These tests were, as has heretofore been explained,originally so incorrectly reported that the public obtained exactly the opposite impression.

In our chapter on "Plumbing Laws," we made the criticism that, in a misdirected effort to obtain security against siphonage, a complicated system had in some cities been prescribed, which was immeasurably less effective than the simple one which had there been forbidden, not only in possessing less power of resistance at the outset, but in its prone-ness to lose the power after short usage.

The anti-siphon trap, just described, affords the means of securing this simple system, it having shown itself to be not only absolutely anti-siphonic in plumbing, but capable of retaining this power undiminished indefinitely.* appreciable, and, with properly constructed traps, it may be disregarded altogether.*

Under powerful and long-continued siphonage, the seal will be lowered until ample room is left for the rapid escape of the air above the water. Under the very most adverse possible conditions, which, however, need never be encountered in plumbing, the water level may be even reduced so low as to give, at first sight, an impression of insecurity, since the full depth of the seal cannot be seen from the outside through the glass body. But a careful measurement of the amount of seal remaining, after nearly all the water has been drawn out of the movable section or body, will still show ample water below the body for protection, provided evaporation, produced by trap venting, is not allowed to act upon it. The seal descends considerably below the bottom of the body, and this can never be withdrawn. With un-vented traps it is found that evaporation is practically un-

* This assertion is based on experiments on its use under kitchen sinks, the most trying tests to which a trap can be subjected.

Now the conditions which would produce such extraordinary siphonage need never be allowed to exist. A siphonage powerful enough to lower the water to the centre of the body is probably as great as ever need be encountered in good plumbing.

It would, therefore, be much simpler and better for those who have not yet learned to place entire confidence in anti-siphon traps to debar those conditions which produce an unnecessarily severe siphonage, rather than insist upon a universal system of trap-venting which is admitted by all to be expensive, dangerous and unreliable.

For instance, a very severe siphonic action may be produced by discharging certain forms of water-closets with large outlets into long vertical runs of soil-pipe. But the outlet of a water-closet should never be quite as large as the soil-pipe and the discharge need never be effected by means of a valve or plunger.

The great siphoning action of these forms of closets is due chiefly to the fact that the outlet is hermetically closed immediately after the discharge, and the small overflow passage is of insufficient capacity to supply air fast enough behind the discharge to break the partial vacuum formed by the friction of the soil-pipe extension.

Many of the improved hopper closets produce a flush equally effective for scouring purposes without this defect, for they allow the necessary air to follow the discharge to break their siphoning force.

With lavatories and all other plumbing fixtures, the overflow passage is nearly or quite as large as the outlet, so that the supply of air can never be suddenly shut off after the discharge, and unless their waste-pipes are of extraordinary length and direct fall, the siphoning action produced cannot be very strong on branches connecting with them. Now, small branch waste-pipes of great length should always be

* See articles on "Sanitary Plumbing," published in The American Architect and Building News for 1S83, '84 and '85; Heading, "Traps;" Chapter on "Evaporation." avoided for this as well as for other reasons. But when, by any chance, they are necessitated by peculiar circumstances, all that is needed is either to enlarge those which have very great fall at and below the points where other branches connect with them, or else to carry such other branches independently into the main soil-pipe. It would be easy, by means of a few careful and systematic experiments, to determine just what proportions and arrangements would be safe for a given standard of resistance, and to establish a few simple rules for their government in practice.

In the tests for siphonage at Worcester, a combination capable of producing an extraordinary siphonage was employed, which probably never is and certainly never need be encountered in plumbing practice, but, as the avowed object of these tests was to determine the comparative power of certain traps and systems of trapping, the strongest possible test that could be devised was made.

The fixture employed had an outlet large enough to fill its waste-pipe full bore (see Fig. 25). The waste-pipe was per pendicular, and the discharge was suddenly arrested alter a given quantity of the water in the tank had escaped, by means of a solid plunger, in such a manner as to seal the outlet hermetically and form a vacuum above the descending water-plug as perfect as could in any way be obtained by water falling in a pipe. The siphonage produced in this case was described by the chairman of the test committee as strong enough to break the seal of a fully vented S-trap with vent only ten feet long, and to crush up and twist out of shape a tin cylinder.

Nevertheless the unvented "Sanitas" trap could not, in any case, be unsealed by this test, even though it was repeated, without refilling the trap, three times in succession.

In our Chapter VII (Traps), under the heading "Back Pressure," we intimated that certain simple precautions, which would hereafter be explained, might be taken to afford protection against this agency. Let us see how this can be done.

Now that our soil-pipes are always, in good plumbing work, properly vented, by being opened to the outer air at both upper and lower ends, it is only under certain rare conditions, such as when a trap is situated near the bottom of a tall stack of pipe and close to a sudden bend, as shown in Fig. 25, that back pressure is produced. The bend in the soil-pipe prevents, by friction, the escape of the air below as fast as it accumulates above under the falling water-plug.