Transportation on Land 261 



engine, a windmill, a railroad car, or any similar bearing, we 

 find that it comprises two parts, one the cast steel frame, and 

 the other the brass or gun metal bush, as it is called, which 

 provides an exceptionally smooth surface. The brass bush is 

 often grooved inside in the direction of the axle. The grooves 

 are filled with a white metal alloy, which, while fitting closely 

 about the axle, yields sufficiently to weight or pressure and 

 causes less friction than cast steel bearings. When worn out, 

 the alloy may be renewed without difficulty. 



Exercise. If convenient, a student may be assigned to visit a 

 factory or mill where babbitting is done, and report to the class on the 

 process. He may report also on other means employed in the factory 

 in reducing friction in machinery. 



As an instance of means employed to overcome useless fric- 

 tion, the bearings of a modern bicycle may be examined. 

 Students may be expected to explain their construction, op- 

 eration, and mechanical advantages. Evidently the bicycle 

 needs an adjustable bearing which, as may be seen, is secured 

 by the use of one or more rings of case-hardened steel balls 

 running in a groove. Both ball and roller bearings are utilized 

 in automobiles, since they cause less friction than any ma- 

 chine which slides ; and, when speed and power are necessary, 

 means which reduce sliding friction are utilized. 



Exercise. What are the chief applications of the roller in modern 

 machinery ? Compare these applications with its use by the ancients, 

 determining in what particular ways progress has been made. 



Rolling friction. The friction between the wheels and the 

 road is peculiarly interesting, although not quite so obvious 

 on steel rails as on dirt roads. On a highway a little hill of dirt 

 (Fig. 77) is formed in front of the rolling wheels of a loaded 

 vehicle. This is crushed, but it requires energy to do it. As 

 the friction thus encountered is continuous, the expenditure 

 of energy in overcoming it must also be continuous. The same 

 kind of friction is met on the steel rails over which a train passes. 



