15. The fresh-air inlet affords free entrance for air, and, as the main lines of pipe are open above the roof, the difference in length of the pipes will give rise to a constant current of fresh air through them, in one direction or the other. (See “Bad Odors,” 1 c.) This results in the oxidation of whatever filth may cling to the pipes, and dilutes offensive gases so that they are rendered harmless. It has also been shown that the various microscopic organisms, which are believed to be the specific cause of certain diseases, increase in virulence, when they propagate in the absence or with a deficient supply of oxygen, while the intensity of their action is diminished if they grow where oxygen is plentiful.
16. Ventilation in chimney-flues. (See “Bad Odors,” 1, c.)
17. Vertical pipes of iron. Iron resists corrosion and the assaults of rats better than lead, and lead pipes are sometimes occluded by pressure and settling. Branch wastes are generally of lead because it is easily cut and bent, so as to be used in difficult situations.
Return-bend or cowl. To prevent senseless persons from throwing things down the pipe, and thus obstructing it.
20. Diameter of pipes. The inside diameter is always meant.
21. D-pipe. (See Appendix B.)
22. No traps on vertical pipes,i. e., at the foot of such pipes, because they would prevent the free circulation of air, and thus render the fresh-air inlet practically useless. Also to prevent “cushioning.”
23. Cast-iron pipes are made in five-foot lengths, with an enlargement at one end, called the hub or bell, and a bead around the edge of the other end (the spigot-end). The spigot-end of one pipe fits loosely into the hub of another, sufficient room being left for calking. (See 26.)
24. Pipes are tarred by being heated to 500° Fahr. and then dipped perpendicularly into a hot bath of coal-tar pitch mixed with a small proportion of heavy coal-oil. This coating prevents corrosion, or at least greatly retards it.
25. Wrought-iron pipes, with screw joints, are used by the Durham House Drainage Company, the joints being gas-tight. They have the advantage of being very durable and impervious to gases, but the disadvantage of being too permanent. If it is desired to change the position of a fixture, or to connect a new one, it is easy to break a hole in a cast-iron pipe, or to remove a portion of it, but where wrought-iron pipe is used this is almost impossible. The Durham system also costs about twenty-five per cent more than the common one, and it is not strictly correct to say that its use renders a house absolutely secure against leakage of sewer-air, because it does not do away with lead branch-pipes, in which frequent leakages occur, and all irregular portions of pipe, with branches, etc., in this system are made of cast-iron with calked joints, as in the ordinary system.