THIRD ORDER OF LEVER. 



53 



which the wheel rests being the fulcrum ; the barrow, the weight ; and the 

 arms of the person who lifts 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 



^ 



w 



Fig. 7. — Second Order of Lever. 



tlie first order of lever will serve our turn liere, if we imagine the power to 

 be exerted in propelling the body to the front, through the tibia (the bone 

 that is situated between the hock and stifle), while the toe rests on the 

 fulcrum formed by the ground {see Fig. 31). 



Third Order. — -W.P.F. {see Fig. 8). This form of lever occurs in a fish- 

 ing rod, with which a man tries to lift a heavy trout out of the water. The 

 weight is at the point of the rod ; the fulcrum, at the butt, is formed by one 

 hand ; and the power is supplied by the other hand, a little above the butt. 

 We have this lever in the bones below the hock, when the horse bends that 



u 



/ F 



w 

 Fig. S. 



Third Order of Lever 



joint by lifting his feet off the ground {see Fig. 32). Here, the power is 

 derived from the muscle which is placed in front of the tibia ; the weight is 

 that of the limb below the hock ; and the fulcrum is formed by the tibia. I 

 may remark in passing that the os calcis does not come into this lever, except 

 in counterbalancing, to some slight extent, the weight of the leg below the 

 hock. 



