The pipe that conveys foul water away from the house must end in the common sewer of the district, and where it joins the street sewer a very common plan, and not at all a bad plan, is to put a heavy metal flap hung on hinges at the mouth of it; the foul water running down from the house pushes open the flap and falls into the sewer, but as soon as it has done running, the flap shuts. That simple contrivance works very well so long as it is in order, and it can be examined from the inside of the sewer; it is used very largely in London.

Upon the house sewer it is usual to put some kind of water-trap. Now, the water-trap, whatever be its form, and there are a great many varieties, is essentially a bend in a pipe which will hold water; the simplest is the so-called syphon, or U shaped bend; when water is poured down it will remain in the bend up to a certain level, so that the water will prevent air from coming into the house through the pipe, from the sewer. But remember always this, that water-traps are things that cannot be relied on: they will prevent great draughts of air from coming out of the pipes when there is water; but suppose there is water in them, then foul matters in the air on one side are absorbed into the water and given out from the surface on the other side into the house; that has been shown perfectly clearly by experiment. Matters of various kinds can be absorbed on one side of the water-trap and given off on the other side, so that you cannot rely upon water-traps alone to prevent foul matters in the sewers getting up into the house. You must rely on something else, and that is ventilation, a means of escape, or, if possible, a forced escape, of the air from the sewer into the open air, so that from every house sewer there ought to be one or more four-inch pipes rising up outside the house from the highest points of the sewer, and going above the tops of the houses into the open air. These pipes may be continuations of the soil-pipes, but must not be rain water-pipes unless the top of these pipes is at a considerable distance from any window.

It is frequent now, and a very good practice, to make an air opening at about the level of the ground, leading into the drain just on the house side of the trap which separates the drain or house sewer from the main sewer, and this is a very good plan provided the opening is not too near to a window, as where it is adopted foul air cannot accumulate in the drain.

I have already spoken about the waste-pipe of the cistern, and that ought always to end outside the house in the open air. The other pipes that ought to end in the same way outside the house, and that very seldom do, are the waste-pipes from the sinks, from the bath, and all waste-pipes of that kind; they all, as a rule, go into the drain. You will say, if you allow these pipes to end over the yard, the yard gets into a great mess, but then that is better than to have half a dozen pipes going into the drain, and allowing foul air to get into the house through them. You can, however, avoid that in several ways. There must be some kind of trap to carry away the rain water. Mow, sometimes the syphon trap is used, a trap with an open gully, and then a syphon-pipe leading from it, so that it holds water up to a certain level, and a very excellent kind of trap it is. If you want to make a sink-pipe or bath-pipe end away from the drain, you need not have the end over the surface of the yard, but you can have one of these syphon-traps with a hole in the side which is above the level of the water in the gully, so that the wastepipe comes in under the grating, but above the water in the trap, and is thus completely disconnected from the drain.

In order to prevent foul-smelling air coming up sink waste-pipes, it is well to have a trap of some kind below the sink, not for the sake of keeping out drain air, but for the sake of preventing a draught from coming in tainted with foul matter from the waste-pipe itself. There ought, then, to be a syphon-trap immediately below the sink, and it is a good plan to have one with an opening closed by a screw cap for the purpose of clearing the trap in case it should get stopped up. Sometimes what is called a D trap, from its shape, is used for this purpose. The pipe ought to be carried through the wall and made to end in the side of a syphon-trap underneath the grating, but above the level of the water in the trap, so that practically the pipe is cut off from the drain. For downstairs sinks there is a trap which does admirably well, and requires no trap to be placed upon the sink-pipe; it is called the Mansergh-trap. The sink-pipe goes in through an aperture in the side of it into one compartment, and is turned downwards, so that the sink-pipe is trapped itself in the water contained in the first compartment; then the water flows from the first compartment through an aperture into the second compartment, and over these two compartments there is a loose iron lid with openings in it, and then from the second compartment the water flows under a partition into a third, which is closed, and from the third compartment it flows through an outlet into the drain, and there is another outlet in the third compartment to which a ventitilating-pipe may be attached.