=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).
Traps were formerly placed at the upper end of the waste-pipe of the sinks when this was directly connected with the drain. Of these the Bell-trap and Antill’s trap were most common. The Bell-trap has been described above. In the Antill-trap the trap is not removable, and the water-seal is deeper than with a bell-trap. This trap is sometimes used instead of a gully-trap, but is not so good.
=Efficiency of Traps.=—Eassie has said “honestly speaking, traps are dangerous articles to deal with; they should be treated merely as auxiliaries to a good drainage system.”
(1) The trap may have been imperfectly laid to begin with.
(3) Unless the precautions already mentioned (pages 172 to 173) are adopted, the flushing of one trap may empty another.
(4) The water of the trap may become impregnated with foul gases, and these then escape on the house-side of the trap. When a sewer becomes suddenly charged with a large amount of water, as during heavy rain, sewer-gases may force their way through the intercepting trap. With a ventilated drain and soil-pipe these dangers are so small that they may be ignored.
=Unsyphoning= of traps has been already mentioned. It occurs particularly when there are several water-closets one over another, connected with the same soil-pipe. The method of preventing it is shown in Fig. 28.
=The Examination of Drains and Sanitary Appliances.=—This examination will involve the detection of (_a_) any deviations from the details of construction and ventilation of drain and soil-pipe, form of w.c., disconnection of waste-pipes, already insisted on; and (_b_) any defect or leakage in any part of these.
1. _Testing of Water-closet._—The interior of the basin or pan may be painted with a mixture of lamp-black, size and water. If the usual flush applied immediately afterwards clears this off, the form of pan and the flushing power are satisfactory. By removing the wood-work around the w.c., leakage or spillings of slop-water around the w.c. can be detected.