an it is ordinarily thought, is called "back pressure." By
"back pressure" is therefore understood the _forcing back_, or, at
least, the _fouling_, of the water in traps, due to the increased
pressure of the air within the pipes back of the traps; the increase
in air pressure being due to heating of pipes by the hot water
occasionally circulating within them, or by the evolution of gases due
to the decomposition of organic matter within the pipes.

[Illustration: FIG. 21.

NON-SYPHONING TRAP.

Copyright by the J. L. Mott Iron Works.]

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 serv