es 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.

The vent-pipe system is also an additional means of ventilating the
plumbing system of the house, already partly ventilated by the
extension of the vertical pipes above the roof and by the fresh-air
inlet. The principal objection urged against the installation of the
vent-pipe system is the added expense, which is considerable; and
plumbers have sought therefore to substitute for the vent pipes
various mechanical traps, also nonsiphoning traps. The vent pipes are,
however, worth the additional expense, as they are certainly the best
means to prevent siphonage and back pressure, and are free from the
objections against the cumbersome mechanical traps and the filthy
nonsiphoning traps.

CHAPTER VII

=Plumbing Pipes=

=The House Drain.=--All waste and soil matter in the house is carried
from the receptacles into the waste and soil pipes, and from these
into the house drain, the main pipe of the house, which carries all
waste and soil into the street sewer. The house drain extends from the
junction of the soil and waste pipes of the house through the house to
outside of the foundations two to five feet, whence it is called
"house sewer." The house drain is a very important part of the
house-plumbing system, and great care must be taken to make its
construction perfect.

_Material._--The material of which house drains are manufactured is
extra heavy cast iron. Lighter pipes should never be used, and the use
of vitrified pipes for this purpose should not be allowed.

_Size._--The size of the house drain must be proportional to the work
to be performed. Too large a pipe will not be self-cleansing, and the
bottom of it will fill with sediment and slime. Were it not for the
need of carrying off large volumes of storm water, the house drain
could be a great deal smaller than it usually is. A three-inch pipe
is sufficient for a small house, though a four-inch pipe is made
obligatory in most cities. In New York City no house drains are
allowed of smaller diameter than six inches.

[Illustration: FIG. 22.

SYSTEM OF