112 FRICTION. 



support, using the first machine alone, considering the 

 result as a new power acting upon the second machine, 

 and so on. j 



212. What is Friction ? The chief impediment 

 to the motion of machinery arises from friction, which may 

 be defined as the resistance which a moving body 

 meets with from the surface on which it moves. 



213. The Cause of Friction. It is impossible, 

 by any known means, to produce a perfectly smooth sur- 

 face. Even a polished surface contains minute projec- 

 tions which fit into corresponding depressions on the cor- 

 responding surface. To produce motion of one surface on 

 the other, these projections must be lifted out, bent down, 

 or broken off. 



214. Eight Facts Concerning Friction. 



The following facts have been determined by experiment, 

 and may be easily illustrated in the same way : 



(1.) Friction is greatest at the 'beginning of motion. 

 After surfaces have been in contact for some time, 

 so that the projections of one have had opportunity 

 to sink deeper into the depressions of the other, the 

 resistance offered by friction is considerably in- 

 creased. Every teamster and street-car driver is 

 familiar with the fact. 



(2.) Friction increases with the roughness of the 

 surfaces. 



(3.) Friction is greater between soft bodies than 

 hard ones. 



(4.) Friction is nearly proportional to pressure. 

 (a.) Place a brick upon a horizontal board. Around it fasten one 



end of a cord and pass the other end over a pulley so that it may 



hang vertically. Add just weights enough to keep the brick in 



