mechanical work is performed by means of bony levers. Having seen how
the head is moved as a lever of the first order, we are now to choose a
part which will show us the plan on which levers of the second order
work, and there are many reasons why we should select the foot. It is a
part which we are all familiar with; every day we can see it at rest and
in action. The foot, as we have already noted, serves as a lever in
walking. It is a bent or arched lever (Fig. 6); when we stand on one
foot, the whole weight of our body rests on the summit of the arch. We
are thus going to deal with a lever of a complex kind.

[Illustration: Fig. 5.--Showing a chisel used as a lever of the second
order.]

In using a chisel to pry open the lid of a box, we may use it as a lever
either of the first or of the second order. We have already seen (Fig.
1) that, in using it as a lever of the first order, we pushed the handle
downwards, while the bevelled end was raised, forcing open the lid. The
edge of the box served as a rest or fulcrum for the chisel. If, however,
after inserting the bevelled edge under the lid, we raise the handle
instead of depressing it, we change the chisel into a lever of the
second order. 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