It has of late been clearly shown that the special vent pipe leads to a gradual destruction of the seal of traps through evaporation.

Evidently, it either causes a current of air to circulate through the outlet-pipe from the trap to the crown of which it is connected, or it does not. If it does, it licks up the water-seal of the trap with a rapidity proportional to its success in performing its functions. If it does not, it fails in the work for which it is constructed.

As to its unreliability in preventing the siphonage of traps, to its costliness, and to its success in complicating the plumbing, nothing further need be said. They form serious objections to its instalment, and so much so that it seems incredible how it should have so long found advocates among thinking persons. Indeed, an apology seems almost needed for occupying so much space as we have in the discussion of the subject, and yet, when we reflect that several large cities have actually made special trap ventilation a legal requirement, and thereby set the pernicious example to the entire country, any amount of space devoted to showing the folly of such an enactment seems admissible.

It is gratifying to see that, within a few months, at least two of our largest cities have come to a realization of their mistake, and have revised their plumbing laws, withdrawing this requirement.

Whether the trap be ventilated or not, it is important that it should be so formed as to be anti-siphonic, inasmuch as otherwise the powerful suction caused by the discharge of properly constructed plumbing fixtures, which are designed to fill the waste-pipes "full bore," is sometimes found sufficient to destroy their seals even when a vent-pipe is applied. Since, too, the mouth of a vent-pipe is often closed by deposit, particularly when it is used under sinks, or its purpose be nullified by other cause, whatever protection it may afford when new, may at any time be cut off without warning after a short usage.

It must be accepted, then, as a primary condition with traps that they should be anti-siphonic in construction; that is, they should be made in such a way that their own outlet or waste-pipe becomes their vent-pipe.

This cannot be accomplished by the use of an S-trap, because the body of air which rushes through the water-seal to supply the vacuum has nothing to prevent its carrying the water away with it.

Nor can it be accomplished by using valves or balls in the trap, because these open in the same direction with the outflowing water. A ball or valve placed as shown in Fig. 14 possesses little or no resistance to siphonic action.

Fig. 14. - Mechanical Seal-Trap.

If the trap be constructed so that the valve shall stand on the mouth of the outlet instead of the inlet pipe, the outflow of the water would be obstructed, and this is the reason why traps so made have never been successful. Neither can a trap be made anti-siphonic by simply increasing the size of the upcast limb of the trap and making of it a pot or reser voir trap, because by so doing the scouring force of the water in passing through the trap is destroyed, and deposits will collect in the trap, especially when it is placed under sinks. These will collect until the upcast limb is reduced in size to that of the flushing stream - a size which is the same as that of the inlet pipe or downcast limb - and the trap becomes converted into an ordinary S-trap having putrefying matter instead of metal for its walls, and loses its power of resistance to siphonage.

Moreover, these large cesspool or reservoir traps are very heavy when filled with water, and are, in consequence, generally set by the plumber so as to rest on the floor below the fixture. The large body of cold water within them congeals grease and other fatty matter which passes into them, and causes it to adhere to the body of the trap. Yet it cannot arrest all the greasy matter. A special grease-trap many times larger than the largest reservoir or pot-trap would be necessary for this. Consequently, both the trap and the pipes are clogged, though the house-owner labors under the agreeable delusion that his pot-trap will form a "nice grease-trap" and protect his waste-pipes from all annoyance on this score.

A properly constructed anti-siphonic trap will be small enough to permit of setting close up under the fixture, and it will contain but little water. The grease cannot then be cooled in passing through the trap, but will escape into a suitable, especially provided grease-trap beyond. The grease-trap itself should be placed outside of the walls of the house, but as near to them and to the sink as is practicable, and the pipes leading to it should be provided with clean-out caps in such a manner that they may be easily cleansed in case of clogging.

From this it will be seen that all forms of cess-pool or reservoir traps must be rejected as well for the reasons heretofore given, as because they cannot be relied upon to be permanently anti-siphonic.

In short, a trap to be permanently anti-siphonic must contain neither mechanical seal nor cess-pool. It must, therefore, rely entirely upon its form to attain its purpose, and its waterway must throughout be substantially the same in area as the inlet and outlet pipes, so that it shall be permanently self-scouring.

Recapitulation:

From what has been said it will be seen that our second principle of "Thorough Ventilation" requires (a) that all the main lines of piping should be thoroughly ventilated from end to end. This having been accomplished we are enabled to provide (b) that none of the smaller branch wastes from lavatory traps shall be vented by special air-pipes. This again having been established, we are forced to provide (c) that all lavatory or smaller traps shall be so constructed as to be permanently and perfectly anti-siphonic without external aid, and (a) that all lavatories shall be so constructed as to fill the waste-pipes "full-bore" when discharged.

These four rules embrace the main principles of Sanitary Plumbing.