erefore adopted of placing a water trap under the water
closet basin or the sink, etc., in direct communication with the
drain. The capacity of water to absorb sewer gas is very great,
consequently the water in the trap would absorb this gas. When the
water became warm from increase of temperature, it would give out the
gas into the house; when it cooled down at night, it would again
absorb more gas from the soil pipe, and frequent change of temperature
would cause it to give out and reabsorb the gas continually.

These objections have led to the present recognized system--viz., 1st,
to place a water trap on the drain to cut off the sewer gases from the
foot of the soil pipe; and, next, to place an opening to the outer air
on the soil pipe between the trap and the house to secure efficient
disconnection between the sewer and the house. It is, moreover,
necessary to produce a movement of air and ventilation in the house
drain pipes to aerate the pipe and to oxidize any putrescible products
which may be in it. To do this, we must insure that a current of air
shall be continually passing through the drains; both an inlet and an
outlet for fresh air must be provided in the portions of the house
drain which are cut off from the main sewer, for without an inlet and
outlet there can be no efficient ventilation. This outlet and inlet
can be obtained in the following manner: In the first place, an outlet
may be formed by prolonging the soil pipe at its full diameter, and
with an open top to above the roof, in a position away from the
windows, skylights, or chimneys. And, secondly, an inlet may be
obtained by an opening into the house drain, on the dwelling side of
and close to the trap, by means of the disconnecting manhole or
branch-pipe before mentioned, or where necessary by carrying up the
inlet by means of a ventilating pipe to above the roof. The inlet
should be equal in area to the drain pipe, and not in any case less
than 4 in. in diameter. If it were not for appearance and the
difficulty of conveying the excreta without lodgments, an open gutter
would be preferable to a closed pipe in the house. This arrangement is
based on the principle that there should be no deposit in the house
drains. Therefore the utmost care should be taken to lay the house
drains in straight lines, both in plan and gradient, and to give the
adequate inclination.

The following are desirable conditions to observe in house drains: 1.
As to mater