342 



OUR PHYSICAL WORLD 



The bone of the forearm hinges near one end on the bone of 

 the upper arm, the bearing serving as a fulcrum (Fig. 178). The 



big muscle at the back 

 of the upper arm, at- 

 taching to the short 

 end or power arm that 

 projects back from 

 the elbow joint fur- 

 nishes power. When 

 the muscle contracts, it 

 straightens the arm, 



FIG. 178. The arm showing the triceps muscle 



and the hand moves 

 very rapidly. It weighs 



much less, however, than the equivalent of the energy that is 



applied by the muscle. 



Levers are of three sorts. Levers of the first class are those 



in which the fulcrum lies between the power and the weight. 



Levers of the second class are those in which the fulcrum is at one 



/VfUcrum 

 First Claw 



/Xfukrum J\veigtt- power 



Second Class 



fulcrum 



pcmr 



Third Class 



FIG. 179. Levers of three classes 



FIG. 1 80. A hammer as 

 a bent lever. 



end, the power at the other, and the weight between. Levers 

 of the third class have the fulcrum at one end, the weight at 

 the other, and the power between. (See Fig. 179.) But in 

 all cases the weight times the weight arm will equal the power 



