t depend on
the principle of physics that warm air rises and that if the warm air of
a room is to be removed, air must in some way be supplied to take its
place. The two essentials for ventilation are opportunity for the
ingress and the egress of air--ingress for fresh air and egress for
polluted air.

_Fresh-air inlet._

In the construction, of a dwelling house, special and adequate
preparation for the admission of fresh air is seldom provided, so that
the existing openings must be used for the purpose. This means that in
the summertime an open window will furnish all the fresh air which a
room receives and, when the temperature of the outside air is
approximately that of the living room, such provision is ample and
satisfactory. But in the wintertime, when the outside air is cold, the
average person will prefer to suffer from the bad effects of impure air
rather than admit cold air which may cause an unpleasant draft.

[Illustration: FIG. 14.--Letting in fresh air.]

One of the simplest and best methods of providing an inlet for fresh
air, without at the same time allowing blasts of wind to enter the room,
is to fasten in front of the lower part of the window a board which
shall just fill the window opening; then, raising the lower sash a few
inches will allow fresh air to enter both at the bottom, where the board
is placed, and at the middle of the window between the sashes (see Fig.
14). Persons sitting close by a window thus arranged may feel a draft
even under these conditions, since the cold air thus admitted will sink
at once to the floor and then gradually rise through the room to the
ceiling, but unless one sits too near the window, this is an admirable
method of admitting fresh air.

Another method, where steam or hot-water radiators are placed in the
room, is to connect the outer air, either through the lower part of the
window or through the wall of the room just below the window opening,
with a space back of the radiator, so that the cold air entering will
pass around and through the radiator and so be warmed as it enters.

[Illustration: FIG. 15.--Ventilating device.]

The picture (Fig. 15, after Jacobs) shows the arrangement of the
radiators in one of the buildings of the University of Pennsylvania. A
is the opening in the wall below the window; _D_ is a valve which
regulates the amount of air entering through the opening; _R_ is the
radiator; _B_ is a tin-lined box which surrounds the radiator;