IN THE LAST HALF-CENTUKY. 79 



Taking this as a very broad and gen- 

 eral statement of the essential facts of the 

 case, the raising of the stone is intelligible 

 enough, as a case of the communication 

 of motion from one body to another. But 

 the potential energy of the raised stone 

 is not so easily intelligible. To all ap- 

 pearance, there is nothing either pushing 

 or pulling it towards the earth, or the 

 earth towards it ; and yet it is quite cer- 

 tain that the stone tends to move towards 

 the earth and the earth towards the stone, 

 in the way defined by the law of gravita- 

 tion. 



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 or another, 

 without any discernible impact of other 

 bodies, is termed a 'force,' which is called 

 'attractive' in the one case, and 'repul- 

 sive' in the other. And such attractive 

 or repulsive forces are often spoken of as 



