ACTION AND REACTION, 



67 



is concentrated in a very small compass, with a great velocity. A gun recoila 

 more with a charge of fine shot, or sand, than with a bullet. The explanation 

 of this is, that with a ball the velocity is communicated to the whole mass 

 at ance, but with small shot, or sand, the velocity communicated by the ex- 

 plosion to those particles of the substance immediately in contact with the powder, 

 is greater than that received at the same instaiit by the outer particles ; con- 

 sequently, a larger proportion of explosive force acta momentarily in an oppo- 

 site direction. 



FiQ. 35. 



We have an illustration of this same principle, when we attempt to drive a 

 nail into a board having no support behind it, or not sufficiently thick to oCFer 

 the necessary resistance to the moving force of the hammer, as is repre- 

 sented in Fig. 35. The blows of the hammer will cause the board to unduly 

 yield, and if strong enough, will break it, but will not drive in the nail. The 

 object i3 attained by applying behind the board, as in Fig. 3G, a block of wood, 



