opposed to sanitary requirements. Double traps are not permissible in any case for a single plumbing fixture, since their use greatly retards the outflow of water and waste matter, thereby causing accumulations of filth that are dangerous and offensive. It should be observed, too, that in these closets the second trap gives no additional protection against sewer-air, since a relief-pipe from the air-space between the traps opens directly into the apartment in which the closet is placed through a concealed orifice above the flushing-tank. The main trap is also completely hidden from view in a part of the closet entirely inaccessible.
Figs. 12 and 13 show a trap-jet closet devised by Mr. J. Pickering 
Fig. 12.—Showing the Principle of a Trap-Jet Water-Closet. For convenience of illustration the inlet and outlet are here represented on opposite sides of the bowl. Putnam to conform to the requirements of the essential principles already stated. Its construction is extremely simple. It has a deep and perfectly protected trap-seal exposed to plain view in the bowl of the closet. The flushing-pipe stands always full of water, equilibrium being maintained by atmospheric pressure. Should the water in the bowl be lowered by evaporation or siphonage, air will enter the flushing-pipe through one of the opening* at the lower end, and water immediately descends to restore the loss to the trap-seal.
Fig. 14 shows a simple illustration of this principle in the sketch of an inverted bottle with the mouth submerged in a basin of water. The contents of the bottle remain undisturbed so long as the level of the surface of the fluid in the basin is constant; but if this be lowered, so that air enters the mouth of the bottle, a corresponding volume of water escapes from the bottle into the basin to restore equilibrium. The application of this principle of protection to houses which are closed and left untenanted during several months of the year is manifestly of great importance. Our city residents, after a summer passed at the sea-shore, or in
