98 OUTLINES OF NATURAL PHILOSOPHY. 



This follows from 99. DYNAMICS ; friction being a 

 force by which motion is uniformly retarded. 



a. Some paradoxical appearances are explained on this 

 principle. When a carpenter would drive the handle 

 of an axe into the head, he strikes against the handle 

 after it is slightly inserted into the head, holding it 

 loosely in his hand, without any firm support, and 

 the head perhaps downward ; at each blow the handle 

 is driven farther than if the blow had been given to 

 the head itself. The reason is, that the handle is 

 the lighter body, so that the velocity which a blow 

 communicates to it is greater than that which the 

 same blow would communicate to the head. The 

 friction is of consequence more effectually overcome. 



Thus, too, a nail is driven by a blow of no great force, 

 into a piece of wood where the mere friction is suffi- 

 cient to retain it against a great force applied to draw 

 it out. 



b. The same thing is exemplified in the method some- 

 times practised, of raising a stone by means of a 

 tackle fastened to it by an iron plug, driven into a 

 circular hole cut in the stone. A few blows of a 

 small hammer are sufficient to fix the plug, so that 

 it will serve not only to suspend the stone, though of 

 several hundred weight, but to draw it out of the 

 earth, in which it lies perhaps half-buried. When 

 the stone is raised, a few blows of the hammer given 

 obliquely to the plug, are sufficient to disengage it. 

 The stones on which this experiment has been made 

 have always possessed great toughness and indura- 

 tion. The whole is explained by the great power of 



percussion 



