equate
for the necessary work. It is very important that there be the freest
possible channel for the movement of air; and nothing will suffice for
this save the continuing of the pipe, at its full size, to its very
outlet. Indeed, angles and bends in a pipe by increasing friction form a
serious obstruction.

The arrangement of the soil-pipe here indicated, although excellent and
efficient, is susceptible of further improvement by the use of a
ventilating cowl or hood at its top. There are many forms of such cowls
in use which are effective whenever there is a sufficient current of
wind; but most of them require a certain force to bring them into
action, and when this force is absent they usually retard the flow they
are intended to increase. This is true of a recent invention known as
"Banner's ventilating cowl," which so long as the wind blows is a most
effective device. When the air is perfectly still, however, it offers by
its curved air-way a certain resistance to the current, and in the case
of baffling winds and flaws the air may blow directly into its opening.

Among the various inventions of this sort nothing seems so free from
objection as the old arrangement known as the "Emerson" ventilator,
shown in Figure 7. This gives a straight outlet, protected by a disk far
enough above it not to prevent its delivery of air; and it becomes an
effective suction cowl, with the least movement of the wind from any
side or from above or below. No eddy caused by the angles of gable
roofs can give it a backward draught; and if a pipe armed with it be
held toward the strongest gale a puff of smoke blown into its other end
will be instantly drawn through. As the patent for this invention has
run out, it is competent for any tinsmith to make it, and it is a common
article of manufacture.

[Illustration: FIG. 7.--THE EMERSON VENTILATOR.]

2. What is said above concerning the ventilation of the soil-pipe from
end to end relates to the interest of the private owner. The interest of
the public gives an equally strong argument in its favor. The sewer
should be as far as possible removed from the condition of an
"elongated cesspool." There must be no halting of its contents, and no
deposit of filth or silt at any point. Within the shortest time
possible, every thing received into the sewer must be passed on and
delivered at its outlet. Still, however perfectly this may be
accomplished, there will always be a certain adhesion of sl