112 MECHANICS. 



Six horses, with a circular saw, driven by means of a good 

 horse-power, will saw five times six, or thirty cords, work- 

 ing the same length of time. In this case the loss by 

 friction is about equal to the additional force required for 

 attendance on the machine. 



Again : a man will cut with a cradle two acres of wheat 

 in a day. A two-horse reaper should therefore cut, at the 

 same rate, ten times two, or twenty acres. This has not 

 yet been accomplished. We may hence infer that the 

 machinery for reaping has been less perfected than for 

 sawing wood. It should, however, be remembered, that 

 great force is exerted, and for many hours in a day, in 

 cutting wheat with a cradle, and therefore less than 

 twenty acres a day may be regarded as the medium 

 attainment of good reaping-machines when they shall 

 become perfected. 



Applying the same mode of estimate, a horse-cultivator 

 will do the work of five men with hoes, and a two-horse 

 plow the work of ten men with spades. A horse-rake 

 accomplishes more than five men, because human force is 

 not strongly exerted with the hand-rake. 



In using difierent tools, the degree of force or pressure - 

 applied to them varies greatly with the mode in wliich 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 auj;er, both hands 100 " about 800 " 



" a screw-driver, one hand 8i " 250 " 



" a bench-viee handle 73 " 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 



