Frictim. 137 



But as neither supposition is true in any case, it follows, (1.) 

 That the resistance of friction arises in part from the difficulty 

 of bending the asperities, and in part from the necessity of rais- 

 ing in a degree the body or incumbent mass. (2.) That the 

 asperities having only a limited degree of adhesion, when the 

 force necessary to cause the body to move exceeds this, the as- 

 perities yield, or are broken down, and the surfaces are grad- 

 ually worn. Thus the effect of friction in machines is not only 

 to consume a part of the force employed, but also to destroy the 

 machines themselves. 



It would seem difficult, if not impossible, to establish general 

 rules, sufficiently exact for determining the force of friction. In- 

 deed it will be readily seen, that this resistance must vary ac- 

 cording to the nature and texture of the surfaces in contact, their 

 flexibility, and the adaptation in size and figure of the prominent 

 parts and cavities to each other, and according as the force is 

 greater or less by which the surfaces are pressed together ; 

 moreover, on account of the flexible nature of surfaces, the prom- 

 inent parts are found to penetrate to a greater depth when more 

 time is allowed for enlarging the openings which they tend to 

 enter. 



It belongs to experiment alone to enlighten us upon these 

 points, and to teach us the proportional effect due to each. The 

 information, however, derived from this source, is not yet so 

 perfect and complete as could be wished, though it is such as may 

 be useful on many occasions. We proceed now to make known 

 some of the results of experiments, as also the method of applying 

 them in calculating the effect of friction in the different kinds 

 of machines, and the different kinds of motion. 



235. (1.) When the surfaces which are to rub the one upon 

 the other, are of the same kind of matter, the resistance of fric- 

 tion, other things being the same, is greater than when the sur- 

 faces are of different kinds. Thus, two pieces of wood of differ- 

 ent kinds slide upon each other with less difficulty than two of 

 the same kind. Iron rubbing on copper has less friction than 

 iron on iron or copper on copper. This is supposed to be owing 

 to the prominent parts and cavities being more nearly fitted to 

 each other in the latter case than in the former. 

 Mech. 18 



