We have already referred to the most notorious illustrations of the cess-pool order of traps; namely, the old D and the common "pot" or "round" trap, as it is sometimes called. Other cess-pool or reservoir traps are shown in Figs. 10 and 11. These mechanical seal-traps require no comment. They possess no advantages to offset their defects, and may be dismissed from service without hesitation. Mercury is superfluous in a trap, because a water-seal properly constructed and guarded is perfectly safe. Moreover it would be evidently impractical to adopt a mercury seal in all the large water-closet traps of a house, and any advantage there might be in its employment in the smaller ones would therefore be frustrated.

Fig. 10. - Cess-pool Trap, with Mercury Seal, showing the manner in which filth may collect.

Fig. 11. - Cess-pool Trap - Common Bell Trap.