strength.  It could lift a 24-lb. weight placed on the forearm directly over its attachment as easily as a single pound weight placed on the palm.  But, then, there is this advantage:  the 1-lb. weight placed in the hand moves with twenty-four times the speed of the 24-lb. weight situated near the elbow.  What is lost in strength is gained in speed.  Whenever Nature wishes to move a light load quickly, she employs levers of the third order.

[Illustration:  Fig. 9A.—­A chisel used as a lever of the third order.  W, weight; P, power; F, fulcrum.]

We have often to move our forearm very quickly, sometimes to save our lives.  The difference of one-hundredth of a second may mean life or death to us on the face of a cliff when we clutch at a branch or jutting rock to save a fall.  The quickness of a blow we give or fend depends on the length of our reach.  A long forearm and hand are ill adapted for lifting heavy burdens; strength is sacrificed if they are too long.  Hence, we find that the laboring peoples of the world—­Europeans and Mongolians—­have usually short forearms and hands, while the peoples who live on such bounties as Nature may provide for them have relatively long forearms and hands.

[Illustration:  Fig. 9B.—­The forearm and hand as a lever of the third order.]

Now, man differs from anthropoid apes, which are distant cousins of his, in having a forearm which is considerably shorter than the upper arm; whereas in anthropoid apes the forearm is much the longer.  That fact surprises us at first, especially when we remember that anthropoids spend most of their lives amongst trees and use their arms much more than their legs in swinging the weight of their heavy bodies from branch to branch and from tree to tree.  A long forearm and hand give them a long and quick reach, so that they can seize distant branches and swing themselves along safely and at a good pace.  Our first thought is to suppose that a long forearm, being a weak lever, will be ill adapted for climbing.  But when you look at Fig. 10, the explanation becomes plain.  When a branch is seized by the hand, and the whole weight of the body is supported from it, the entire machinery of the arm changes its action.  The forearm is no longer the lever which the brachial muscle moves (Fig. 10), but now becomes the base from which it acts.  The part which was its piston cord now serves as its base of fixation, and what was its base of fixation to the humerus becomes its piston cord.  The humerus has become a lever of the third order; its fulcrum is at the elbow; the weight of the body is attached to it at the shoulder and represents the load which has to be lifted.  We also notice that the brachial muscle is attached a long way up the humerus, thus increasing its power very greatly, although the rate at which it helps in lifting the body is diminished.  We can see, then, why the humerus is short and the forearm long in anthropoid apes; shortening the humerus makes it more powerful as a lever for lifting the body.  That is why anthropoids are strong and agile tree-climbers.  But then watch them use those long hands and forearms for the varied and precise movements we have to perform in our daily lives, and you will see how clumsy they are.