42 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 imglit 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. 



First Order. — P.F.W. (Fig. 15), as when two persons make a fee-saw 

 by sitting on the opposite ends of a plank which rests on some convenient 



W F 



Fig. 15.— First Order of Lever. 



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. 39). 



Second Order. — P.W.F. (Fig. 16). A wheel-barrow, when lifted in the 

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

 which the wheel rests being the fulcrum j the baiTow, the weight ; and the 

 arms of the person who hfts the handles, the force. We have another example 

 in an oar employed to row a boat ; the water being the unstable fulcrum, 

 and the rowlock being the point through which the weight (the boat) is 

 pushed forward. The bones and muscles which I have taken to illustrate 



