The lid is not now forced up on the bevelled edge, but is
raised on the side of the chisel, some distance from the bevelled edge,
which thus comes to represent the fulcrum. By using a chisel in this
way, we reverse the positions of the weight and fulcrum and turn it into
a lever of the second order. Suppose we push the side of the
chisel--which is 10 inches long--under the lid to the extent of 1 inch,
then the advantage we gain in power is as 1 to 10; we thereby increase
our strength tenfold. If we push the chisel under the lid for half its
length, then our advantage stands as 10 to 5; our strength is only
doubled. If we push it still further for two-thirds of its length, then
our gain in strength is only as 10 to 6.6; our power is increased by
only one-third. Now this has an important bearing on the problem we are
going to investigate, for the weight of our body falls on the foot, so
that only about one-third of the lever--that part of it which is formed
by the heel--projects behind the point on which the weight of the body
rests.
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