112 MECHANICS. 



Therefore, according to the rule of virtual velocities, 

 one ounce of force at the rim of the wheel will over- 

 come twenty ounces of friction at the axle ; or if the 

 axle were twice as large, then, according to the same 

 rule, it would requu-e two ounces to overcome the same 

 friction acting between larger surfaces. 



For this reason, large wheels in wheel-work for mul- 

 tiplying motion, if not made too heavy, run with less 

 force than smaller ones, the power acting upon a larger 

 lever. Horse-powers for thrashing-machines, consist- 

 ing chiefly of a large, light crown-wheel, well stiffened 

 by brace-work, have been found to run with remarka- 

 ble ease ; a good example of which exists in what is 

 known as Talpin's horse-power, when made in the 

 best manner. 



FRICTION-WHEELS. 



On the preceding principle, friction-wheels or fric- 

 tion-rollers are constructed, for lessening as much as 

 possible the friction of axles in certain 

 cases. By this contrivance, the axle, a 

 {Fig. 89), instead of revolving in a sim- 

 ple hole or cavity, rests on or between 

 Friction-wheels. ^^^ ^^^^^ ^f ^^^^ ^^^iey: whccls. As the 



axle revolves, the edges turn with it, and the rubbing 

 of surfaces is only at the axles of these two wheels. 

 If, therefore, these axles be twenty times smaller than 

 the wheels, the friction will be only one twentieth the 

 amount without them. This contrivance has been 

 strongly recommended and considerably used for the 

 cranks of grindstones {Fig. 90), but it was not found 

 to answer the intended purpose so well as was expect- 



