80 THE ADVANCE OF SCIENCE 



if they were real things, capable of exert- 

 ing a pull, or a pnsh, npon 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 sum- 

 mit 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 opera- 

 tion, 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, ex- 

 cept the bob, were abolished, it would 

 move for ever in the direction of a tangent 



