To provide for diverting the flow from the siphon chamber first into one of the two portions into which the sub-surface system is divided, and then, after an interval of three days or a week, into the other portion of the system, at the point where the main carrier is to branch, a ten-inch iron pipe casting (see Fig. 34), with its lower portion forming the body of a double Y-branch of six-inch or eight-inch pipe, may be placed, having a swinging blade or gate attached inside in a vertical position. When, for example, the effluent has been passed for a week into section B of the sub-surface system, the gate _C_, shown in Fig. 34, may be swung to the dotted position and the effluent, at each discharge of the siphon chamber, will then pass through the branch carrier _A_ to section A of the sub-surface system; or a double Y-branch of iron pipe (see Fig. 35) or a cross may be placed at this point on the main carrier when there are to be three sections of the sub-surface system, and valves may be placed on the three branches of the main carrier thus formed to permit of alternately shutting off the flow to the various sections of the sub-surface tiling system (see also Fig. 30). Perhaps the simplest and most serviceable device, however, for alternately resting different portions of the irrigation field is a diverting manhole with stop planks or wooden sluices sliding in grooves in the concrete walls or in a wooden frame, as shown in Fig. 27. (See also Fig. 43, Chapter V.) Where the ground-water level is not very deep below the surface, or a clay or hardpan stratum occurs at a depth of a few feet, it is advisable to underdrain the irrigation field by lines of open-jointed tiling laid at right angles to the lateral distributing tiling and spaced about fifteen feet apart. (See Fig. 36.) [Illustration: FIG. 34.—Special Casting of Double Y-Branch with Swinging Gate.] [Illustration: FIG. 35.—Double Y-Branch with Valves on Branches of Main Carrier.]