82 MECHAXICS. 



It is familiar to most persons, that when two surfaces 

 slide over each other while pressing together, the minute 

 unevenness or roughness of their surfaces causes some ob- 

 struction, and more or less force is required. This resist- 

 ance is known as f net ion. 



EOLLING FEICTIOX. 



The term is also applied to the resistance of one body 

 rolling over another. This may be observed in various 

 degrees by rolling an ivory ball successively over a carpet, 

 a smooth floor, and a sheet of ice ; the same force which 

 would impel it only a few feet on the carpet would cause 

 it to move as many yards on a bare floor, and a still 

 greater distance on the ice. The two extremes may be 

 seen by the force required to draw a carriage on a deep 

 sandy or loose-gravel road, and on a rail-road. 



NATUKE OF FEICTIOT^. 



If two stiff bristle brushes be pressed with their faces 

 together, they become mutually interlocked, so that it 

 will be quite difiicult to give them a sliding 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 roughly-sawed wood, asperities interlock 

 in the same way, but less in degree ; a diminished force is 

 consequently required in moving the two surfaces against 

 each other. On smoothly planed wood the friction is still 

 less; and on polished 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 be- 

 tween different substances, the following simple and in- 



