AND THE NATURE OF FORCE. 607 



in turn to other bodies, and so on. Ic does not assert that when the 

 prime mover ceases to act, the bodies which resisted the motion will 

 then drive the body back to its original position. 



Take for instance the illustration just mentioned, of drawing a 

 piston in an air-tight tube against the pressure of the atmosphere. 

 If we draw the piston partly up the tube, we communicate a certain 

 amount of actual energy to the air, which energy becomes diffused 

 through the atmosphere. If we let go the piston the pressure of the 

 atmosphere will drive it to the bottom of the tube, but this is not in 

 consequence of the energy which had been previously communicated 

 to the atmosphere ; that energy is by this time diffused through space, 

 and the principle of conservation asserts that it is still in existence 

 and never can be destroyed. It cannot assert that when, the piston 

 is let go it will be forced to the bottom of the tube. That depends 

 upon the pressure of the air being continued, about which we do not 

 necessarily know anything. It happens that it does so in this 

 instance, and we have a case of so-called potential energy. In' boring 

 a hole with an auger, we do not expect that when we let go the 

 handle of the auger it will be immediately turned round and worked 

 out of the hole, acquiring an amount of energy equal to that which 

 had been expended. Why is not energy restored in this case as well 

 as the other. It is plainly owing to circumstances that the principle 

 had nothing to do with. What the principle does assert is, that in 

 both cases a definite amount of actual energy has been imparted to 

 the resisting parts, and that this energy is always in existence some- 

 where as actual energy. So if it were possible for the molecules of 

 bodies in cooling to remain apart, the principle of conservation 

 would be in nowise affected thereby. 



Thus the convertibility of natural forces can only be taken to 

 mean the transferences of actual energy from one portion of matter 

 to another which are incessantly going on. 



In conclusion, we may sum up our results as follows : 



On the theory of action at a distance — 

 (a) The principle of conservation cannot hold in nature. 

 ( 6 ) The term potential energy, from its definition, is incapable of 



strict application to any natural system of bodies. 

 ( c ) The principle of conservation, regarded merely as expressing 

 periodical restorations of actual energy, cannot hold in nature. 



