This consists in connecting a vent pipe at or near the highest part of the trap, as shown in Fig. 71. The action of this arrangement is evident; in place of the waste pipe receiving the air necessary to fill it, through the basin, after the solid column of water has passed down, it is drawn in through the vent pipe, as shown by the arrows, and the seal remains, or should remain, unbroken. It also prevents "self-siphonage" by breaking the column of water and admitting atmospheric pressure at the highest point or crown of the trap. The vent not only prevents the seal from being broken, as described, but allows any gases which may form in the waste pipe to escape above the roof of the house. In order to be effective, the back vent should be large, but even when of the same size as the waste pipe, the flushing of a closet will oftentimes break the seal, especially if the vent pipe is of considerable length. The vent often becomes choked, either with the accumulation of sediment near the trap or by frost or snow at the top; in this case its effect is of course destroyed. Another disadvantage of the back vent is the hastening of evaporation from the trap and the unsealing of fixtures which are not often used.

Back Venting 1000277

Fig. 70.

The second method of guarding against the loss of seal by siphonage is to make the body of the trap so large that a sufficient quantity of water will always adhere to its sides after siphoning to restore a seal. The pot or cesspool trap shown in Fig. 72 is based on this principle.

Back Venting 1000278

Fig. 71.

Back Venting 1000279

Fig. 72.

The third method consists in the use of a trap of such form that it will not siphon, and will at the same time be self-cleaning. Among other types the centrifugal trap, shown in Fig.. 34 and 35, is claimed to fulfil these conditions. The pot trap, while less affected by the siphoning action, is more or less objectionable on account of retaining much of the sediment and solid part of the sewage which falls into it.