lifted the moment that a pound of sugar has been placed in the opposite scale—­the sugar thus representing the power.  If, however, we move the knife-edge or fulcrum so that it is only 1 inch from the sugar end of the beam and 9 inches from the weight end, then we find that we have to pour in 9 lb. of sugar to equalise the 1-lb. weight.  The chisel used in prying open the box lid was 10 inches long; it was pushed under the lid for a distance of 1 inch, leaving 9 inches for use as a power lever.  By using a lever in this way, we increased our strength ninefold.  The longer we make the power arm, the nearer we push the fulcrum towards the weight or resistance end, the greater becomes our power.  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.  The fulcrum on which it moves is the atlas—­the first vertebra of the spine (Fig. 2).  When a man stands quite erect, with the head well thrown back, the ear passages are almost directly over the fulcrum.  It will be convenient to call that part of the head which is behind the ear passages the post-fulcral, and the part which is in front the pre-fulcral. Now the face is attached to the pre-fulcral part of the lever and represents the weight or load to be moved, while the muscles of the neck, which represent the power, are yoked to the post-fulcral end of the lever.  The hinder part of the head serves as a crank-pin for seven pairs of neck muscles, but in Fig. 2 only the chief pair is drawn, known as the complex muscles.  When that pair is set in action, the post-fulcral end of the head lever is tilted downwards, while the pre-fulcral end, on which the face is set, is turned upwards.

[Illustration:  Fig. 2.—­The skull as a lever of the first order.]