be provided with a water-seal
of about an inch.

_Trap-vents._

In city plumbing it is customary to vent traps; that is, to carry
another system of pipes from the top of the trap nearest the fixture up
to and through the roof. On most roofs, where modern plumbing has been
installed, are seen two pipes projecting, one the soil-pipe and the
other the vent-pipe, indicating the location of a bath-room below (see
Fig. 61). In a single house, however, and particularly in view of
experiments made recently on the subject of trap siphonage, these
trap-vents seem hardly necessary. They were formerly insisted upon
because of the feeling that by the passage of a large amount of water
down the soil-pipe, sufficient suction might be induced to draw out the
water from some small trap on the way, thereby opening a passage for
sewer gas into the room. Experiments have shown that it is practically
impossible to draw off the water from a trap in this way, and that the
system of vent-pipes does little more than add to the cost.

The traps themselves, however, are essential, and great care should be
taken to see that each trap is in place and has a seal of the depth
already mentioned. The best trap to use in any fixture is the simplest,
and a plain S trap answers every purpose. It is always wise to have a
clean-out at the bottom of the trap; that is, a small opening which can
be closed with a screw plug, so that when the trap becomes clogged, it
can be easily opened and cleaned (see Fig. 62).

[Illustration: FIG. 62.--A trap.]

_Water-closets._

A great many kinds of water-closets have been made and used, with
various degrees of success. The old-fashioned pan-closet becomes easily
clogged, allows matter to decompose in the receptacle under the valve,
and, in spite of its being cheaper, should not be used. The long-hopper
closet is also objectionable, for the same reason. A recent bulletin of
the Maine State Board of Health, which gives the relative merits of the
different forms now available, very directly and briefly, is here
repeated:--

"The choice of a water-closet should be made from those which have the
bowl and trap all in one piece, which are simple in construction, are
self-cleansing, and have a safe water-seal. None should be considered
except the short-hopper, the washout, the washdown, the syphonic, and
the syphon-jet closets.

"Short-hopper closets not many years ago were considered desirable, but
other styles costin