56 



CHAPTER VII. 



LEVERS. 



Definition— First Order— Second Order— Third Order— Relations between the 

 Power and Weight in Levers— Comparisons between Power and Weight in 

 Muscular Levers— Directions in which the Power and Weight respectively Act. 



The movements of the limbs are due to the working of various levers, 

 formed by bones and acted upon by muscles. 



Definition.— A lever is a bar which has a fulcrum, or fixed 

 point, so arranged that movement can be communicated to a weight 

 at another point on it, by a power acting on a third point on the bar. 

 Agreeably to the relative positions of the fulcrum (F), weight (W), and 

 power (P), we have the three following orders of levers. 



C^ 



w 



Fig. 21. — First Order of Lever. 



First Order. — P.F.W. {Fig. 21), as when two persons make a 

 see-saw by sitting on the opposite ends of a plank which rests on some 

 convenient fulcrum. We have this order of lever in the bones from 

 the point of the hock to the foot, when a horse kicks out with a hind 

 leg (Fig. 46). 



P 



a- 



Fig. 22. — .Second Order of Lever. 



Second Order. — P.W.F. (Fig. 22). A wheel-barrow, when 

 lifted in the usual manner, furnishes us with an instance of this lever ; 



