pressure which are results of
the variations in temperature, humidity, etc. Whenever air is warmer
in one place than in another, the warmer air being lighter and the
colder air outside being heavier, the latter exerts pressure upon the
air in the room, causing the lighter air in the room to escape and be
displaced by the heavier air from the outside, thus changing the air
in the room. This mode of ventilation is always constant and at work,
as the very presence of living beings in the room warms the air
therein, thus causing a difference from the outside air and effecting
change of air from the outside to the inside of the room.

=Methods of Ventilation.=--The application of these principles of
ventilation is said to be accomplished in a natural or an artificial
way, according as mechanical means to utilize the forces and
properties of air are used or not. But in reality natural ventilation
can hardly be said to exist, since dwellings are so constructed as to
guard against exposure and changes of temperature, and are usually
equipped with numerous appliances for promoting change of air.
Windows, doors, fireplaces, chimneys, shafts, courts, etc., are all
artificial methods of securing ventilation, although we usually regard
them as means of natural ventilation.

=Natural Ventilation.=--The means employed for applying the properties
of diffusion are the materials of construction. A porous material
being favorable for diffusion, some such material is placed in several
places within the wall, thus favoring change of air. Imperfect
carpenter work is also a help, as the cracks and openings left are
favorable for the escape and entrance of air.

Wind, or the motion of air, is utilized either directly, through
windows, doors, and other openings; or indirectly, by producing a
partial vacuum in passing over chimneys and shafts, causing suction of
the air in them, and the consequent withdrawal of the air from the
rooms.

The opening of windows and doors is possible only in warm weather; and
as ventilation becomes a problem only in temperate and cold weather,
the opening of windows and doors cannot very well be utilized without
causing colds, etc. Various methods have therefore been proposed for
using windows for the purposes of ventilation without producing
forcible currents of air.

The part of the window best fitted for the introduction of air is the
space between the two sashes, where they meet. The ingress of air is
mad