ADVANTAGES OF FRICTION. 115 



sistance,* a slight variation in the effects in the lubri- 

 cating substance is of less importance than retaining 

 its place. In more complex machinery, as horse-pow- 

 ers for thrashing-machines, friction becomes a very 

 large item, unless the parts are kept well lubricated 

 with the best materials. 



Leather and hemp bands, when used on drums for 

 wheel-work, should possess as much friction as possi- 

 ble, to prevent slipping, thus avoiding the necessity of 

 tightening them so much as to increase the friction of 

 the axles. Wood with a rough surface has one half 

 more friction than when worn smooth ; hence moisten- 

 ing and rasping small drums may be useful. Facing 

 with buff leather or with coarse thick cloth also ac- 

 complishes a useful purpose. It often happens that 

 wetting or oiling bands will prevent slipping, by keep- 

 ing their surfaces soft, and causing them to fit more 

 closely the rough surface of the drum. 



ADVANTAGES OF FRICTION. 



Although friction is often a serious inconvenience, or 

 loss, in lessening the force of machines, there are many 

 instances in which it performs important offices in na- 

 ture and in works of art. " Were there no friction, all 

 bodies on the surface of the earth would be clashing 

 against each other ; rivers would dash with an un- 

 bounded velocity, and we should see little besides col- 



* If the friction at the axle be one twelfth of the force, and the 

 diameter of the wheels ten times as great as the diameter of the axle, 

 the friction at the axles will be reduced to one twelfth of a tenth, or 

 one hundred and twentieth part of the force, according to the law 

 of virtual velocities as applied to the wheel and axle. 



