MEANS OF TRANSPORT. 



such as a house-wall. Now put a chalk mark 

 on the lowest part of the wheel and on the 

 ground immediately beneath it. Now back the 

 machine a little from the wall ; and when the 

 greater part of it has moved an inch, the 

 marked part of the wheel will not appear to 

 have moved at all. As the machine is slowly 

 backed a little further, the first visible move- 

 ment of the bottom of the wheel is upwards 

 from the ground, while it is still impossible 

 to see that the chalk mark has backed from 

 the wall to the smallest extent. 



It is, in fact, true of all wheels, and at all 

 speeds, that the part immediately in contact 

 with the ground has no movement at all, so 

 that even when a train is travelling at the 

 speed of a mile a minute, each wheel has at 

 every moment a part namely, the lowest 

 part resting at that moment on the rail which 

 does not at that moment move forward at all. 



This being established, take a bristle, twice 

 as long as the mark A C, and attach it with 

 gum in the position shown at D G, half its 

 length projecting beyond the edge. Now place 

 the disc in the same position as before, with 

 the point F against the edge of the ruler, and 

 the outer half F G of the bristle reaching 

 over it. Roll the disc very slightly back- 

 wards and forwards as before, watching the 

 point G at the outer end of the bristle. It 

 will be seen to move always in the opposite 



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