54 



ELEMENTARY PHYSIOLOGY 



LESSON IX. 

 THE LEVERS OF THE BODY. 



A lever is a rigid bar which is capable of turning freely about 

 a fixed point called the fulcrum. In fig. 60, the bar WP is free to 

 swing or turn on the fulcrum F. If we apply a downward pulling 

 force at the point, P, that end of the lever will move downward ; 



Fig. 60. Illustrating the Lever (First Order). 



w, a weight suspended at one end ; F, the fulcrum ; p, the point at which the 

 power is applied. 



but this pulling force may be resisted by suspending a weight at 

 the other end of the lever. Thus we have two forces acting on the 

 bar one applied at the point P, and the other, the downward 

 tendency of the weight suspended, acting at the point W. 



Levers are classified under three orders according to the 

 relative positions of the fulcrum, the point at which a power is 

 applied, and the point at which this power, is counteracted by a 



Fig. 61. A Crowbar, used as a Lever of the First Order. 



weight or any resisting body. The three orders are known as the 

 first, second, and third. 



Levers of the first order are those in which the fulcrum is 

 between the power and the weight. The following are a few 

 familiar examples : 



