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.

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