In Fig. 38 is shown a sand filter layout with three beds. In this drawing are shown the sewer leading from the house, the settling tank, the siphon chamber, in which are placed two siphons, the effluent sewers, and the diverting manhole, from which three pipe lines convey the sewage to the filter beds. In Fig. 39 is shown also a view of the three filter beds, one of the beds being shown in section. Figs. 43 and 44 show a plan and view of the diverting manhole. Where sand must be carted in to form the filters, the embankments to retain the sand should generally be formed by excavating for a depth of two feet the whole area upon which the beds are to be placed. The material thus excavated will usually be sufficient to form the embankments. The embankments should usually be at least two feet wide on top and should have side slopes of one and a half to one; that is, the bottom width of the embankment should be two feet plus three times the height. In clay soils the pits for the filter beds may be excavated with the sides vertical, or nearly so. The bottom of each bed, as it is prepared for the placing of the sand which is to compose the filter, should slope slightly from the sides toward the centre line of the bed. [Illustration: FIG. 41.] Where the character of the underlying strata of soil or the presence of ground water requires that sand filters, whether natural or artificial, should be underdrained, this may be accomplished by laying a longitudinal main drain through the centre of the bed at a depth of at least three or four feet below the surface, with branches each way at intervals of about fifteen feet. The main underdrain should be six inches in diameter, of agricultural tile or of vitrified sewer pipe, laid with open joints, and should have a fall of at least six inches per hundred feet. The branches may be of three-inch agricultural tile. [Illustration: FIG. 42.] [Illustration: FIG. 43.—View of Diverting Manhole.]