beside a warm chimney. Yet if the air it contains should sometimes move
downward it will do no special harm; anything is better than
stagnation. Into this open pipe, which should be not only water-tight
but air-tight through its entire length, all waste-pipes from the house
should empty as turbid mountain torrents pour into the larger stream
that flows through the valley. (Fig. 1.) Now, unless the upward draught
through this large pipe is constant and strong, you will see at once
that the air contained in it (which we must treat as though it were
always poisonous) would be liable to come up through these branches
into the rooms, where they stand with open mouths ready to swallow
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 blo