To resist this pressure it is only necessary to have a sufficient body of water in the trap, and to set the trap at a distance below the fixture it serves sufficient for this water to form in the pipe, when subjected to back pressure, a column from twelve to sixteen inches long. The weight of such a column is ample to withstand any back pressure ever now encountered in good plumbing. It will be seen, by examining Figs. 33 and 35, that our trap has been constructed with this in view.

We have stated that a trap should be designed in such a manner that it shall be secure against the effect of capillary attraction.

Fig. 36. - Trap resisting capillary "attraction; transverse section through the body.

Fig. 37. - Longitudinal section through the body.

Numerous elaborate experiments have been made within a few years on this insidious enemy to the life of the water-seal of traps, and it has been found that there is a limit to the vertical and horizontal distance to which water can be carried by substances liable to collect in traps.

We find this limit to be three inches for vertical extension, and less when considerable horizontal is added to the vertical extension.

We must, therefore, construct our trap in such a manner that the water shall be obliged to travel more than this distance before its seal can be broken. Figs. 36 and 37 show the manner in which this has been accomplished in the design of the trap under consideration.

In our first chapter we established as one of our ten plumbing axioms, that every part of our plumbing should be visible and accessible. To carry out this principle in the construction of a trap, it is not only necessary to provide a means of easily opening it, but it is equally important to see that it may as easily be closed again without danger of subsequent leakage and without special tools or expert aid. In order to be certain that the movable cap shall always be airtight at its point of junction with the body of the trap, this point should always be underneath the normal level of the water therein.

In the ordinary pot-trap this clean-out cap stands at the top, as shown in Fig. 26, where, if an unsound joint occurs, it will not be announced by an escape of water, as it should be in order that the defect may be at once noticed and remedied. The most dangerous elements of sewer gas are odorless, and a leakage may be too small to be detected by the sense of sight or smell, and yet be large enough to admit disease into the house.

It is better to endanger the floors or plastering by a leakage of water than the health or life of the householder by a leakage of gas.

Fig. 38. - 3/4 "Sanitas" Trap.

Fig. 39. - 3/4 "Sanitas" Trap, opened.

It is found to be exceedingly difficult to make a large threaded joint, like the one ordinarily used, water and gas tight. The usual flat washer is pushed out of place and injured when the cap is screwed down, and the flat washer is always difficult to compress to the point of tightness. Moreover, the washer adheres to metal surfaces tenaciously after having been pressed against them for some time, particularly in hot weather, and prevents them from being unscrewed without injury to the parts. Finally, a large opening is apt to have its threading injured by a crossing of the thread.

The manner in which these difficulties have been overcome is shown in Figs. 38 and 39, which represent our anti-siphon trap in its 3/4 S form, the latter with the cap and reflecting diaphragm removed to show the interior construction.

We see that the plane of junction of the parts comes below the normal water level. The cap is connected with the body, not by the usual large threading, but by three small bolts which screw into three brass nuts cast in the body of the trap to correspond with the three ears in the cap. A rubber washer, round in section, as shown in the drawing, Fig. 39, fits into a groove around the opening in the body, and all that is necessary is an ordinary screw-driver to make, with the arrangement, a perfectly and permanently tight joint, which can easily be taken apart at any time by the owner, and safely put together again in a few moments without help.

We have established as another leading principle, that the waste-pipes and their traps should be substantially self-cleansing. The severest trial to which a trap can be subjected for this is under an ordinary kitchen or pantry sink, where common traps, of the reservoir or cesspool kind, like the D-trap, pot and bottle traps, soon become clogged with grease. A trap which will resist this ordeal without becoming stopped up with sediment may be safely trusted under all other circumstances.

The body of the reservoir trap is so large and heavy that it is placed on the floor below the sink for proper support.

The great volume of water in the trap thus placed cools the grease as it passes through it and causes it to congeal upon the sides and top of the trap until it reduces the body to about the size of the inlet arm. The rapidity of the flow of the water through this large body is diminished in pro-portion to the increase of its size. Hence, the scour of the water is insufficient to drive the grease out as it congeals.

The body of our improved trap, on the contrary, is small and it may be set close to the sink so that the grease is not cooled in its passage through it, but escapes in its liquid condition into the proper grease receptacle beyond. Hairs, lint, dirt, and even full length matches are easily carried through this trap by a powerful water flow, such as would be obtained by the discharge of any plumbing fixture properly constructed as to its outlet. When this outlet is large enough to fill the waste-pipe "full bore," the fixture acts as a flush tank and scours the waste-pipes and trap alike. The sides of the trap (as well as of the waste-pipe itself) will, of course, become clouded in time and ultimately thinly coated with sediment, but never to a dangerous extent or in such a manner as to destroy its effectiveness. The reflecting partition or diaphragm within the trap fits closely to the sides of its body, and never obstructs its flow or collects sediment; for the water flows around the end of the partition and not against it.

The Sanitas Universal or Adjustable Trap. Figs. 40 and 41 represent a form of the Sanitas trap, designed for convenient adjustment to any position of the waste-pipe connections. For this purpose the arms of the trap are made to swivel in any desired direction, before being made fast by the bolts and nuts.

Fig. 40. - Sanitas Universal Trap. Running Trap.

Fig. 41. - Sanitas Universal Trap, adjusted for a left-hand waste-pipe.