A condition somewhat similar, but acting in a reverse way, is presented in what is commonly termed "siphonage." Just as well as the seal in traps may be forced back by the increased pressure of the air within the pipes, the same seal may be _forced out_, pulled out, aspirated, or siphoned out by a sudden withdrawal of a large quantity of air from the pipes with which the trap is connected. Such a sudden withdrawal of large quantities of air is occasioned every time there is a rush of large column of water through the pipes, e. g., when a water-closet or similar fixture is suddenly discharged; the water rushes through the pipes with a great velocity and creates a strong down current of air, with the result that where the down-rushing column passes by a trap, the air in the trap and, later, its seal are aspirated or siphoned out, thus leaving the trap without a seal. By "siphonage" is therefore meant the emptying of the seal in a trap by the aspiration of the water in the trap due to the downward rush of water and air in the pipes with which the trap is connected.
To guard against the loss of seal through siphonage "nonsiphoning" traps have been invented, that is, the traps are so constructed that the seal therein is very large, and the shape of the traps made so that siphonage is difficult. These traps are, however, open to the objection that in the first place they do not prevent the fouling of the seals by back pressure, and in the second place they are not easily cleansable and may retain dirt in their large pockets. The universal method of preventing both siphonage and back pressure is by the system of vent pipes, or what plumbers call "back-air" pipes. Every trap is connected by branches leading from the crown or near the crown of the trap to a main vertical pipe which runs through the house the same as the waste and soil pipes, and which contains nothing but air, which air serves as a medium to be pressed upon by the "back-pressure" air, or to be drawn upon by the siphoning, and thus preventing any agitation and influence upon the seal in the traps; for it is self evident that as long as there is plenty of air at the distal part of the seal, the seal itself will remain uninfluenced by any agitation or condition of the air within the pipes with which the trap is connected.