MECHANICS, 108 



p. 155. Mr K. FITZGERALD has described the con- 

 struction of friction wheels, by which he reduced the 

 power of friction from 425 Ib. to two and three- 

 fourths. 



Rollers put under a heavy body, diminish the friction in 

 the greatest degree possible, if they are spheres or cy- 

 linders, without any axis on which they are constrain- 

 ed to move. The conversion of friction from sliding 

 to rolling is then complete. 



The wheels of carriages are contrivances of the same 

 kind : but in them the conversion above referred to 

 is imperfect : for though the friction at the circum- 

 ference of the wheel is that of rolling, at the axis, 

 it is that of rubbing ; yet as, in this last, the dis- 

 tance from the centre of motion is small, the mo- 

 mentum of the friction is small in the same pro- 

 portion. 



170. In wheel carriages, it is more advantageous 

 that the wheel should turn on a fixed axle, than 

 that the axle should turn with the wheel, because 

 in the latter case, when the direction of the motion 

 changes, one of the wheels must go backward, and 

 must oppose the motion of the other, so that the 

 friction of rubbing is substituted for that of rolling. 



171. The plane on which a carriage moves, and 

 the line of draught, being both horizontal, the ad- 

 vantage for surmounting an immoveable obstacle, of 

 a given height, is as the square root of the radius of 

 the wheel. 



Let 



