=Ventilation of the house drain= from end to end is important, a free escape of foul gases out-of-doors being induced. The _exit_ is provided by carrying the soil-pipe up full bore above the eaves, and remote from windows. One opening alone would not induce a current of air, and the other end of the drain being trapped from the sewer (B, Fig. 29) it is necessary to provide an _inlet for fresh air_ at E. This may be placed a few feet above the ground or at the ground-level. Usually a mica-flap valve is provided at its upper end, which closes whenever a puff of foul air attempts to escape from the drain. The necessity for this is doubtful. Ordinarily air enters at the inlet and circulates through the drain, escaping at the upper end of the soil-pipe ventilator.
It has been advocated that the soil-pipe ventilator should form a means of ventilating the sewer as well as the house-drain, the intercepting trap in the latter (B, Fig. 29) being removed. This is inexpedient, an element of risk being introduced, in view of the possibility of the drain or some of the connections of internal sanitary fittings being defective.
The drain as ordinarily arranged is trapped from the sewer by an =intercepting trap=. This is not merely a trap, but a trap provided with ventilation at its end nearest the house. A form commonly employed is shown in Fig. 32. B is the junction with the house-drain, at D is the water-seal, while at C fresh air enters the house-drain. E is a cleaning eye, through which any chokage can be cleared towards A, leading to the public sewer. It is important that the intercepting trap should be accessible in the event of accidental stopping. This is provided by an =inspection chamber= or =man-hole=. This if close to the house is provided with an air-tight cover, the inlet ventilator being conveyed above ground to a convenient point. The man-hole itself is built with brick set in cement and lined with cement. Note that in the man-hole itself half-channel pipes convey the sewage instead of complete pipes.
Sometimes more than one drain-pipe converges to the same man-hole, and then a more elaborate arrangement than that shown in Fig. 29 is required, the branch-pipes converging into half-channel pipes in the man-hole.