52 LEVERS. 



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 the bones and acted upon by the muscles. 



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

 so arranged that movement can be communicated to a weight at another 

 point on it, by d. 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. 



First Order. — P.F.W. {see Fig. 6), 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 



w V f 



U 



w p 



Fig. 6. — First Order of Lever. 



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

 Fig. 3°)- 



Second Order. — P.W.F {see Fig. 7). A wheel-barrow, when lifted in 

 the usual manner, furnishes us with an instance of this lever ; the ground on 



