THE CONQUEST OF NATURE 



to be transformed into the long sweep of the implement, 

 so that with small expenditure of energy the desired 

 work is accomplished. Similarly, the sledge and the 

 axe lengthen out the lever of the arms, so that great 

 momentum is readily acquired, and with the aid of 

 inertia a relatively enormous force can be applied. 

 It will be observed that a laborer in raising a heavy 

 sledge brings the head of the implement near his 

 body, thus shortening the leverage and gaining power 

 at the expense of speed; but extends his arms to their 

 full length as the sledge falls, having now the aid of 

 gravitation, to gain the full advantage of the long arm 

 of the lever in acquiring momentum. 



Even such elaborately modified implements as the 

 treadmill and the rowboat are operated on the principle 

 of the lever. These also are mechanisms that have 

 come down to us from a high antiquity. Their utility, 

 however, has been greatly decreased in modern times, 

 by the substitution of more elaborate and economical 

 mechanisms for accomplishing their respective pur- 

 poses. The treadmill, indeed which might be likened 

 to an overshot waterwheel in which the human foot 

 supplied the place of the falling water in giving power 

 has become obsolete, though a modification of it, 

 to be driven by animal power, is still sometimes used, 

 as we shall see in a moment. 



All these are illustrations of mechanisms with the aid 

 of which human labor is made effective. They show the 

 devices by which primitive man used his ingenuity in 

 making his muscular system a more effective machine 

 for the performance of work. But perhaps the most 



[58] 



