TO examine and criticise the various traps fixed by plumbers throughout the United Kingdom would need more space than is contained between the covers of this book; but having in the previous chapter discussed at some length the non-cleansing class, the traps which, like ponds, are filth-accumulators; and the traps which, when defective in certain of their parts, to the eye, even on close examination, would show no defect; as well as that most illusory of traps, the bell-trap; it will not occupy much space to discuss the self-cleansing class, the traps through which matters pass, when properly flushed, like jets of water through the air, leaving no trace behind.

2. Before discussing certain self-cleansing traps in use, it may be better to lay down the principles on which traps for plumbers' work and drainage work should be constructed, and the conditions on which they should be fixed. In doing this I extract largely from my " Lectures."

(a) The trap should be free from all angles, corners, and places where filth could accumulate.

(b) A free way should be made for the discharges to pass through the trap without breaking their form, i.e., the traps should be like a round pipe, so made or bent as to form a water-seal of about 1 1/2 or 2 in. deep.

(c) The body of the trap should be smaller than its inlet, so as to hold as small a quantity of water as possible, consistent with the position in which it will be placed and the work it will have to do, to admit of easy changing every time a flush of water is sent through it.

(d) The minimum-sized trap should be used consistent with circumstances, but governed to some extent by the size of the waste-pipe or drain on which it is fixed, and the flush of water likely to be sent into it. A trap, though of a self-cleansing form, may become a little cesspool if the size is greater that can be cleansed by an ordinary flush of water from the "fitting," or "fixture" - wash-hand-basin, sink, or water-closet - on which it is fixed.

(e) The water-way into a trap for fixing to flat-bottomed vessels with a grating over its mouth, should be larger than its body part, or than the waste-pipe with which its outlet may be connected, so as to be able to send efficient water-flushes through the trap to cleanse it and its waste-pipe. When the trap is smaller than the waste-pipe, no good flushes can be sent through the pipe to cleanse it. (See figs. 79 and 80, showing such traps; or the plumber can easily cone a piece of lead pipe for receiving a larger grating or plug, or washer, and solder this to the inlet of a syphon-trap.)

(f) The inlet, or mouth of the trap, should be so arranged that the water-flushes shall fall upon the "standing water" of the trap with a vertical pressure, so as to drive everything foreign out of the trap, and to entirely change its previous contents.

(g) The inlet side of all traps fixed upon drains outside the house should be open to the atmosphere, so that any bad air rising from foul matter decomposing in the trap, or coming through it from the drain or sewer, may readily pass into the open air, or be largely diluted with fresh air before passing into any waste-pipe, soil-pipe, or drain emptying into such traps.

Fig. 79.-2 in. S-Trap, with Enlarged Mouth.

Fig. 80. - 2 in. Half-S-Trap, with Enlarged Mouth.

In cold countries, where the water standing in such open traps would be liable to freeze in severe frosts, the mouth of the trap should be sealed over, and the "foot-ventilation," or fresh air-induct, should be taken into the waste-pipe, soil-pipe, or drain some little distance up or away from the water-seal of the trap, to prevent the cold aircurrents playing upon it and freezing it. In this country, in sheltered places, there is little or no risk; and if the trap (for disconnecting waste-pipes, soil-pipes, or drains) is kept well down into the ground in exposed places, there is no danger from frosts, though in very severe frosts it is well to throw a little straw upon the surface-gratings of such traps.

Another advantage is gained by keeping the fresh air-induct pipe some little distance up (say 15 ins.) from the bottom of a trapped soil or waste-pipe, for when a full and rapid discharge of water is sent into a soil or waste-pipe it does not get away as fast as it enters, but accumulates in the bottom of the pipe, and, rising up in the pipe, would readily flow into the foot-ventilating or air-induct pipe if kept too low down, and perhaps stop it up with foreign matter.

Fig. 81. - View of " Anti-D-Trap," for Closets.

Fig. 82. - Section.

3. Of all the traps that are made there is none so easy to cleanse as the round-pipe trap commonly called the syphon-trap, when its body or lower part is contracted. With its lower part the same size as its inlet, i.e., when it is of equal bore right throughout, for closets and sinks the " Anti-D-trap " is the more self-cleansing of the two. Nor is this difficult to understand. In the "Anti-D-trap," as shown in figs. 81 and 82, the lower part of the trap is purposely much reduced in size for the flushes to pass through it in the form of a water-plug, to scour out the whole of the interior of the trap. But in the round-pipe trap, fig. 83, whether Beard and Dent's cast-lead trap, or the "Du Bois" hydraulic drawn-lead trap, the lower part of the trap being of equal bore with the inlet, no frictional force is brought to bear upon the lower part of the trap, by an ordinary flush of water, to wash out any sediment.