14. =Trap in house-drain.= Intended to cut off all communication between the house and the street sewer. If a contagious disease occurs in one’s own house, he can be sure that the excreta are disinfected, and that disease-germs can not escape into the air, even if the pipes happen to be defective; but he can not be sure that such excreta are properly disinfected in other houses. The only safe way, therefore, is for each house to be cut off from all others by the means described. The objections to the trap on the house-drain are, that it forms a slight obstruction to the flow of drainage, and is liable to be obstructed by deposits, and that it favors “cushioning,” i. e., the forcing of smaller traps, when the air in the drain is compressed between this trap and a down-coming mass of water. (See “Bad Odors,” 1, _d_.) The first of these objections is met by making the trap a half S one, so that the fluids fall down one limb with sufficient force to wash out the bottom of the trap thoroughly with each discharge, and the second by the fresh-air inlet, which relieves the pressure that otherwise might force the traps. 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=,, 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.)