I have no wish to extol the human machine unduly, nor to run down the
motor cycle because of certain defects. There is one defect, however,
which is inherent in all motor machines which man has invented, but from
which the human machine is almost completely free. We can illustrate the
defect best by comparing the movements of the heel with those of the
crank-pin of an engine. One serves as the lever by which the
gastrocnemius helps to propel the body; the other serves the same
purpose in the propulsion of a motor cycle. On referring to Fig. 7, A,
the reader will see that the piston-rod and the crank-pin are in a
straight line; in such a position the engine is powerless to move the
crank-pin until the flywheel is started, thus setting the crank-pin in
motion. Once started, the leverage increases, until the crank-pin stands
at right angles to the piston-rod--a point of maximum power which is
reached when the piston is in the position shown in Fig. 7, B. Then the
leverage decreases until the second dead centre is reached (Fig.
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