should not be far below the temperature of the room, or if so, the
entering current should be introduced towards the ceiling, that it may mix
with the warm air.

Draughts must be avoided. If the circuit from entrance to exit is short,
draughts are likely to be produced, and impure air has less chance of
mixing by diffusion with the pure air. The current of air introduced
should be constant, otherwise the balance may occasionally be in favor of
vitiated air. If a mode of ventilation prove successful, it should not be
interfered with by other means of entrance. Thus, an open door may prevent
the incoming air from passing through its proper channels. It is desirable
that the inlet be so arranged that it can be diminished in size or closed
altogether. For instance, when the outer air is very cold, or the wind
blows directly into the inlet, the amount of cold air entering it may
lower the temperature of the room to an undesirable degree.

In brief, it is necessary to have a thorough mixing of pure and impure
air, so that the combination at different parts of the room may be fairly
uniform. To secure these results, the inlets and outlets should be
arranged upon principles of ventilation generally accepted by authorities
on public health. It seems hardly necessary to say that due attention must
be paid to the source from which the introduced air is drawn. If it be
taken from foul cellars, or from dirty streets, it may be as impure as
that which it is designed to replace.

Animal Heat.

223. Animal or Vital Heat. If a thermometer, made for the purpose, be
placed for five minutes in the armpit, or under the tongue, it will
indicate a temperature of about 98-1/2 degrees F., whether the surrounding
atmosphere be warm or cold. This is the natural heat of a healthy person,
and in health it rarely varies more than a degree or two. But as the body
is constantly losing heat by radiation and conduction, it is evident that
if the standard temperature be maintained, a certain amount of heat must
be generated within the body to make up for the loss externally. The heat
thus produced is known as animal or vital heat.

This generation of heat is common to all living organisms. When the mass
of the body is large, its heat is readily perceptible to the touch and by
its effect upon the thermometer. In mammals and birds the heat-production
is more active than in fishes and reptiles, and their temperatures differ
in degree even in dif