Fig. 26 shows in plan and section a sub-surface irrigation system. The section, which is drawn to a larger scale than the plan, shows the settling tank and the adjoining siphon chamber. From this siphon chamber the effluent sewer carries the discharge from the siphon to the diverting manhole, at which point the effluent is diverted to the different portions of the sub-surface tiling. In Fig. 27 is shown in plan the diverting manhole and a small portion of the sub-surface tiling system together with a section through the diverting manhole and one of the lines of distributing tiling. The main carrier should be of vitrified tile sewer pipe with cemented joints, and should always have two or more branches at the irrigation field in order to allow the use of different portions of the field in turn for three days or a week at a time, thus allowing one of the portions of the field to be resting for corresponding periods. The branch carriers should be of vitrified tile also, and should have cemented joints. If the diameter of the siphon is 5 inches, the main carrier should be of 8–inch vitrified tile with a fall of at least 6 inches per 100 feet in order to quickly carry the dose from the siphon chamber to the several lines of sub-surface tiling forming the distributing system. With 3–inch siphons, a 6–inch main carrier may be used, but the gradient or fall of the main carrier should then be at least 12 inches per 100 feet, owing to the smaller capacity of the 6–inch pipe. In placing the siphon in position, when the siphon chamber is being built, care should be taken to see that the trap or U-shaped pipe is set plumb or in a vertical position. Concrete should then be placed around the siphon to hold it in proper position and at the proper height, and the trap should be filled with water before the bell is placed in position. The bell should then be placed in position over the long leg of the trap to prevent the materials used in construction from being dropped into the siphon. The siphon should be set so that the lower edge of the bell, or of that portion of the bell under which the effluent is to flow, is three inches above the floor of the siphon chamber. [Illustration: FIG. 28.—Y-Branch of Vitrified Tile Pipe.] In laying the distributing system, every second or third length of the branch carriers, according to the porosity of the soil and the spacing of the lines of distributing tiling, should consist of a Y-branch (see Fig. 28), to which a one-eighth bend (see Fig. 29) should be fitted if the lines of lateral tiling are to be laid at right angles to the main carriers, as shown in Fig. 27; or the lateral tiling may be fitted directly to the Y-branch if the lateral lines are to be led away from the carrier at an angle of 45°, as shown in Fig. 30. The Y should branch from the lower portion of the pipe, as shown in Fig. 28. [Illustration: FIG. 29—Eighth Bend of Vitrified Tile Pipe.] [Illustration: FIG. 30.—Sub-surface Tiling.]