d if we assume that a man of sixty in the same temperature is
only able to respire so much air as shall cause him to evolve so much
heat as would raise forty pounds of water from 32 deg. to 212 deg., we see a
general reason why the older man should feel an effect from a sudden
change in the temperature of the air which the younger would not feel;
and if we assume, further, that a man of eighty could in the same time
produce as much heat as would raise only twenty pounds of water from
32 deg. to 212 deg., we see a good reason why the oldest should suffer more
from a decrease of external temperature than the other two. It is
necessary, however, to know more than this general statement of an
approximate fact; we ought to understand the method by which the
reduction of temperature influences, and the details of the
physiological process connected with the phenomena. When a human body
is living after the age when the period of its growth is completed and
before the period of its decay has commenced, it produces, when it is
quite healthy, by its own chemical processes, so much heat or force as
shall enable it, within given bounds, (1) to move its own machinery;
(2) to call forth, at will, a limited measure of extra force which has
been lying latent in its organism; and (3) to supply a fluctuating
loss that must be conveyed away by contact with the surrounding air,
by the earth, and by other bodies that it may touch, and which are
colder than itself. There is thus produced in the body, _applied_
force, _reserve_ force, and _waste_ force, and these distributions of
the whole force generated, when correctly applied, maintain the
perfect organism in such balance that life is true and steady. So much
active force carries with it the power to perform so much labor; so
much reserve force carries with it the power to perform a measure of
new or extra labor to meet emergencies; so much waste force enables
the body to resist the external vicissitudes without trenching on the
supply that is always wanted to keep the heart pulsating, the chest
breathing, the glands secreting or excreting, the digestive apparatus
moving, and the brain thinking or absorbing.

Let us, even in the prime of manhood, disturb the distribution of
force ever so little, and straightway our life, which is the resultant
of force, is disturbed. If we use the active force too long, we become
exhausted, and call on the reserve; if we continue the process, the
result is