OUR BONES ARE LIVING LEVERS 49 



or weight. A pair of ordinary weighing scales also ex- 

 emplifies the first order of levers. The knife edge on 

 which the beam is balanced serves as a fulcrum ; it is 

 placed exactly in the middle of the beam, which we shall 

 suppose to be 10 inches long. If we place a i-lb. weight 

 in one scale to represent the resistance to be overcome, 

 the weight will be 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 prising 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 



4 



