To make a trap with what is known as a water-seal, the U pipe is turned into an S fallen forward, [symbol s fallen forward]. Under the basin in the bathroom one can see the waste pipe and can imagine where the water is inside. It flows out of the basin into the first loop of the S, rises into and flows over the other loop until the basin and pipe are emptied as far as _a_. The water has then no power to force itself beyond the loop _b_, and stays in the first loop, forming a water seal between the sewer and the outlet of the basin. Through that loop water gases and odours cannot come, and across the loop at _b_, impure substances and water cannot force their way back from a lower level. It is well to rinse a basin, sink or tub after it is used, and one must be particular that the closet flushes generously, for the water left in the loop should be clean water. If one merely allows the water from tub or basin to sink through the outlet some of that water remains in the loop, and it is water which contains impurities washed from clothes or bodies. You see the practice of rinsing bathtubs and basins has more in it even than courtesy. Occasionally, clean, hot, strong suds should be emptied down basins, tubs, sinks and closets and allowed to stand in the loop, as this cleanses the pipe from impurities or grease which may have adhered to its sides. Disinfectant may be used in the same way when it is thought to be needed. Strong disinfectant should not be left long in a trap as it may eat the joints or even the substance of the pipes. On the other hand, in judging the quantity to use, allowance must be made for the fact that disinfectant poured down pipes goes into water, into a good deal of water in the case of the closet. [Illustration] Two things sometimes make traps ineffectual. One is that the loop may not be deep enough. An S like this, for instance, is useless. So much water can flow out that an air passage is left at _a_, and the pipe is not sealed. The other thing is that sometimes the suction of water rushing down from an upper story will draw the water out of the traps it passes on the way. Either of these difficulties can only be remedied by an alteration in the plumbing arrangements. The concern of the housekeeper in the matter is not to rest if the waste pipes give off the least odour, and to get as reliable a person as possible to inspect them. To have good plumbing is worth going without much. In truth, it is the last thing in which to exercise economy. In building a new house, it is better to have no rugs, no table-linen, and to leave two rooms unfurnished or unbuilt than to put in _cheap_ plumbing.