All appliances on the drainage pipes of a house, such as water-closets, sinks, etc., must be supplied with a trap. The different forms of traps in use are the S-trap, the

FIG. 39.-Mason's trap.

bell-trap, the anti-D trap, and Mason's trap (Figs. 36, 37, 38, and 39).

Soil Pipe. The pipes for carrying aw y the sewage

A

FIG. 40.-Method of connecting soil pipe with house drain.

from a house are called soil pipes. For an ordinary dwelling the soil pipes should be constructed of heavy iron tubing, with tight joints, circular in shape, and 10 centimeters in diameter. The interior of the soil pipe must be smooth, so as not to impede the flow of the sewage. The soil pipe is ventilated through the warming of its contained air, causing an upward current, the fresh air entering

through a ventilator opening on the outside of the house next the point of disposal, and takes its exit through the upper end of the pipe, which is carried up over the roof of the building. The soil pipe should have an S-shaped trap between the ventilator opening and the sewer. All connections of drainage pipes with the soil pipe must be absolutely tight, and should be made at an acute angle, not at a right angle with the soil pipe (Fig. 40).

Where several closets on different floors discharge into

the same soil pipe, the suction of the water in the soil pipe causes the trap of the other closet to become unsealed. To obviate this defect the traps are supplied with a separate ventilating pipe of small diameter, which enters into the soil pipe above the highest appliance of the system. These extra pipes also serve to ventilate the traps and pipes, and for this reason this is frequently spoken of as the "back-airing" of traps. It serves to

supply fresh air to the pipes, and thus serves to prevent the growth of anaerobic bacteria in the unventilated portion of the traps. This prevents the generation of disagreeable odors. The method of ventilation of soil pipe and the traps is represented in Fig. 41.

The required amount of fall for house drains may be determined according to the following rule: Multiply the diameter of the drain in centimeters by 4; thus a 10centimeter drain should have a fall of 1 in 40; a 15-centimeter drain I in 60, and so on. If the distance from the

appliance to the soil pipe is too great to obtain the requisite amount of fall in the limited space between floor and ceiling, it will be necessary to have extensions from the soil pipe from the basement to the roof to receive these drains.

Another method of sewage removal is usually spoken of as the dry method. The pail system and the dryearth closet are the principal types of the dry methods of sewage removal. In the pail system the excreta are simply received in boxes or tanks, and these are emptied whenever necessary. In the dry-earth closet a receptacle