56 



MECHANICS. 



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 pre- 

 cisely where the wheels rest on the ground, but if the 

 road ascend or descend, it falls elsewhere; this ex- 

 plains the reason why it will run by its own weight 

 down a slope. 



"Whenever a stone or other obstruction occurs m a 

 road, it becomes requisite to raise the centre by the 

 force of the team and by means of oblique motion, so 

 Fie. 29. as to throw the wheel over it, 



as shown by Fi^. 29. One 

 of the reasons thus becomes 

 very plain why a large wheel 

 Fig. 30. will run with 



^^^~" more ease on 

 a rough road 

 than a small- 

 er one ; the 



larger one mounting any stone or obstruction without 

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

 shown by the dotted lines in the annexed figures {Figs. 

 29 and 30). Another reason is, the large wheel does 

 Fig. 31. Fig. 32. not siuk iuto the smaller 



cavities in the road. 



A self-supporting fruit- 

 ladder [Figure 31) (the 

 centre of gravity, when 

 in use, being at or near 

 the top) must have its 

 A dangerous- Icgs morc wldcly Spread, 

 uddlr^'^' to be secure from fall- 



A Jirmly-set fruit-ladder. 



