We see here a very rapid diminution of the seal. The average loss per diem exceeded one-third of an inch, or exactly four-elevenths of an inch. The smallest loss is one-eighth of an inch, and the largest nearly seven-eighths of an inch. The fixture side of the trap was closed during the tests.

(b) A second series of experiments was made with an ordinary 1" cast lead trap having a seal 1" deep. The trap was connected with the heated flue at a point 3" beyond the crown. Four tests were made. The loss of seal was much slower than in the former tests because of the distance of the mouth of the vent-pipe from the crown of the trap. The rate of evaporation, however, in these four tests averaged one-seventh of an inch a day, the greatest loss in any one day being three-eighths of an inch. In all these experiments on evaporation it was found to make no material difference in the results whether the fixture end of the trap was open or closed, showing that evaporation at this point was inappreciable.

In the experiments on evaporation with the cold ventilating flue, in the first experiment with the vent at crown, the anemometer recorded an average rate of movement of the ventilating current of 112,567', or 20 miles, every twenty-four hours, or, with the correction for friction applied, of 94' per minute.

In the second test, with vent at crown, the average was 85' per minute; with vent 2" from crown the average was 109'.

The velocity of the current during the cold months of the year was quite uniform. In the summer months, however, it was exceedingly variable, sometimes equaling that of the cold season, and sometimes ceasing entirely, or even retrograding.

In the cold months the relation between the rapidity of evaporation and the velocity and dampness of the air current was not accurately determined, the rate of evaporation being quite uniform in spite of considerable barometric fluctuation and change of velocity.

But in summer a change of the conditions of the atmosphere produced a very marked change in the rate of evaporation. On a few occasions of damp or rainy weather in the summer months, where the cold brick flue was used without a ventilating cap on top, the seal actually gained slightly in depth, from condensation on the cold flue of the damp air of the soil-pipe, or from an actual fall of rain or moisture down the chimney. These accretions were, however, very rare, not occurring more than three times in the whole duration of the experiments.

The scientific investigation of this branch of the subject would require more elaborate apparatus and much more time than have been at our disposal, yet what records we have made have been made with accuracy.

Deductions from the Experiments on Evaporation.

From these experiments we fined (1) that a rapid evaporation of the water seal of traps takes place when they are ventilated at or near the crown, and that the evaporation goes on both in winter and in summer, and in ordinary unheated flues, as well as in flues artificially heated. The evaporation is most rapid in winter or with flues artificially heated, and slowest in summer, especially in damp weather. Hence it may be stated generally that the rapidity of the evaporation depends upon the velocity, temperature and hygrometric condition of the atmosphere.

(2) That in winter the evaporation produced by ventilation is so rapid as to destroy the seal of an ordinary 1" machine-made S-trap in from four to eleven days, according to the nature of the current.

(3) That without ventilation, or with the ventilating flue taken from a considerable distance below the crown, the evaporation of the water seal of traps is exceedingly slow, and that unventilated traps having a considerable water capacity may be considered perfectly secure against this danger unless they are left unused for years at a time.

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 absolutely saturated with moisture, 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. The apparatus used in the above recorded tests was fitted up exactly as is customary to fit it up 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 closing the inlet end of the trap against loss of its seal through these agencies, and 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' from the traps tested. Hence 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 result of our experiments in this direction accords with the experience of many sanitary engineers, health inspectors and plumbers who have recently had occasion, since the enactment of the plumbing laws in various parts of the country, to observe the effect of the provision requiring branch ventilation on the water seal of the traps.

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

From the foregoing experiments we deduce the following: (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, such as is used for lavatories and bath-tubs, even when hand-made, and 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 pipe and of no unusual or excessive length, are incapable of resisting the severe action of siphonage produced by the simultaneous discharge of certain forms of plunger 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 siphonic 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 expensive, and are open to the objections attending all cesspools. The positions of their clean-out caps are faulty, inasmuch as they are above the water line, and would fail to announce, by a leakage of water, a faulty adjustment of the cap.

(Signed) J. P. Putnam, L. Fredk. Rice. 4 Pemberton Sq., June, 1884.