When you have water closet and drainage, the great object to be attained in house drainage is to prevent the sewer gas from passing from the main sewer into the house drain. It was the custom to place a flap at the junction of the house drain with the sewer; but this flap is useless for preventing sewer gas from passing up the house drain. The plan was therefore 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.