A proper trap should possess the following characteristics :
(1) It should do its work by means of a water seal alone.
(2) It should be self-scouring.
(3) It should be capable of resisting the severest strains of siphonage, momentum and back pressure that can ever possibly be brought to bear upon it in properly constructed plumbing, even when this plumbing is unscientifically used by the occupants, and this without the aid of special trap ventilation.
(4) It should contain a body of water large enough to be practically proof against evaporation.
(5) It should be simple, of durable material and economical to manufacture, with smooth porcelain enameled surfaces.
(6) It should be so constructed that its interior can be inspected without removing the trap.
(7) It should have a tight-fitting, easily accessible clean-out cap, to admit of removing easily any valuable or foreign substance that may have lodged in any part of it.
(8) It should offer the minimum of resistance to the flow of water through it.
(9) It should be ornamental in appearance.
(10) Finally, it should be independent of the fixture to which it is attached and should be easily connected or disconnected.
At first thought it would seem as if some of the above requirements were incompatible or even positively antagonistic. How can a trap which is perfectly self-scouring and simple be made to resist the most powerful action of siphonage, momentum or back pressure without the aid of some mechanical seal ? It is nevertheless possible to obtain this result, and the manner in which it is done will be shown hereafter.