MEN AND ANIMALS AS PRIME-MOVERS.
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to give the required motion to the machine. His foot thus forms a part of the link c'. The special formula of the second mechanism is therefore (C7)dr c , and it is to be noticed also that the link a swings only, and does not rotate. We find then that the worker makes a portion of his own body into a mechanism, which he brings into combination, that is chains kinematically, with the mechanism to be driven.
A workman applying both hands to turn a crank in a case where there is large expenditure of effort, chains the mechanism formed by his limbs to that of the crank in a very complex
FIG. 356.
manner. He alters at will the action of the force-closure which brings certain joints into, or throws them out of, use, as becomes necessary at each instant.
The motion of the human body in driving a tread-wheel, or still more that of an animal in a "gin," is complicated in the same way. Always, however, we have the same union, by kinematic chaining, of the living mechanism with that of the machine, while no receptor, in the hitherto accepted sense of the word, can be distinctly recognised. Our investigations, then, lead us to the conclusion: that the receptor does not form an essential part of the complete machine.
