The dimensions of siphon chambers to effectively deliver the effluent in proper volumes to the sub-surface irrigation system are given in the following tables, which indicate widths of siphon chambers to agree in general with the widths of the settling tanks to serve a given number of persons, as shown in Chapter II. These tables of dimensions for siphon chambers provide for two different capacities where the same number of persons are served by the sewer, depending on the total lengths of sub-surface tiling required, which in turn depend on the character of the soil in which the sub-surface system is laid. The tables provide for a division of the sub-surface tiling system into two parts up to a system for 100 persons, and into three parts for a greater number of persons. These tables also show the total length of lateral distributing tiling in the sub-surface irrigation system necessary to distribute over a sufficient area at the irrigation field, in both sandy soils and in the heavier loams, the various quantities of sewage to be treated in the different-sized tanks and discharged from the siphon chambers. The tables also indicate the diameter of the siphon and the discharging depth of each siphon. As discussed in Chapter III, the siphon, in discharging, may draw upon the upper 4 to 8 inches of sewage in the settling tank without interfering with the efficiency of the tank. The dimensions of siphon chambers for 75 or more persons in Table II, and for 35 or more persons in Table III (see page 59), provide for such a draught upon the settling-tank contents of from 4 to 8 inches when the siphon discharges. This will decrease the cost of the plant somewhat and provide for a more efficient form of siphon chamber. The last column in each table provides for the proper height of dividing-wall between the settling tank and siphon chamber to allow the drawing down of the settling-tank contents as noted above. [Illustration: FIG. 26.—Plan and Section of Sub-surface Irrigation System.] [Illustration: FIG. 27.—Plan and Section of a Portion of a Sub-surface Irrigation System.] The sub-surface irrigation or distributing system consists of a main carrier or effluent sewer leading away from the siphon chamber to the irrigation field, of two or more branches of this main carrier, and of parallel lines of lateral distributing tiling extending at intervals of 4 to 6 feet from the branch carriers, or, in some locations, from each side of the branch carriers. The frontispiece shows the relation between the several portions of a sub-surface irrigation system. The house sewer is shown leading to the settling tank, and from the siphon chamber adjoining the settling tank the main carrier or effluent sewer is shown leading to a diverting manhole from which the effluent is carried at each discharge of the siphon to the lateral lines of sub-surface tiling by the two branch carriers.