HAVING explained the principles which should govern the construction of plumbing appliances and shown how widely they have been disregarded in practice, it remains now to point out the comparatively few cases in which they have been observed.

Where the co-existence, in any one appliance, of all the desiderata enumerated, was not to be found among the apparatus in use, the writer has endeavored, during his special study of the subject, and his professional practice, to supply the deficiency by constructing new ones himself, and where this has been successfully accomplished, he will not be deterred by such authorship from justly describing them in their proper connections. Indeed no other course would be justifiable.

A perception of the existence of defects was the principal cause of his experiments and of the resulting improvements,, and it would be of little use to stir up and alarm the public by finding fault with existing appliances and methods without the ability to furnish practical cures for their defects.

The trap referred to in the last article as devised by the writer serves to illustrate this point. No simple water-seal trap, having all the requirements needed to render its use perfectly safe under all circumstances without external aid, was to be found.

There were self-cleansing traps, and anti-siphon traps, and traps designed to resist back pressure or evaporation or capillary action; but there were none which were capable of withstanding every variety of adverse influence at once.

Various contrivances were devised to aid traps externally in the performance of their duties, but without success. All kinds of devices for mechanically restoring a water-seal destroyed by any cause were tried, but failed for want of simplicity and reliability.

Finally the vent-pipe was conceived of, and for a time it was supposed that the great remedy had been attained. A few rough and unsystematic laboratory tests made on siphonage, which seemed to corroborate this idea, at once gave rise, in several large cities, to a law rendering special trap-venting obligatory, and not alone this, but requiring it in all cases even when the fixtures above were of such a kind and setting that they could not by any possibility produce a siphoning action on traps below, and when ample aeration and scouring were afforded by other means.

At the time this law was enacted the common round or pot trap of large size had shown itself to be capable of resisting siphonage when new and clean, but it was known that clogging was liable soon to deprive it of this power.

The object of the vent-pipe was to afford protection against siphonage without the use of cesspools. But while it compels venting it does not prohibit cesspools. The practical result is, that since the enactment of the law, cesspools have become more prevalent than ever. Not only has the use of the pot-trap continued undiminished, but the mouth of the vent-pipe has added a cesspool to traps which were otherwise self-cleansing. The waste water, especially under sinks, gradually deposits in this mouth a coating of grease and filth which ultimately clogs it up exactly as it does the unscoured corners of the pot-trap.

Thus, not only is the original purpose of the law frustrated, but the very evil it was intended to remove has been actually augmented by it. The pot-trap is converted by grease into an S-trap. The vent-pipe was applied to protect the S-trap, but is itself destroyed by the very same agency which destroyed the pot, and the only wonder is that this inevitable result was not anticipated before the law was passed. Nevertheless the need of some form of protection was urgent, and the eagerness of the public to obtain security at any price was evinced by the readiness with which they submitted to the heavy burden of trap-venting so long as they believed in its efficiency.

We have now found this belief to be fallacious. Safety must be sought in some other direction. Siphonage must be guarded against, not by adding to the trap a limb of indefinite length and connecting it with the external air, but by forming the trap itself in such a manner that its own waterway shall serve as an air passage and permit the air of the room to supply the partial vacuum in the soil-pipe without driving the water out of the trap before it.

In constructing the trap provision must be made also for resisting back pressure, evaporation, capillary action, leakage and other adverse influences.

To obtain these results, without internal complication or external aid, is only possible by taking the fullest advantage of the various laws which govern the action of fluids. The difference in the specific gravity of air and water, and the consequent difference of momentum of the two fluids under equal rapidity of motion, give us reliable means of separating the air from the water in their passage along the inner walls of the trap as simply and unfailingly as chaff is separated from the grain in the winnowing machine. The relative, attractive and cohesive forces of the particles of the two fluids form other characteristics which must be made use of.

Before we can do this, we must examine the forces which will bring these characteristics into play in plumbing.

Siphonage is caused by a partial vacuum formed in the waste-pipe with which the trap is connected, and is equivalent to a diminution of the atmospheric pressure on the drain side of the trap. The partial vacuum is occasioned by a body of water discharged into the pipe. This water, falling like a plug, rarifies the air behind it, and compresses that in advance of it, to a degree proportional to the velocity and volume of the falling mass. The water in an ordinary trap, say an S-trap, offers very feeble resistance to this atmospheric disturbance. Air passes through it, and in its passage drives out the water before it, destroying the seal. Our problem is to discover how the form of the trap may be modified to cause the air to pass through the water without driving it out in advance of it.

Back pressure takes place when a sudden bend in a vertical soil-pipe, or the inertia of the water in the main house-trap, prevents the free escape of air in front of a body of water falling rapidly in the pipe. A trap connecting with the pipe just above the bend or house-trap would lose its seal under the effort of this compressed air to regain its normal condition, unless certain precautions hereafter to be explained are taken to guard against this accident. Formerly, back pressure was occasioned also by the winds, tides and heat or chemical action in the sewers. But the custom of thoroughly ventilating the sewers and main soil-pipes has now removed this danger.

Evaporation, with unvented traps, goes on with extreme slowness, and with traps containing a considerable body of water, no danger from this cause need be anticipated, unless the building is left unoccupied and unwatched for years at a time. It is, therefore, only necessary in the construction of our trap to see that it has as large a water capacity as is possible, consistent with other requirements.

Capillary action is the gradual attraction of water along the surfaces of fine hairs or similar substances. When lint, hair, or other fibrous matters collect in traps, they may, under certain circumstances, draw off their water-seal by capillary attraction, unless this danger be guarded against in some manner in the construction of the trap.