172 MECHANICS. 



which the muscles are exerted. The following table 

 gives the results of experiments with human strength, 

 variously applied, for a short period : 



Force of the hands Force of the tool 



on the tool. on the object. 



With a drawing-knife 100 lbs. 100 lbs. 



" a large auger, both hands 100 " about 800 " 



" a screw-driver, one hand 84 " 250 " 



" a bench-vice handle 72 " about 1000 " 



" a windlass, with one hand 60 " 180 to 700 " 



" a hand-saw 36 " 36 " 



" a brace-bit, revolving 16 " 150 to 700 " 



Twisting with thumb and fingers, but- 

 ton-screw, or small screw-driver. ... 14 " 14 to 70 " 



The force given in the last column will, of course, 

 vary with the degree of leverage applied ; for example, 

 the arms of an auger, when of a given length, act v/ith 

 a greater increase of power with a small size than 

 with a large one. This degree of power may he calcu- 

 lated for an auger of any size, by considering the arms 

 as a lever, the centre screw the fulcrum, and the cut- 

 ting-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. 

 JFor this reason, all kinds of hand-labor, with the arms 

 pulling toward or pushing directly from the shoulders, 

 are most easily performed, 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 



