38 WHEEL CH. Ill 



carriages are made to track the same, but in different 

 localities this track varies ; for instance, in New York 

 it is 4 ft. 8 in., while in Pennsylvania it is 5 ft. 2 in. 



Wheel. — The wheel is a simple mechanical con- 

 trivance for transferring the friction between the 

 moving body and the ground, to a surface which 

 reduces that friction, and also for diminishing the 

 resistance opposed by an obstacle on the ground. 



If a weight is drawn along on the ground on a 

 sled, the friction between the runner of the sled and 

 the ground is great, and is due to the roughness 

 of both surfaces, as is shown by the fact that when 

 both are very smooth, as in a steel runner on ice, 

 the friction becomes very small. When a wheel is 

 introduced, as it does not slide on the ground, the 

 friction is changed to a rolling friction, and the rub- 

 bing friction is transferred to the axle, which can 

 have its surface so polished and supplied with a 



lubricant that this 

 rubbing friction is 

 greatly diminished. 

 As to the ob- 



stacle : it is obvious 

 that the wheel has 

 to be lifted over it, 

 and the draught, 

 acting in the line 

 A B (Fig. 17), pulls on a bent lever, BCD, raising 

 the weight, which may be considered as concentrated 



