II THE PROGRESS OF SCIENCE 91 



towards the earth and the earth towards the stone, 

 in the way defined by the law of gravitation. 



In the currently accepted language of science, 

 the cause of motion, in all such cases as this, when 

 bodies tend to move towards or away from one 

 another, without any discernible impact of other 

 bodies, is termed a " force," which is called " at- 

 tractive " in the one case, and " repulsive " in the 

 other. And such attractive or repulsive forces 

 are often spoken of as if they were real things, 

 capable of exerting a pull, or a push, upon the 

 particles of matter concerned. Thus the potential 

 energy of the stone is commonly said to be due to 

 the " force " of gravity which is continually 

 operating upon it. 



Another illustration may make the case plainer. 

 The bob of a pendulum swings first to one side 

 and then to the other of the centre of the arc 

 which it describes. Suppose it to have just 

 reached the summit of its right-hand half-swing. 

 It is said that the "attractive forces" of the 

 bob for the earth, and of the earth for the bob, set 

 the former in motion ; and as these " forces " are 

 continually in operation, they confer an accelerated 

 velocity on the bob ; until, when it reaches the 

 centre of its swing, it is, so to speak, fully charged 

 with kinetic energy. If, at this moment, the 

 whole material universe, except the bob, were 

 abolished, it would move for ever in the direction 

 of a tangent to the middle of the arc described. 



