262 Introduction to the Study of Science 



The rails are depressed ever so slightly by the wheels, and each 

 wheel must be pulled continuously from the bottom of the 



^ depression. This adds to the labor of the 



locomotive. Resistance of this kind is known 

 as rolling friction. 



The rolling friction of wagons and auto- 

 mobiles is reduced by the use of wheels with 

 broad tires, which prevent their sinking 



deeply into the road. In order to eliminate 



FIG. 77. Diagram m l ar g e P ar t the rolling friction of railroads, 

 showing rolling fric- roadbeds are built solidly of crushed, closely 



tion on road (greatly 1111 j i i_ i 



exaggerated) . packed rock, heavy ties or sleepers being used ; 



the size, weight, and hardness of the steel 

 rails are increased ; and smooth, properly tempered surfaces 

 are provided on the wheels of engines and cars. Examination 

 of a track will show to what extent these means are utilized. 



SUMMARY 



By means of transportation the whole earth may be made to con- 

 tribute to the needs of any community. By travel individual and 

 social progress is accelerated. 



The topography and climate of a country largely control and modify 

 the means of transportation. 



A primitive vehicle of transportation is the pole drag. It is based 

 on the principle of the lever. 



The forces in a lever are inversely as the length of the arms. 



The use of the lever is illustrated in such appliances as (1) the 

 teeter-board and equal-arm balance, (2) the crowbar, (3) the claw- 

 hammer, (4) pliers, (5) the nutcracker, (6) the windlass, (7) the loco- 

 motive driving wheel. 



The use of the roller substitutes rolling friction for sliding friction. 



Friction of a wheel on the ground, or rolling friction, is reduced 

 by the use of a hard roadbed. Friction of a wheel on its axle is re- 

 duced by means of oil, by babbitting, and by roller or ball bearings. 



Sliding friction decreases when an object begins to slip. 



REVIEW QUESTIONS 



1. Mention several facts that show the importance to your com- 

 munity of transportation. 2. Mention several food products of com- 



