CENTRE OF GRAVITY. EXAMPLES. 



39 



chanfjing its place, still continues to fall on the very point 

 on which the round body rests. 



But if the level floor is exchanged for a slope or inclin- 

 ed plane (tig. 28), the line of direc- Fig 28. 

 tion no longer falls at the touching- 

 point, but on the side from it down- 

 ward ; the ball will therefore, by its 

 mere weight, commence rolling, and 

 continwe to do so until it reaches the 

 bottom of the slope. 



Wheel-carriages owe their comparative ease of draught 

 to the fact that the centre of gravity in the load is moved 

 forward by the rolling of the wheels, on a level, or paral- 

 lel with the surface of the road, just in the same way that 

 the round ball rolls so easily. Each wheel supporting its 

 part of the load at the hub, the same rule applies to each 

 as to a ball or cylinder alone. Hence, on a level road, the 

 line of direction falls precisely where the wheels rest on 

 the ground, but if the road ascend or descend, it falls else- 

 where ; this explains the reason why it will run by its own 

 weight down a slope. 



Whenever a stone or other obstruction occurs in a road, 

 Fis. 2f. it becomes requisite to raise the 



centre by the force of the team- 

 and by means of oblique motion, 

 -\ so as to throw the wheel over it, 

 as shown by fig. 

 29. One of the 

 reasons thus 

 becomes very 

 plain why a 

 large wheel will 

 run with more ease on a rough road than a smaller one ; 

 the larger one mounting any stone or obstruction without 

 liftins: the load so much out of a level or direct line, as 

 sh6wn by the dotted lines in the annexed figures, (figs. 29 



Fig. 30. 



