BEST WAY TO APPLY STRENGTH. 113 



with the degree of leverage applied; for example, the 

 arms of an auger, when of a given length, act Avith a 

 greater increase of power with a small size than with a 

 large one. This degree of power may be calculated for 

 an auger of any size, by considering the arms as a lever, 

 the centre screw the fulcrum, and the cutting-blade as the 

 weight to be moved. The same mode of estimate will 

 apply to the vice-handle, the windlass, and the brace-bit. 

 Every one is aware that a heavy weight, as a pail of 

 water, is easily lifted when the arm is extended downward, 

 but with extreme difficulty when thrown out horizontally. 

 In the latter case, the pail acts with a powerful leverage 

 on the elbow and shoulder-joint. For this reason, all 

 kinds of hand labor, with the arms pulling toward or 

 pushing directly from the shoulders, are most easily per- 

 formed, while a motion sidewise or at right angles to the 

 arm is far; less effective. Hence great strength is applied 

 in rowing a boat or in using a drawing-knife, and but little 

 strength in turning a brace-bit or working a dasher-churn. 

 Hence, too, the reason that, in turning a grindstone, the 

 pulling and thrusting part of the motion is more powerful 

 than that through the other parts of the revolution. 

 This also explains why two men, working at right 

 angles to each other on a windlass, can raise seventy 

 pounds more easily than one man can raise thirty 

 pounds alone. This principle should be well understood 

 in the construction or selection of all kinds of machines 

 for hand labor. 



CHAPTER IX. 



MODELS OF MACniXES. 



Serious errors might often be avoided, and sometimes 

 gross impositions prevented, by understanding the differ- 

 ence between the working: of a mere model, on a miniature 



