=Safe-wastes= and the =overflow-pipes= of tanks and cisterns may be connected with the soil or waste pipe. When they are, they are usually trapped. It will often be found that the traps are empty and useless, so that offensive gases escape from the pipes. With tank-overflow pipes this is generally the case. Safe-waste traps are sometimes provided with small feed-pipes, intended to discharge water into them every time the fixtures are used. It will often be found, however, if the end of the feed-pipe is pulled up out of the waste, that no water runs through it, and it is useless, sometimes because it is too small and is choked with dust and sediment, and sometimes because it is wrongly attached to the source of supply. Safe-wastes, discharging into the kitchen or cellar, may convey to the rooms odors of cooking or of articles stored in the cellar (onions, turnips, etc.), or from the servants’ water-closet, which is often offensive. If urine or other offensive liquids from leakage flow through them, the upward currents of air will be impregnated with odors from the filth that has clung to the pipe long after the leak has been repaired. In any event these safe-wastes constitute a direct communication between different rooms of a house, which is not always desirable. It is better to do without them. They are rarely of any use. If required, however, they should either be trapped under the safe, and means taken to insure a constant water-seal, or they may be closed by a piece of paper pasted over the lower opening, which will prevent the entrance of any odors from cellar or kitchen, but will give way in case of leakage. The safe-wastes of water-closets often discharge into the trap of the water-closet. This is a bad plan, because, if there is an obstruction in the trap, the safe-waste is, of course, useless. If offensive odors are traced to safe-wastes, the openings had better be sealed. If overflow-pipes are the source, they must be disconnected from the soil or waste pipe and made to discharge elsewhere (e. g., into some sink or water-closet). [Illustration: Fig. 29.—Cushioning illustrated.] (_d._) =Traps.=—If two traps are so arranged that the air is compressed between them by an oncoming rush of water, the trap containing the shallowest water-seal will be forced, and a certain amount of air will escape through it from the interior of the pipe. This result is due to “cushioning,” as it is called, and is illustrated in Fig. 29. Such a fault is to be remedied by connecting the crown of one of the traps with a ventilating-pipe, so that any pressure of the kind is immediately relieved without disturbing the seal of the trap. [Illustration: Fig. 30.—Double-trapped waste-pipe (air-bound).] [Illustration: Fig. 31.—Two sinks with but one trap.]