THE POWER AND WEIGHT IN LEVERS. 57 



the ground on 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 have been taken to illustrate the first order of lever will serve our 

 purpose here, 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 (Fig. 47). 



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Fig. 23. — Third Order of Lever. 



Third Order. — W.P.F. (Fig. 23). This form of lever occurs in 

 a fishing 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 joint by lifting his foot off the ground 

 (Fig. 4<S). 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. The os calcis does not come into 

 this lever, except in counterbalancing, to a very slight extent, the 

 weight of the leg below the hock. 



Relations between the Power and Weight in Levers. — 



The farther the power is from the fulcrum, the greater will be the 

 mechanical advantage at which it will act ; and vice versa. Thus, if 

 one arm of a see-saw is longer than the other, a comparatively light 

 weight at the end of the former will counterbalance a heavy one 

 at the extremity of the latter. Also, the longer an oar is " in-board," 

 as in an outrigger, the greater will be the power which a rower will have. 

 If we apply this principle to the horse, we shall see that the longer is 

 the OS calcis (Figs. 46 and 47), the greater will be the mechanical advantage 

 at which the muscles of the gaskin will act in kicking or in propelling 

 the body forward. As length of os calcis gives increased leverage in 

 the hind limb, so does length of pisiform afford it in the fore leg. 



