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Modern sanitary plumbing establishes two lines of defence against the intrusion of sewer air into our dwellings: 1. An exterior line consisting of a water trap on the house drain, with a fresh air inlet to the soil pipe on the house side of the trap. 2. An interior line consisting of a water trap beneath each fixture with a vent pipe on the soil-pipe side of each trap. The first prevents the pipes in the interior of the building from being in any way affected by varying air pressure in the sewers and establishes a ventilating current in the soil pipe itself. The second protects the interior from the intrusion of soil-pipe air and insures the traps against siphonage. The plumbing regulations of most of our cities require each trap to have a vent pipe of suitable size at or near the crown of the trap and extended to
PLUMBING SIMPLIFIED.. JAMA. 1897;XXVIII(15):708–709. doi:10.1001/jama.1897.02440150038007