If there are two traps on the same line of pipe (Fig. 30), so that a part of the interior of the pipe is cut off from the external air in both directions, it becomes “air-bound,” and one of two results will follow, viz.: water discharged into the fixture above the higher trap will remain in the bowl and not run down, on account of the compression of air between the traps; or, if it does run out of the fixture, it will displace an equivalent bulk of foul air, which will bubble up through the upper trap and cause offensive odors. In such a case, the lower trap should be removed. If =two= or =more waste-pipes= (as in sets of tubs, in sinks, and often in urinals) are provided with but =one trap= for all (Fig. 31), there will be a constant current of air along the pipes, sometimes emerging from one opening and sometimes from another. This air will be contaminated by the filth that lines the pipes, and will often be quite offensive. In such cases, each waste-pipe should be independently trapped, or (as in the case of bath-tubs or urinals) the overflow-pipe should be connected with the trap of the waste-pipe below the water-seal, as in Fig. 25. When a body of water sufficiently large to nearly fill a pipe is discharged into it, it drags air along with it, and exhausts the air in all branch-pipes to a greater or less extent. When this exhaustion amounts to a diminution of the pressure in the pipe of about 1/350, the water in an ordinary trap (with a one-inch seal) will be forced out of it by the pressure of the external atmosphere, or in other words will be sucked out of the trap into the pipe, leaving no water-seal in the trap, and, therefore, allowing free exit for sewer-air. This effect is known as the “siphoning” of the trap, and is most likely to occur when the main pipe is not open at its upper extremity (is not ventilated), and when the branch wastes are of the same size with the main one. This fault is remedied by the ventilation of the traps, and of the main pipes, as described in the regulations above quoted (17 and 33). 2. =To defects in the pipes.= (_a._) =Drains.=—Bad odors in cellars often come from broken earthenware or tile drains, cracked by settling, or from rat-holes in cemented drains. Earthenware or brick drains inside a house should be replaced by iron ones. (_b._) =Joints.=—=Cement= or =putty= joints are often cracked. =Water-closets= sometimes become loosened from the floor and leak fluids and gases. The covers of =hand-holes= of traps sometimes get loose and leak gas. Sometimes =trap-screws=, instead of being at the bottom of the trap, where they are covered by water, are at the top; if these are loose, gases may escape. All such defects should be repaired.