SAFETY IN HOUSE-DRAINAGE. 299 



that of a water-seal will ever be devised for the protection of our 

 houses from the poisonous air of drains and sewers. But, having 

 determined this point, we find that there are still serious difficul- 

 ties to overcome in making traps that will not lose their seal. In 

 our house-drains there are always certain influences that tend to 

 lower the water in the traps, or even to draw it forcibly out, and 

 then, of course, with the seal destroyed, there is no protection 

 against sewer-air. Siphonage, capillary attraction, and evapora- 

 tion are among the most potent of these hostile agencies. Certain 

 atmospheric disturbances also, sucli as strong draughts of air, 

 may produce tlie same effect, and tlie trouble which has resulted 

 from causes of this kind has, for the most part, been ignorantly 

 attributed to the supposed easy permeability of the water as re- 

 gards the dangerous gases of decomposition in our sewers. 



Various means have been devised to prevent siphonage of 

 traps. One of the most common is that of trap-ventilation or back- 

 venting, so called. In its most common form, it consists of a 

 system of air-tubes connected with the crown of each trap, and 

 running by one or several lines of larger pipes to the outer air 

 above the roof of the building. It was expected that this device 

 would prevent entirely the siphonage of traps, and in several 

 cities plumbing laws have been framed requiring all traps to be 

 vented in this manner. But in hundreds of houses to-day, where 

 this method of protection has been adopted, we find sewer-air 

 entering freely through traps that continually lose their seal. 

 And it will be easily seen, after a little reflection, that this fal- 

 lacious remedy causes quite as serious evils as those it is designed 

 to obviate. In the first place, if the ventilation of the trap by 

 this metliod is effective, a current of air is introduced close to the 

 water-seal, and this circulation must induce evaporation of the 

 water in the trap, rapid in proportion to its efficiency as a venti- 

 lator, so that the seal is soon destroyed. This frequently happens 

 to the traps of basins and other plumbing fixtures that are not in 

 every-day use. Again, it is found that the friction of the air in 

 the vent-pipes is sometimes so great as to prevent them from per- 

 forming their proper function. In this case, where there is the 

 slightest retardation in the passage of air through the vent, 

 siphonage takes place and the seal of the trap is destroyed as 

 readily as if the vent-pipes had not been used. Again, the lower 

 orifice of the air-pipe is frequently obstructed by filtli thrown up 

 to the crown of the trap as the waste- water passes rapidly out. 

 In time, this obstruction increases to such an extent that the vent- 

 pipes can not give a free passage for the air, and there is conse- 

 quently no relief for the seal when siphonage occurs. It should 

 be observed, also, that in modern houses, where trap- ventilation io 

 considered to be most perfect, there is still another set of air- 



