This we shall
find is a discovery which Nature made use of many millions of years ago
in fashioning the body of man and of beast. When we apply our force to
the long end of a lever, we increase our power. We may also apply it, as
Nature has done in our bodies, for another purpose. We have just noted
that if the weight end of the beam of a pair of scales is nine times the
length of the sugar end, that a 1-lb. weight will counterpoise 9 lb. of
sugar. We also see that the weight scale moves at nine times the speed
of the sugar scale. Now it often happens that Nature wants to increase,
not the power, but the speed with which a load is lifted. In that case
the "sugar scale" is placed at the long end of the beam and the "weight
scale" at the short end; it then takes a 9-lb. weight to raise a single
pound of sugar, but the sugar scale moves with nine times the speed of
the weight scale. Nature often sacrifices power to obtain speed. The arm
is used as a lever of this kind when a cricket ball is thrown.
Nothing could look less like a pair of scales than a man's head or
skull, and yet when we watch how it is poised and the manner in which it
is moved, we find that it, too, acts as a lever of the first order.
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