[Illustration: Fig. 28.--Four methods of protecting open joints in distribution lines--an all-important work. Sketches show cross-section and longitudinal views; the depth from the surface of the ground to the top of the tile is about 10 inches] 1. _A_, Subsoiled ground; _B_, 3 or 4 inch drain tile; _C_, strip of tarred paper about 6 inches wide and extending three-fourths the distance around the tile, allowing sewage to escape at the bottom; _D_, coarse sand, gravel, broken stone or brick, slag, cinders, or coke, the coarsest material placed around the tile (where the ground is naturally very porous and well drained, special filling in the trench may be omitted); _E_, natural soil. 2. Drain tile covered with a board laid flat, leaving the entire joint open. 3. Drain tile laid in stoneware gutter pieces and the joint covered with stoneware caps; gutter and cap pieces are inexpensive commercial products; their radius is longer than that of the outside of the tile, thus leaving open most of the joint space; the gutter aids in keeping the tile in line. 4. Vitrified sewer pipe with hubs facing downhill; the spigot end should be centered in the hub with a few small chinks or wedges. [Illustration: Fig. 29.---Close soils should be deeply subsoiled and underdrained. Porous, well-drained, air-filled soil is absolutely necessary. _A_, Subsoiled ground; _B_, 3 or 4 inch distribution tile; _C_, depth variable with the climate, 1¼ to 3½ feet; _D_, 4-inch underdrain; _E_, depth such as would prepare land for good crop production, generally 3½ to 4 feet; _F_, stone or other coarse material; _G_, gravel grading upward to coarse sand; _H_, loose soil] Type 2 calls for one or more diverting boxes, according to the number of lateral distributors, and readily permits of wasting sewage at widely separated elevations and distances. The outlet pipes enter the box at slightly different elevations, for the reason already stated. With either type, should the outlets not be set at the right elevations, partial plugging of the holes and a little experimenting will enable one to equalize or proportion the discharges. [Illustration: Fig. 30.--Two systems of distribution on steep slopes--use of diverting box. _A_, Direction of slope; _B_, contour of field; _C_, 4, 5, or 6 inch sewer from tank; _D_, diverting box; _E_, 3-inch or 4-inch distribution tile] =Sewage switch.=--The clogging of filters and soils after long-continued application of sewage has been previously referred to. It is, therefore, desirable to arrange the distribution system in two units with a switch between them, so that one area may drain and become aerated while the other is in use. This procedure is especially desirable where the soil is close and the installation of considerable size. It adds to the life and effectiveness of the distribution area and permits use of a plant in case it is necessary to repair, extend, or relay the tile in either unit.