=Operation.=--Attention must be given to every plant to insure success. Unusual or excessive foulness should be investigated. No chemicals should be used in a septic tank; garbage, rags, newspaper, and other solids not readily soluble in water should be kept out of sewers and tanks. The plant should be inspected often, noting particularly if the siphon is operating satisfactorily. If scum forms in the settling chamber it should be removed, and the sludge should be bailed or pumped out yearly. Frequently tanks are not cleaned out for three or four years, resulting in large quantities of solid matter going through to the distribution system and clogging it. Clogging may occur in the tile or in the adjacent soil. In either case the tile should be dug up, cleaned, and relaid. In some cases it has been found advantageous to relay the tile between the former lines. When sewage is applied to fairly porous land at the slow rate here recommended and the plant is well handled the tile lines should operate satisfactorily for many years. Liming heavy soils tends to loosen and keep them sweet. =Field data.=--As a basis for outlining or designing a suitable installation the following data should be known: 1. State, town, and whether in or near an incorporated municipality. 2. Usual number of persons to be served. 3. Average daily consumption of water in gallons. 4. Kind and depth of well, depth to water surface. 5. Character of soil, whether sandy, gravelly, loamy, clay, or muck. 6. Condition of soil as to drainage. 8. Character of underlying rock and, if known, its depth below the surface. 9. Depth to ground water at both house and field where sewage is to be distributed. 10. Minimum winter temperature and approximate depth to which frost goes. 11. Number and kind of buildings to be connected with the sewer. 12. Number and kind of plumbing fixtures in each building. 13. Whether plumbing fixtures are to be put in the basement. 14. Depth of basement floor below ground. A plan to scale or a sketch with dimensions showing property lines, buildings, wells, springs, and drainage outlets should be furnished. The direction of surface drainage should be indicated by arrows. The slope of the land (vertical fall in a stated horizontal distance) should be given or if possible a contour plan (showing lines of constant elevation) should be furnished. GREASE TRAPS.