96 



WELLS'S NATURAL PHILOSOPHY. 



Owing to its mechanical disadvantages, this class of levers 

 circumstancL^*^ ^^ rarely used, except where a quick motion is required, rather 

 do we employ than great force. The most striking examples of levers of the 

 thirrclafs ?"'^ third class are found m the animal kingdom. The Umbs of 

 animals are generally levers of this description. The socket 

 of the bono, a, Fig. G5, is the fulcrum ; a strong muscle attached to the bone 



near the socket, c, and extend- 

 ing to d, is the power ; and the 

 weight of the limb, together 

 with whatever resistance, w, is 

 opposed to its motion, is the 

 weight. A very slight con- 

 traction of the muscle in this 

 case gives considerable motion 

 to the limb. 



The leg and claws of a bird, 

 are examples of the third class 

 of levers, the whole arrange- 

 ment being admirably adapted to the wants of the animal. "When a bird rests 

 upon a perch, its body constitutes the weight, the muscles of the leg the 

 power, and the perch the fulcrum. Now, the greater the weight of the body, 

 the more strain it exerts upon the muscles of the claws, which, in turn, grasp 

 the perch more firmly: consequently, a bhd sits upon its perch with the 

 greatest ease, and never falls off in sleeping, since the weight of the body is 

 instrumental in sustaining it. 



207. A Compound Lever is a combination 



Compound Le- of scveral simplc levers, so arranged that the 



shorter arm of one may act upon the longer 



arm of another. In this way, the power of a small force 



in overcoming a large resistance is greatly multiplied. 



Fig. 66. 



An arrangement of compound levers is shown in Fig. 66. Here, by means 

 of three simple levers, 1 pound may be made to balance 1000; for if the long 

 arm of each of the levers is ten times tlie length of the short one, 1 pound 

 at the end of the first one will exert a force of 10 pounds upon the end of the 

 second one, which will in turn exert ten times that amount, or TOO pounds, 

 upon the end of the third one, which will balance ten times that amount, or 

 1000 pounds, at the other extremity. 



