=Varieties of Traps.=—Traps are placed at various points of the house-drainage system to prevent the admission of currents of foul air into the house. They are all constructed so as to intercept a water-seal between the drain and the house or yard at the upper end of the trap. Traps are placed in four positions in connection with the drainage of a house: (1) near the junction of the house-drain with the sewer; (2) under the pan of each w.c.; (3) in the open air at the ground level to receive waste water from bath, sink, and lavatory basin; and (4) in the waste-pipe close under the bath, sink, or lavatory, when the waste-pipe is long and apt to become foul; (5) inside sinks at the upper end of their waste-pipes. _Intercepting traps_ between the drain and sewer have already been described. They must always be ventilated (C, Fig. 32 and Fig. 29). A syphon shape with a water-seal of 3 inches is required, and the trap should be self-cleansing, that is, whenever the w.c. is used, the fæces ought to be carried beyond the intercepting trap into the sewer. Other forms of intercepting trap were formerly used, one of the worst of which is shown in Fig. 33. With such a trap as this, an accumulation of filth is inevitable. [Illustration: FIG. 33. DEEP DIPSTONE TRAP, WITH ACCUMULATION OF FILTH IN IT. =A=—Drain entering trap. =B=—Drain leaving trap. =C=—Dipstone.] =Water-Closet and Slop-Closet Traps= are of the syphon or anti-D type. The water-seal in these must be at least 3 inches deep, and the trap must be ventilated by an upright extension of the soil-pipe, otherwise the water in the trap may be syphoned out when the w.c. is used. Hellyer’s “anti-D” trap is a lead syphon trap, the calibre of which is diminished at its bent portion, while the portion of the trap nearest the soil-pipe or drain is square instead of circular. The constriction increases the force of the flush of water and thus cleanses the whole trap, while the square shape impedes the free flow of water, and thus diminishes the risk of syphonage. Various forms of trap are shown in Fig. 19 to 34. The most objectionable of these is the old-fashioned D trap (Fig. 22), the corners and angles of which become fouled, and consequently the lead becomes corroded. [Illustration: FIG. 34. BAD FORM OF TRAP: BELL TRAP. BAD FORM OF TRAP: ANTILL TRAP.] =Gully-traps= are placed in the yard, for the discharge over them of waste-pipes (Fig. 20). A complete disconnection from the drains is thus effected. Formerly _bell-traps_ were used for this purpose. In the Bell-trap not much water can get through, the space A becomes blocked with dirt, the cover B is often taken off and lost, and then the drain is untrapped; and even without this, the water-seal is very slight, and the water quickly evaporates. =Traps under sinks, etc.=, have been already described (page 166).