MECHANICAL PARADOXES. 



centre to the edge, and lay it flat on the table 

 against the edge of a thin flat ruler, or some 

 object which is thinner than the disc itself, in 

 such a way that the end C of the line touches 

 the ruler. Now roll the disc slightly back- 

 wards and forwards against the ruler, so that 

 the point B moves an eighth of an inch or 

 less. The most careful observation will not 



FlQ. 3. BACKWARD MOTION OF PART OF RAILWAY CARRIAGE 

 WHEEL 



detect any movement of the point C, at the 

 very end of the line. If the disc be rolled a 

 little further, so that B moves a quarter of 

 an inch or more, the first visible movement 

 of the point C will be sideways from the edge 

 of the ruler. For some time after this has 

 occurred it will be quite impossible to see any 

 backward or forward movement of this point, 

 lengthways of the ruler. 



This fact may be examined on a larger 

 scale by wheeling a bicycle forward till the front 

 wheel is stopped by some vertical obstacle, 



6 



