IX THE LAST IIALF-CEXTURY. 77 



cannot be explained, but must be taken 

 as an ultimate fact about which, explica- 

 ble or inexplicable, there can be no doubt. 

 Strictly speaking, we have no direct ap- 

 prehension of any other cause of motion. 

 But experience furnishes innumerable ex- 

 amples of the production of kinetic energy 

 in a body previously at rest, when no im- 

 pact is discernible as the cause of that en- 

 ergy. In all such cases, the presence of a 

 second body is a necessary condition ; and 

 the amount of kinetic energy, which its 

 presence enables the first to gain, is strict- 

 ly dependent on the relative positions of 

 the two. Hence the phrase energy of po- 

 sition, which is frequently used as equiva- 

 lent to potential energy. If a stone is 

 picked up and held, say, six feet above 

 the ground, it has potential energy, be- 

 cause, if let go, it will immediately begin 

 to move towards the earth ; and this en- 

 ergy may be said to be energy of position, 

 because it depends upon the relative posi- 



