37. =Overflow-pipes= from fixtures. It is better to connect them with the trap below the water-level. (See “Bad Odors,” 1, _d_.) 38. =Safes= are sheets of metal (generally lead) with turned-up edges, placed beneath basins, sinks, water-closets, etc., to protect the floor in case of leakage. The waste-pipes of safes are common sources of nuisance. (See “Bad Odors,” 1, _c_.) 39. To prevent contamination of the contents of the refrigerator. 41. When water-closets are flushed directly from the common supply-pipe of the house, as in the different kinds of valve-closets, the water in the pipes, under certain circumstances, will be contaminated: e. g., if a person opens a faucet on one floor at the same moment that the valve is opened to flush a water-closet on a higher floor, water will be sucked back from the valve, and the air of the closet will follow it. Check-valves have been used to prevent this, but they should not be depended on. [Illustration: Fig. 22.] [Illustration: Fig. 23.] [Illustration: Fig. 24.] Various forms of =tanks= or =cisterns= for water-closets are here shown. Fig. 22 represents a tank, _b_, which discharges water as long as the handle (attached to the chain _a_) is raised. It gives a flush also of the entire contents of the service-box _c_, after the handle is lowered. Fig. 23 shows a waste-preventing tank, which is divided into two compartments, _b_ and _d_. It will be seen that this allows only the contents of _d_ to be discharged, with an after-flush from _c_. Fig. 24 shows a tank, in which the chain _a_ is attached to the seat, and is pulled down when the seat is depressed. It will be seen that no water can flow while the closet is in use, but, when the seat is released, the contents of _c_ are discharged with great force. In all these figures, _e_ is a ball-cock. This cock is controlled by a hollow copper ball, which floats on water, and is attached to the end of a lever. When the water falls, the ball falls with it, and opens the cock. When it is floated up to a certain point, it closes the cock and stops the flow. [Illustration: Fig. 25.—Drainage plan for dwellings.] 45. =Leaders= should not be used as soil-pipes, because, during a rain, the ventilation of the pipes is interfered with by the downward flow of water, which will also empty unventilated traps by siphoning. The necessity of having leaders open flush with the roof also interferes with proper ventilation. 47. If =steam= enters a soil-pipe, it heats the water in the traps, injures joints on account of the extreme changes of temperature induced, and hastens corrosion of the pipes. Fig. 25 shows a system of house-drainage for a city house planned in accordance with the above rules. [Illustration: Fig. 26.—Field’s flush-tank.]