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 difficulties 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 evaporation are among the most potent of these hostile agencies. Certain atmospheric disturbances also, such as strong draughts of air, may produce the same effect, and the 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 regards 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 fallacious 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 method 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 ventilator, 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 performing 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 filth 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 consequently no relief for the seal when siphonage occurs. It should be observed, also, that in modern houses, where trap-ventilation is considered to be most perfect, there is still another set of air--
