SIZE OF WHEELS CX ROADS. .89 



axle, will run more easily as the axle is made smaller. 

 This is not owing to the rubbing surfaces being less in 

 size, as some mistakenly suppose, for it has just been 

 shown that this makes very little or no difference, pro- 

 vided the pressure is the same; but it is owing to the 

 leverage of the wheel on the friction at the axis ; and the 

 smaller the axle, the greater is this leverage ; for, if the 



Fig. 97. 



axle, a (fig. 97), be six 

 inches in circumference, 

 and the Avheel, h c, be 

 ten feet in circumference, 

 then the outer part of 

 the wheel will move 

 twenty times further 

 than the part next the 

 axle. Therefore, accord- 

 ing to the rule of virtual 

 velocities (already ex- 

 plained,) one ounce of 

 force at the rim of the 

 wheel will overcome twenty ounces of friction at the 

 axle ; or if the axle were twice as large, then, according 

 to the same rule, it would require two ounces to over- 

 come the same fiiction acting between larger surfaces. 



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

 plying motion, if not made too heavy, run with less force 

 than smaller ones, the power acting upon a larger lever. 

 Plorse-powers for thrashing-machines, consisting chiefly of 

 a large, light crown-wheel, well stiffened by brace-work, 

 have been found to run with remarkable case; a good 

 example of which exists in wliat is 

 horse-power, when made in the best manner. 



FRICTION-WnEELS. 



On the preceding principle, friction-wheels or friction- 

 roUers are constructed, for lessening as much as possible 



