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 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. The strength of the muscles which act on the heel will be increased only by about one-third. We have already seen that a double engine, made up of the _gastrocnemius_ and _soleus_, is the power which is applied to the heel when we walk, and that the pad of the foot, lying across the sole in line with the ball of the great toe, serves as a fulcrum or rest. The weight of the body falls on the foot between the fulcrum in front and the power behind, as in a lever of the second order. We have explained why the power of the muscles of the calf is increased the more the weight of the body is shifted towards the toes, but it is also evident that the speed and the extent to which the body is lifted are diminished. If, however, the weight be shifted more towards the heel, the muscles of the calf, although losing in power, can lift their load more quickly and to a greater extent.