First install the tile disposal field where there will be least danger of polluting water supplies, at least 100 feet from water sources if possible and always at a lower surface elevation. This is of greatest importance. Even though selecting a more distant location would result in greater initial cost, it would be a good investment as protection against diseases that might result from pollution of water sources. The site should slope away from the house and away from the source of water. Gentle unshaded slopes free of trees or shrubbery are best. Root-free locations are important because the open-jointed tile cannot be "rootproofed." Porous, well-drained, gravelly, or sandy soil allows greater purification. Do not have the disposal field in vegetable gardens, under roadways, in swampy land, in muck soils, or in areas having rock substrata sloping toward the water supply. Allow sufficient area, where available, to enlarge the field later if needed. The septic tank may be close to the house, but a more distant site would reduce the likelihood of odors if leakage occurs. The tank should also be kept 50 feet or more from any source of water supply and at a lower elevation. It should not be placed under driveways, pavements, or flower beds, as these would make it not readily accessible for periodic inspection. Care should be taken to insure that surface drainage from the area around the tank will not reach the vicinity of the water supply. THE HOUSE SEWER Material Vitrified salt-glazed clay or well-made concrete sewer pipe and cast-iron soil pipe are the standard materials for house sewers on farms. Asphalt-impregnated fiber pipe, of a type designed especially for house sewers, appears to be satisfactory for this purpose. Cast-iron soil pipe with leaded joints should be used when the sewer is within 50 feet of a well or suction line from a well, within 10 feet of any drinking-water supply line under pressure, within 5 feet of basement foundations, or when laid beneath driveways with less than 3 feet of earth covering the pipes. When within 15 feet of large trees or shrubs, the sewers should have root-tight joints. Size For house sewers, 4- and 6-inch pipes are generally used. Where a 4-inch pipe is used, cast-iron is commonly recommended. Grades with little fall require larger pipes. The large sizes are also less liable to become clogged. Clay pipe is made in pieces 2 or 2-1/2 feet long, whereas fiber-pipe sections are 4 feet long and cast-iron pipe 5 feet long, so that there are fewer joints. The minimum number of joints is desirable, as there is less danger of stoppage. Alinement