e possible whenever the lower sash is raised or the upper one is
lowered. In order to prevent cold air from without entering through
the openings thus made, it has been proposed by Hinkes Bird to fit a
block of wood in the lower opening; or else, as in Dr. Keen's
arrangement, a piece of paper or cloth is used to cover the space left
by the lifting or lowering of either or both sashes. Louvers or
inclined panes or parts of these may also be used. Parts or entire
window panes are sometimes wholly removed and replaced by tubes or
perforated pieces of zinc, so that air may come in through the
apertures. Again, apertures for inlets and outlets may be made
directly in the walls of the rooms. These openings are filled in with
porous bricks or with specially made bricks (like Ellison's conical
bricks), or boxes provided with several openings. A very useful
apparatus of this kind is the so-called Sheringham valve, which
consists of an iron box fitted into the wall, the front of the box
facing the room having an iron valve hinged along its lower edge, and
so constructed that it can be opened or be closed at will to let a
current of air pass upward. Another very good apparatus of this kind
is the Tobin ventilator, consisting of horizontal tubes let through
the walls, the outer ends open to the air, but the inner ends
projecting into the room, where they are joined by vertical tubes
carried up five feet or more from the floor, thus allowing the outside
air to enter upwardly into the room. This plan is also adapted for
filtering and cleaning the incoming air by placing cloth or other
material across the lumen of the horizontal tubes to intercept dust,
etc. McKinnell's ventilator is also a useful method of ventilation,
especially of underground rooms.

[Illustration: FIG. 5.

HINKES BIRD WINDOW. (TAYLOR.)]

[Illustration: FIG. 6.

ELLISON'S AIR INLETS. (KNIGHT.)]

[Illustration: FIG. 7.

SHERINGHAM VALVE. (TAYLOR.)]

[Illustration: FIG. 8.

THE TOBIN VENTILATOR. (KNIGHT.)]

[Illustration: FIG. 9.

McKINNELL'S VENTILATOR. (TAYLOR.)]

To assist the action of winds over the tops of shafts and chimneys,
various cowls have been devised. These cowls are arranged so as to
help aspirate the air from the tubes and chimneys, and prevent a down
draught.

The same inlets and outlets which are made to utilize winds may also
be used for the ventilation effected by the motion of air due to
difference in the specific gravity of out