temperature, and the amount of water it contains in a
state of vapor. The quantity removed is greatest when the air is warm
and dry, and the change, or current, rapid.

_Observations._ 1st. The first discovery of the use of free
evaporation of the perspiration from the skin in reducing the heat of
the body, and the analogy subsisting between this process and that of
the evaporation of water from a rough porous surface, so constantly
resorted to in warm countries, as an efficacious means of reducing the
temperature of the air in rooms, and of wine and other drinks, much
below that of the surrounding atmosphere, was made by Franklin.

2d. In all ages and climes, it has been observed that the increased
temperature of the skin and system in fevers, is abated as soon as
free perspiration is restored. In damp, close weather, as during the
sultry days of August, although the temperature is lower, we feel a
disagreeable sensation of heat, because the saturation of the air with
moisture lessens evaporation, and thus prevents the escape of heat
through the lungs and skin.

3d. It is on the principle of the evaporation of fluids that warm
vinegar and water, applied to the burning, aching head, cools it, and
imparts to it a comfortable feeling. The same results follow if warm
liquids are applied to the skin in the hot stage of fever; and this
evaporation can be increased by constant fanning.

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568. What are the principal means by which a uniform temperature of
the body is maintained? On what does the quantity of heat abstracted
from the system depend? What discovery relative to animal heat is due
to Franklin? What is said of free perspiration in fevers? What
occasions the disagreeable sensation of heat in damp, close weather?

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4th. It is frequently noticed, in very warm weather, that dogs and
other domestic animals are seen with their tongues out of their
mouths, and covered with frothy secretions. This is merely another
mode of reducing animal heat, as the skin of such animals does not
perspire as much as that of man.

569. Under some circumstances, a portion of the heat of the system is
removed by radiation. When cold air comes in contact with the skin and
mucous membrane of the lungs, heat is removed from the body, as from a
stove, to restore an equilibrium of temperature. The removal of heat
from the body is greatest when we are in a current of cold air, or
when a