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
gastrocnemius and soleus. In the standing posture the heel slopes
downwards and backwards, and is thus in a position, as regards its
piston cord, considerably beyond the point of maximum leverage. As the
heel is lifted by the muscles, it gradually becomes horizontal and at
right angles to its tendon or piston cord.
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