side and inside air. Any
artificial warming of the air in the room, whether by illuminants or
by the various methods of heating rooms, will aid in ventilating it,
the chimneys acting as powerful means of removal for the warmer air.
Various methods have also been proposed for utilizing the chimney,
even when no stoves, etc., are connected with it, by placing a
gaslight within the chimney to cause an up draught and consequent
aspiration of the air of the room through it.

[Illustration: FIG. 10.

VENTILATING THROUGH CHIMNEY. (KNIGHT.)]

The question of the number, relative size, and position of the inlets
and outlets is a very important one, but we can here give only an
epitome of the requirements. The inlet and outlet openings should be
about twenty-four inches square per head. Inlet openings should be
short, easily cleaned, sufficient in number to insure a proper
distribution of air; should be protected from heat, provided with
valves so as to regulate the inflow of air, and, if possible, should
be placed so as to allow the air passing through them to be warmed
before entering the room.[14] Outlet openings should be placed near
the ceiling, should be straight and smooth, and, if possible, should
be heated so as to make the air therein warmer, thus preventing a down
draught, as is frequently the case when the outlets become inlets.

[Illustration: FIG. 11.

COWL VENTILATOR. (KNIGHT.)]

=Artificial Ventilation.=--Artificial ventilation is accomplished
either by aspirating the air from the building, known as the vacuum or
extraction method, or by forcing into the building air from without;
this is known as the plenum or propulsion method.

The extraction of the air in a building is done by means of heat, by
warming the air in chimneys or special tubes, or by mechanical means
with screws or fans run by steam or electricity; these screws or fans
revolve and aspirate the air of the rooms, and thus cause pure air to
enter.

[Illustration: FIG. 12.

AN AIR PROPELLER.]

The propelling method of ventilation is carried out by mechanical
means only, air being forced in from the outside by fans, screws,
bellows, etc.

Artificial ventilation is applicable only where a large volume of air
is needed, and for large spaces, such as theaters, churches, lecture
rooms, etc. For the ordinary building the expense for mechanical
contrivances is too high.

On the whole, ventilation without complex and cumbersome mechanisms is