THE ANIMAL MACHINE 



pered steel, and the handle or helve of hickory is given 

 a slight curve that is an improvement on the straight 

 handle formerly employed; but on the whole it may 

 be said that the axe is a surviving primitive implement 

 that has held its own and demonstrated its utility in 

 every generation since the dawn, not of history only, 

 but of barbarism, perhaps even of savagery. 



The saw, consisting essentially of a thin elongated 

 blade, one ragged or toothed edge, is a scarcely less 

 primitive and an equally useful and familiar application 

 of the principle of the inclined plane though it requires 

 a moment's reflection to see the manner of application. 

 Each tooth, however minute, is an inclined plane, cal- 

 culated to slide over the tissue of wood or stone or iron 

 even, yet to tear at the tissue with its point, and, with the 

 power of numbers, ultimately wear it away. 



THE WHEEL AND AXLE 



The primitive friction reducer, which continues in 

 use to the present day unmodified in principle, is the 

 wheel revolving on an axle. Doubtless man had 

 reached a very high state of barbarism before he in- 

 vented such a wheel. The American Indian, for exam- 

 ple, knew no better method than to carry his heavy 

 burdens on his shoulders, or drag them along the ground, 

 with at most a pair of parallel poles or runners to modify 

 the friction; every move representing a very wasteful 

 expenditure of energy. But the pre-historic man of the 

 old world had made the wonderful discovery that a 

 wheel revolving on an axle vastly reduces the friction 



