30 



APPLIED SCIENCE 



up a little, he sticks his crowbar into the ground under the stone 

 and at the same time pushes forward (Fig. 11). In this way he 



moves the stone onward little by 

 little, the ground being the fulcrum. 

 The same principle of leverage ap- 

 plies to the opening of doors or box 

 covers. The oars of boats and the 

 masts of a ship in which the cargo 

 acts as resistance, the bottom of the 

 vessel as the fulcrum, and the sails 

 as the moving power, are also levers 

 of the second class. Nutcrackers 

 (Fig. 12), lemon-squeezers, and de- 

 vices consisting of two legs joined 

 by a hinge are further illustrations of this class of levers. 



33. Levers of the Third Class. In the third type of 

 lever the fulcrum is at one end, the 

 weight at the other, and the force 

 is placed between them (Fig. 13). 

 The advantage of this arrange- 

 ment is that a small force causes 



Fi 



1 1 . A Lever of the Second 

 Class. 



FIG. 12. A Nutcracker. 

 An example of a lever 

 of the second class. 



the extreme point of a long arm to move over a great space. 



The mechanism of the muscles acting on the bones illustrates 



this form of lever. The 

 elbow or joint is the ful- 

 crum, the muscle the mov- 

 ing power, and the weight 

 raised the resistance. The 



FIG. 13. A Safety Valve. muscles of large migrating 



A lever of the third class. ^^ for example) must be 



very powerful in order to sustain the weight of their bodies while 

 they travel for days. 



34. Compound Levers. Levers are said to be com- 

 pounded or compound when their free ends are joined to the 



