e accomplished by mechanical means oftener
than three times in an hour, but a natural change of more than three
times in an hour will ordinarily create too strong a current of air,
and may cause draughts and chills dangerous to health.

In determining the cubic space needed, the height of the room as well
as the floor space must be taken into consideration. As a rule the
height of a room ought to be in proportion to the floor space, and in
ordinary rooms should not exceed fourteen feet, as a height beyond
that is of very little advantage.[13]

=Forces of Ventilation.=--We now come to the question of the various
modes by which change in the air of a room is possible. Ventilation is
natural or artificial according to whether artificial or mechanical
devices are or are not used. Natural ventilation is only possible
because our buildings and houses, their material and construction, are
such that numerous apertures and crevices are left for air to come in;
for it is evident that if a room were hermetically air-tight, no
natural ventilation would be possible.

The properties of air which render both natural and artificial
ventilation possible are diffusion, motion, and gravity. These three
forces are the natural agents of ventilation.

There is a constant diffusion of gases taking place in the air; this
diffusion takes place even through stone and through brick walls. The
more porous the material of which the building is constructed, the
more readily does diffusion take place. Dampness, plastering,
painting, and papering of walls diminish diffusion, however.

The second force in ventilation is the motion of air or winds. This is
the most powerful agent of ventilation, for even a slight,
imperceptible wind, traveling about two miles an hour, is capable,
when the windows and doors of a room are open, of changing the air of
a room 528 times in one hour. Air passes also through brick and stone
walls. The objections to winds as a sole mode of ventilation are their
inconstancy and irregularity. When the wind is very slight its
ventilating influence is very small; on the other hand, when the wind
is strong it cannot be utilized as a means of ventilation on account
of the air currents being too strong and capable of exerting
deleterious effects on health.

The third, the most constant and reliable, and, in fact, principal
agent of ventilation is the specific gravity of the air, and the
variations in the gravity and consequent