ESTIMATING THE POWER OF LEVERS. 67 



used only on hard and smooth floors, and not, like the 

 wheel-barrow, on soft earth, the more nearly the load 

 is placed directly over the wheels, the more easily they 

 will run. 



3. In a lever of the third kind, the weight being 

 further from the fulcrum than the power, it is only 

 used where great power is of secondary importance 

 when compared with rapidity and dispatch. A hand- 

 hoe is of this class, the left hand acting as the fulcrum, 

 the right hand as the power, and the resistance over- 

 come by the blade of the hoe as the weight. ^ A hand- 

 rake is similar, as well as a fork used for pitching hay. 

 Tongs are double levers of this kind, as also the shears 

 used in shearing sheep. The limbs of animals, gener- 

 ally, are levers of the third kind. The joint of the bone 

 is the fulcrum ; the strong muscle or tendon attached 

 to the bone near the joint is the power ; and the weight 

 of the limb, with whatever resistance it overcomes, is 

 the weight, A great advantage results from this con- 

 trivance, because a slight contraction of the muscle 

 gives a swift motion to the Hmb, so important m walk- 

 mg and running, and in the use of the arms. 



SECTION III. 



ESTIMATING THE POWER OF LEVERS. 



The power of any lever is easily calculated by meas- 



Fig- 46. uring the length of 



--r---'|k' its two arms, that 



,,-.;^--'- £\ p w is, the two parts in- 

 to which it is divi- 

 of the first kind. dcd by the wcight, 



