LINE OF DIRECTION. 



55 



Fig. 27. 



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 tliis 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 pomt. If the floor be levels 

 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 being 

 raised at all. This is the reason that 

 a ball, a cylinder, or a wheel is rolled 

 forward so much more easily than any flat-sided or ir- 

 regular body. In all these cases, the line of direction, 

 although constantly changing 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 in- 

 clined plane {Fig. 28), the Ime of 

 direction no longer falls at the touch- 

 ing-point, but on the side from it 

 downward ; the ball will therefore, 

 by its mere weight, commence roll- 

 ing, and continue to do so till 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 parallel with the surface of the road, just in 

 the same way that the round ball rolls so easily. Each 



