26 



MECHANICS. 



Fig. 4. 



on a bush, and jumped one side, when the animal, 

 leaping swiftly on the concealed bush, was carried by 

 momentum over the precipice. 



As a large or heavy body possesses greater moment- 

 um than a small or light one, so any body moving with 

 great speed possesses more than one moving slowly ; 

 for instance, the momentum of a 

 rifle ball is so great as to carry it 

 through a thick plank, wliile, if 

 thrown slowly, it would scarcely 

 indent it. 



This property of bodies is ap- 

 plied with great advantage to 

 many practical purposes. The 

 momentum of the hammer drives 

 the nail into the wood; for the 

 mere pressure of its weight would 

 not do it, if it were a hundred 

 times as heavy. Wedges are 

 driven by employing the same 

 kind of power. 



On a larger scale, the pile-en- 

 gine operates in a similar man- 

 ner. The ram or weight, h {Fig. 

 4), is slowly lifted by means of a 

 pulley and wheel-work, worked 

 by the handles or cranks, b b, un- 

 til the arms of the tongs which 

 hold the ram are compressed in 

 the cheeks, i /, when it sudden- 

 Piie Engine. \y falls with prodigious force on 



Ihe pile or post to be driven. In this way long posts of 



