38 



MECHANICS. 



Every farmer who erects a wall or building, every team- 

 ster who drives a heavy load, or even he who only carries a 

 heavy weight upon his shoulder, may learn something use- 

 ful by understanding the laws of gravity. 



It is familiar to every one, that a body resting upon a 

 broad base is more difficult to upset than when the base 



rig. 26. 



lit' 



A ^' 



is narrow. For instance, the square 

 block (fig. 26) is less easily thrown 

 over than the tall and narrow block 

 of equal w^eight, because, in turning 

 the square block over its lower edge, 

 the centre of gravity must be lifted 

 up considerably in the curve shown 

 by tlie dotted line c / but with a tall, 

 narrow block, this curve being almost 

 on a level, very little lifting is re- 

 quired. Hence the reason that a high load on a wagon is 

 so mucli more easily overturned than a low one. 



Of all forms, a pyramid stands the most firmly on its 

 base. The centre of gravity, c (fig. 26), being so near the 

 broad bottom, it must be elevated in a very steep curve 

 to throw the line of direction beyond the base. For this 

 reason, a stone wall, or the dam for a stream, will stand 

 better when broad at bottom and tapering to a narrow top 

 than if of equal thickness throughout. 



When a globe or round ball is placed upon a smooth 

 floor, it rests on a single point. If the Fig. 27. 



floor be lenel^ the line of direction will 

 fall exactly at this resting-point (fig. 

 27). To move the ball, the centre will 

 move precisely on a level, without be- 

 ing raised at all. This is the reason 

 that a ball, a cylinder, or a wheel is rolled forward so 

 much more easily than a flat-sided or irregular body. In 

 all these cases, the line of direction, although constantly 



