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. 7, C);
from that point the leverage is increased until the second maximum is
reached (Fig. 7, D), whereafter it decreases until the arrival at the
first position completes the cycle. Thus, in each revolution there are
two points where all leverage or power is lost, points which are
surmounted because of the momentum given by the flywheel. Clearly we
should get most out of an engine if it could be kept working near the
points of maximum leverage--with the lever as nearly as possible at
right angles to the crank-pin.

[Illustration: Fig. 7.--Showing the crank-pin of an engine at: A, First
dead centre. B, First maximum leverage. C, Second dead centre. D, Second
maximum leverage.]

Now, we have seen that the tendon of Achilles is the piston cord, and
the heel the crank-pin, of the muscular engine represented by the