The siphon consists primarily of a bent tube, one arm being shorter than the other. After the vacuum has been created, and both arms filled with water, the action continues because the falling of the greater weight of water in the long arm exerts a suction on that in the short arm. If the two arms were of the same length, the weight of each would balance that of the other, and the result would be that the water in each arm would fall by gravity, at once emptying both arms of the siphon. It will be seen, then, that the trap with its outlet, almost always represents the ideal form of siphon, for the middle leg of the trap is short and under atmospheric pressure, and the outlet is generally much longer, and at its lower end often subject to influences which tend to produce a vacuum. In order, then, that the entrance of air may break the siphonic action, the air must be admitted at or near the crown of the trap. That there are many influences in the plumbing system tending to produce a vacuum may be seen in the text under Plate 36, in which this subject is taken up more extensively.

The vent pipe connected at the crown of the trap is the means employed to prevent trap siphonage, and to date it is the only practical means. Various experiments have been tried to prevent trap siphonage without employing an expensive vent system, but to no avail. Having now covered some of the features which apply to traps in general, the consideration of the S-trap will be taken up.

This trap is more extensively used than any other form of trap.

The S-trap and the drum trap may be considered as the fundamental forms of traps, all other traps now in use being based upon one or the other in their operation.

Much debate has arisen as to the relative advantages of these two forms of traps, but it is not the purpose of the author to enter into the controversy. Facts concerning the advantages and disadvantages of each will be given, the reader reaching his own conclusions as to which is the more perfect trap.

The S-trap, owing to its form and to the fact that its passage throughout is of the same size, possesses excellent self-scouring qualities, a most desirable feature in traps.

On the other hand, there is no other trap so susceptible to the action of siphonage as the S-trap, and it would be very unsafe to install this trap without providing it with a vent. Upon the proper application of the vent the successful operation of the S-trap largely depends. The greatest difficulty which the trap vent has to contend with is the accumulation of grease, hair, lint, etc., about the opening of the vent into the trap.

So great is this evil that it is an acknowledged fact that in a very large majority of instances the vents of traps that have been in use for a number of years are undoubtedly inoperative, owing to complete stoppage of the entrance of the vent into the trap.

Patent devices to prevent this have failed. Cleanouts on trap vents, as shown in Fig. D, are seldom used, owing to the fact that the existence of the trouble is usually unknown, and the need of the remedy therefore not appreciated.

The nearest approach to a vent which will not close up is the connection shown in Fig. F, in which the vent is taken from the top of the waste fitting. This method is known as continuous venting, and is of such acknowledged excellence that it is taken up at length under Plates 26, 27, and 28.

S-traps are made in three styles, the full S, three-quarter S, and half S.

In the latter two forms the vent may be taken off at a considerable distance from the seal, as seen in Figs. C and E. Such a connection is preferable to that of either Fig. A, B, or D, for there is not so great a tendency to throw the waste up into the vent as in the three connections named.

There is one other feature which makes the work of Fig. C preferable to that of Figs. A, B, and D.

Air is supplied to the trap seal at such a distance from it, that the rate of evaporation will be materially less than in the case of the other three connections.

The vents in Figs. B and D being taken off further from the trap seal than in Fig. A, their rate of evaporation will be less.

It may be stated, however, that the connection shown in Fig. A is the one most commonly in use. Although evaporation is not so dangerous a factor as siphonage in connection with traps, it is much more to be feared than would appear at first thought.

This is particularly true of traps under fixtures which are seldom used, or traps of fixtures in houses that are vacant, as is often the case during the summer season.

The S-trap, when used to serve the bath tub, is often found very inaccessible when it is desired to clear it of stoppage, for the trap screw, so convenient in most positions, is in this case very difficult to get at.

Flooring must usually be taken up to get at the cleanout.

In Fig. E is shown a very desirable method of providing a clean-out for the bath trap. The cleanout being brought flush with the floor, any stoppage may be removed without taking up the flooring.

The sizes of traps are, viz.:

Traps for water closets, 4 in. diameter.

" " slop sinks, 3 " "

" " kitchen sinks, 1 1/2 or 2 " "

" " laundry tubs, 1 1/2 " "

" bath tubs, 1 1/2 "

" " urinals, 1 1/2 " "

" " lavatories, 1/4" "

" " other fixtures, 1 1/2 " "

Every trap should be provided with a cleanout on its inlet side or below the water level in the trap, and the overflow from each fixture should be connected on the inlet side of the trap. Through carelessness and ignorance the overflow is sometimes found connected to the sewer side of the trap, thereby forming a by-pass through which gases and odors from the drainage and sewer system may enter the house. The trap should always be set level with respect to its water seal. Otherwise the available depth of seal will be lessened, and the seal possibly entirely lost.

Traps located under floors should have cleanouts accessible from above the trap, except in cases where the trap is accessible from the floor below, owing to the form of floor construction, as, for instance, in factory work. The waste from a fixture should never pass through more than one trap before entering the house drain. The effect of passing waste through two traps is to cause air-lock between the two traps, which impedes the natural flow of the waste and results finally in a stoppage of the waste.