CH - v ] MECHANICAL RECREATIONS 



95 



the product of the weight of the 1 bullet and its distance from 

 the vertical through G. 



The idea of the constructors of such machines was that, as 

 the bullet in any compartment would roll under gravity to the 

 lowest point of the compartment, the bullets on the right-hand 

 side of the wheel in the diagram would be farther from the 

 vertical through C than those on the left. Hence the sum of 



the moments of the weights of the bullets on the right would 

 be greater than the sum of the moments of those on the left. 

 Thus the wheels would turn continually in the same direction 

 as the hands of a watch. The fallacy in the argument is 

 obvious. 



Another large group of machines for producing perpetual 

 motion depended on the use of a magnet to raise a mass which 

 was then allowed to fall under gravity. Thus, if the bob of a 

 simple pendulum was made of iron, it was thought that magnets 

 fixed near the highest points which were reached by the bob in 

 the swing of the pendulum would draw the bob up to the same 

 height in each swing and thus give perpetual motion, but the 

 inventors omitted to notice that the bob of the pendulum would 

 gradually get magnetised. 



Of course it is only in isolated systems that the total amount 

 of energy is constant, and, if a source of external energy can be 

 obtained from which energy is continually introduced into the 

 system, perpetual motion is, in a sense, possible ; though eveu 

 here materials would ultimately wear out. Streams, wind, the 



