require to be considered more in detail. The simplest idea of a mechanical filter is a tub, with sand in the bottom and some form of drainage system. Water is run over the sand, passes through it, and is collected by the drainage system. When the sand becomes clogged it is washed by the use of a reverse current of water. This reverse current of water is so rapid as to preclude the use of a drainage system consisting of gravel, tile-drains, etc., such as are used in sand filters operated at lower rates, and instead metallic strainers in some form are used. The sand comes directly against these strainers, which are made as coarse as it is possible to have them, without allowing the sand to pass. The rate of washing is usually from five to seven gallons per square foot per minute. In the Warren filter the openings in the strainers at the bottom are 6 to 8 per cent of the total area, and during washing the water has an average velocity of 0.20 foot per second upward through them. This velocity is so slow that the friction of the water in passing through the openings in the screen is practically nothing. A result of this is that if there is any unequal resistance of the sand to the water, the bulk of the water goes up at the points of least resistance in the sand. [Illustration: FIG. 22.—SECTION OF JEWELL MECHANICAL FILTER USED IN PITTSBURG EXPERIMENTS.] This tendency would be fatal were it not for the revolving rake which loosens and mixes the sand and largely corrects it. The correction, however, is imperfect, and some parts of the filter are washed more than others. The rake is also necessary to prevent the separation of sand into coarser and finer particles. It is practically impossible to get filter sand the grains of which are all of the same size. When a filter is washed the tendency is for the wash water to go up in limited areas. The larger sand grains tend to collect at these points while the finer grains collect in places where there is no upward current, or where it is less rapid. In many filters this tendency is very strong. The revolving rake is necessary to correct it, and to keep the sand thoroughly mixed, otherwise when a filter is put in operation after washing, the frictional resistance through the coarse sand being less, the bulk of the water goes through it, with the result that a part of the area, and the part which is least efficient as a filter, passes nearly all of the water, and with inferior results.