First of all, the water-seal must be deep—never less than four inches—and this seal must be exposed to view for reasons which will be hereafter given. Again, there must be a considerable depth of standing water in the bowl of the closet, to deodorize fecal matter and to secure cleanliness. The outlet of the bowl must also be completely submerged, to prevent its becoming foul and offensive from use. The closet should invariably have some device for maintaining the water-seal against loss from evaporation.
The traps of water-closets are especially exposed to the danger of losing their seal. This happens frequently from the effect of wind and other atmospheric disturbances. An open fire in an adjoining room, or a ventilator near by, may cause this loss of seal from sudden draughts of air, and sewer-air frequently has free entrance into houses for hours and even days at a time when the danger is not suspected by the occupants, since the water-seal of the trap, in badly constructed fixtures, is wholly concealed from view.
In a series of careful experiments, conducted by the writer, it was observed that even in well-ventilated soil-pipes, strong air-currents prevail to such an extent as to cause the water-seal of 
Fig. 9—"Wash-out" Water-Closet, showing how Accidental Obstructions may remain undetected in the Trap which is concealed from View. traps to fluctuate with a quick rise and fall, so that enough water is soon spilled over the outlet of traps to unseal them entirely. The results of these experiments have recently been fully confirmed by other investigators.
Of course, the more shallow the seal, the greater is the danger of its being destroyed from any cause; and for this reason a considerable depth of water is required in the trap of every water-closet, it is necessary, also, that this seal should be plainly exposed to view, in order that any loss of water from the causes just mentioned, or by leakage, may be readily noticed.
But it will be observed that the water-closets in most common use have an extremely shallow seal. The reason of this is that, with the imperfect means employed for flushing them, a greater depth of water in the trap can not be allowed, since it would seriously retard the outward flow of waste-water. The common
