92 MECHANICS. 



friction at the axle is but a small loortion of tlie resistance,* 

 a slight variation in tlie effects in tlie lubricating sub- 

 stance is of less importance than retaining its place. In 

 more complex machinery, as horse-powers for thrashing- 

 machines, friction becomes a large item, unless the parts 

 are kept well lubricated with the best materials. 



Leather and hemp bands, Avhen used on drums for 

 wheel-work, should possess as much friction as possible, 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 moistening and rasping 

 small drums may be useful. Facing with buff leather or 

 with coarse thick cloth also accomplishes a useful purpose. 

 It often happens that wetting or oiling bands will prevent 

 slipping, by keeping 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 nature 

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

 on the surface of the earth would be clashing against each 

 other; rivers Avould dash with an unbounded velocity, and 

 we should see little besides collision and motion. At 

 present, whenever a body acquires a great velocity, it soon 

 loses it by friction against the surface of the earth. The 

 friction of water against the surfaces it runs over soon 

 reduces the rapid torrent to a gentle stream ; the fury of 



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

 eter of the Avheels ten times as j^rcat as the diameter of the axle, the 

 friction at the axles Avill be reduced to onc-lwelftli of a tenth, or one 

 hundred and twentieth iKirt of the force, according to Ihc law of virtual 

 velocities as applied to the wheel and axle. 



