104 MECHANICS. 



NATURE OF FRICTION. 



If two stiff bristle brushes be pressed with their faces 

 together, they become mutually interlocked, so that it 

 will be quite difficult to give them a shding motion. 

 This may be considered as an extreme case of friction, 

 and serves to show its nature. In two pieces of coarse, 

 rough sandstone, or of rouglily-sawed wood, asperities 

 interlock in the same way, but less in degree ; a di- 

 minished force is consequently required in moving the 

 two surfaces against each other. On smoothly-planed 

 wood the friction is still less ; and on poUshed glass, 

 where the unevenness can not be detected without the 

 aid of a powerful magnifying glass, it is reduced still 

 further in degree. 



ESTIMATING THE AMOUNT OF FRICTION. 



In order to determine the exact amount of friction 

 between different substances, the following simple and 

 ingenious contrivance is adopted : an inclmed plane, 

 a b {Fig: 87), is so formed that it may be raised to any 



Fig. 87. 



desired height by means of the arc of a circle and a 

 screw. Lay a flat surface of the substance we wish 

 to examine upon this inclined plane, and another small- 



