What then is animal strength considered in the same light? The animal
is not creative. It can make nothing--it can only transform. Does it
create any strength or force? No. The strength it puts forth or exerts
is merely the outcome of this transformation, which it is the office
of the machine to perform.

What do we find transformed? Simply the energy, or potential,
contained in the fuel or food we put into the machine. Its exact
equivalent we find transformed to another form of energy, known as
animal strength, which is simply heat within the system available for
the working of its mechanical parts. How, then, is this energy which
exists in the shape of animal strength used and distributed? This is
the question the answer of which underlies this whole discussion as a
principle. It is distributed to the different parts of the machine in
proportion to the relative amount of physical work that nature has
made it the office of any particular part to perform.

Let us see how it is with the bird machine. In course of flight he is
called upon to remain in the air, which means that should he cease to
make an effort to do this, i.e., should he cease to expend energy in
doing it, he would fall during the first second of time after ceasing
to make the effort some sixteen feet toward the center of the earth.
But he remains in the air for hours and days at a time. What is he,
then, doing every second of that time? He is overcoming the force of
gravitation, which is incessantly pulling him down. That is, every
second he is doing an amount of work equal to his weight--say 10 lb.
multiplied by 16--say 160 lb. approximately; all this by beating the
air with his wings. Now let us institute a slight comparison--and the
work shall be performed by a man, who climbs a mountain 10,000 feet
high in 10 hours. The man weighs 150 lb.; he climbs 10,000 feet;