It would obviously be impossible to devise a form of apparatus for experimental purposes which should cover all the varying conditions liable to be met with in plumbing practice. The position of the trap on the soil pipe branch, the manner and position of connection of the branch with the main pipe, the amount of usage the pipes sustained, the manner in which the ventilating flues were constructed, would all produce their effects upon the results. Nevertheless, in every case where the ventilating flue performed the office of producing a movement of the air within the pipes for which it was intended, and this air was not absolutely saturated with moisture, an event very rare in well ventilated sewers and house drains, the evaporation must of necessity go on in the manner recorded as the result of these experiments.

How far the variation of the conditions would affect the rapidity of the loss of seal must be left to other investigators to determine, and it is hoped that the National Board of Health will at some early day take up this whole matter, and by a most thorough and judicial series of exhaustive experiments put a final end to all doubt and discussion on the part of the public in this extremely important department of sanitary building.

The apparatus used in our tests for the Boston Board of Health was fitted up exactly as is customary to fit up plumbing for actual use.

The entire length of the soil pipe was kept much of the time wet during the experiments on evaporation, by discharges through it made for the tests on siphonage and back pressure, precaution being of course taken by having the inlet end of the trap always corked up so as to be secure against loss of seal through these agencies. The inlet at the end of the soil pipe system, where the fresh air was taken in to produce the ventilating current above the trap, was distant as much as 60 or 70 feet from the traps tested, so that the air was obliged to traverse a considerable length of damp soil pipe, the greater part being nearly horizontal, on its way to the trap, and it may therefore be assumed it was conducted over as large an area of moist surface as it would ordinarily encounter in practice. Moreover, the results of our experiments in this direction correspond with the experience of many sanitary engineers, health inspectors and plumbers who have had occasion, since the enactment of the plumbing laws in various parts of the country, to observe the effect of the provision re quiring branch ventilation on the water seal of traps.

General Conclusions from the Experiments on Siphonage, Back Pressure and Evaporation.

From these tests I conclude as follows:

(1) The ordinary form of machine-made small S trap with shallow seal and without special ventilation is incapable of resisting the action of siphonage or back pressure, even in a very mild form.

(2) A small S trap even when made of unusually deep seal is incapable without special ventilation of resisting the action of siphonage or back pressure in a mild form.

(3) Small S traps when ventilated at the crown, with vent pipes having a diameter the full size of the bore of the trap, and of no unusual or excessive length, are incapable of resisting the severe action of siphonage produced by the simultaneous discharge of any powerful flushing water closets and ordinary bath tubs under ordinary conditions likely to be encountered in practice. Water closets producing a powerful flushing of the soil pipes when discharged should not be prohibited on account of their siphoning power, because the periodical flushing of the soil pipes by their use is productive of great good, and their siphoning action may be counteracted by other means.

(4) Special trap ventilation when the vent pipe is applied at or near the crown of the trap induces a current of air over the water which rapidly destroys the seal.

(5) Trap ventilation when the vent pipe is applied at a point so far below the crown as to avoid the danger of evaporation leaves the trap open to the danger of self-siphonage, as well as of severe siphoning action. The position of the vent pipe on the trap does not (at least within the limits covered by our experiments) materially affect the action of siphonage.

(6) Pot traps of the ordinary sizes are incapable, without special trap ventilation, of resisting the severest action of siphonage liable to be encountered in plumbing.

(7) Pot traps of the largest size are open to the objections attending all cesspools, and should never be allowed.

(8) Anti-siphon traps, may be constructed in such a manner as to resist permanently, without back venting, all the adverse forces to be encountered in plumbing.