whatever is poured into them.  It is necessary, therefore, to build dams across them that will allow water to go down but prevent air from going up.  These dams are called ‘traps.’  They are intended to catch only hurtful elements that might seek to intrude.  It often happens that those who set them get caught, for they are not infallible.  Whatever the form or patent assumed by these water-dams, they amount to a bend in the pipe rilled with water. (Fig. 2.) Sometimes a ball or other form of valve is used, but the water is the mainstay.

[Illustration:  Fig. 1.]

[Illustration:  Fig. 2.]

“Theoretically, this is the whole machinery of safe, ‘sanitary’ plumbing:  A large open pipe kept as clean and free as possible, into which the smaller drains empty, these smaller drains or waste-pipes having their mouths always full, and being able, so to speak, to swallow in but one direction.  Everything can go down; nothing can come up.  That all these pipes shall be of sound material, not liable to corrosion; that the different pieces of which they are composed shall be tightly joined; that they shall be so firmly supported that they will not bend or break by their own weight, or through the changes of temperature to which they are subject, and that they shall be, if not always in plain sight, at most only hidden by some covering easily removed, are points which the commonest kind of common sense would not fail to observe.

“Practically, there are weak spots in the system, even if plumbers were always as honest as George Washington—–­before he became a man, and as wise as Solomon—­before he became discouraged.  A water barricade, unless it is as wide as the English Channel, is not a safeguard against dangerous invasion.  A slight pressure of air, as every boy blowing soap bubbles can show you, will force a way through a basin full, and the same thing would happen if there should chance to be a backward current of air through these pipes, with this difference, that while the soap bubbles are harmless beauties, these may be filled with the germs of direful diseases.  Still another danger to which this light water-seal is exposed is that a downward rush of water may cause a vacuum in the small pipes, somewhat as the exhaust steam operates the air-brakes, and empty the trap, leaving merely an open crooked pipe.  Both these weak points may be strengthened by a breathing hole in the highest part of the small pipe below the trap.  This must, of course, have a ventilating pipe of its own, which, to be always effectual, should be as large as the waste-pipe itself. (Fig. 3.)

[Illustration:  Fig 3.]

[Illustration:  Fig 4]