We see that the loss averages about an eighth of an inch per diem. It amounts to about a quarter of an inch the first day, and gradually diminishes as the level of the water descends in the trap, and the distance of its surface from the ventilating current increases, to a little less than 1/8 inch per diem. Hence an ordinary S trap having a 1-inch or a 1-inch seal might lose its seal in from nine to eleven days under similar conditions.

Fig. 281. Record of Evaporation with Cold Flues.

Fig. 281. Record of Evaporation with Cold Flues.

Fig. 282. Record of Evaporation with Heated Flues.

Fig. 282. Record of Evaporation with Heated Flues.

The experiment was repeated several times at different parts of the year from the middle of December to the middle of May, with substantially the same results.

The same trap was now tested at the middle opening, whose center was 2 inches below the crown. The rate of evaporation was somewhat slower, as shown. This experiment was carried on only 11 days, inasmuch as by this time 1l/2 inches of the seal had been destroyed, and the seal of many ordinary S traps does not exceed 1 inches or 1 inches. The same trap was now ventilated at the lowest point, i. e., 6 inches below the crown. The evaporation in this case was exceedingly slow, and after the first two or three days was almost inappreciable.

A number of experiments were then made on S traps unventilated, but open at both ends, as is the case in practice. The loss of water was almost inappreciable, not exceeding 1-32nd or 1-16th of an inch in ten days.

(B) Experiments on Evaporation Produced by a Heated Ventilating Flue.

A 1 -inch trap having a seal 3 inches deep was tested. A 1 -inch wrought iron gas pipe 6 inches long connected with the crown of the trap with a brick flue 8x12 inches heated by a stove. (See Fig. 282.)

The diagrams shown in Fig. 282 represent five tests, two made in March, one in October, and two in November. Here again the perpendicular lines represent the depth of the seal of the trap. The upper arc represents conventionally the outlet mouth and the lower arc the inlet mouth of the trap. The horizontal lines show the position of the water level in the trap at the same hour in the morning of each day recorded in figures on the diagram. We see here a very rapid diminution of the seal. The average loss per diem exceeded 1/3 of an inch. The smallest loss is l/8 inch, and the largest nearly 7/8 inch. The fixture side of the trap was closed during the tests.

A second series of experiments was made with an ordinary 1-inch cast lead trap having a seal 1 inches deep. The trap was connected with the heated flue at a point 3 inches below 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 top of the water. 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 3/8 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 the evaporation at this point was inappreciable.

In the experiments on evaporation with the cold ventilating flue, in the first experiment with the vent at the crown, the anemometer recorded an average rate of movement of the ventilating current of 94 feet per minute.

In the second test, with vent at the crown, the average was 85 feet per minute; with the vent 2 inches from the crown the average was 109 feet per minute. 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 altogether, or 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 descent of moisture down the chimney. These accretions, however, were 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 was at our disposal; yet what records we made were made with great care and accuracy.

From these experiments we found (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 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-inch 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 unused for years at a time.