532 Dr. Lodge on Action at a Distance, 



Another mode of expressing the matter is to say that the 

 stone possesses potential energy by virtue of its position, just 

 as a bullet possesses energy of position when placed inside a 

 loaded gun, or as an arrow possesses it when strung on a bent 

 bow. But one has carefully to shut one's eyes and think 

 vaguely if one is to maintain this ground with any degree of 

 comfort ; for it will be soon perceived that it is not the bullet 

 nor the arrow which really possesses the energy before firing, 

 but the powder and the bow, and that it is difficult to see 

 how a passive stone at rest in its ordinary condition, notwith- 

 standing that it is in an elevated position, can properly be 

 said to possess any energy at all. 



The difficulty of supposing that the potential energy belongs 

 to the stone any more than to the earth having been felt, the 

 fact was denied ; and it is now orthodox to say that the 

 energy belongs neither to the stone nor to the earth (neither 

 to the bullet nor to the gun), but to the system in virtue of its 

 configuration. 



Now this is undoubtedly true, and as a mere abstract and 

 mathematical way of stating the facts it is sufficient ; but on 

 examining it more closely one perceives that it is only a 

 mystical way of saying that the energy really belongs to the 

 medium which is driving the two bodies together (or apart) — 

 the. strained aether (as I believe), /or Le Sage's corpuscles, or 

 Mr. Tolver Preston's gravity-gas, or whatever it may be. 



The energy must be possessed by something, unless we go 

 back and say it is only possz6fe energy (which is the most 

 barefaced denial of its conservation); and, according to my 

 view, potential energy can only be possessed by a body 

 exerting force ; so what is there, then, that can possess the 

 energy but the medium which surrounds the two bodies and 

 which is pressing them together ? 



This is what is exerting the force ; and when the stone is 

 allowed to fall, this is what does the work and transfers its 

 energy both to the stone and earth, though practically all to 

 the stone ; so that, just before the blow takes place, the stone 

 is found to possess the whole of the energy in the kinetic 

 form, work having been done upon it during the whole time 

 of fall. 



This statement is not identical with that generally accep- 

 ted ; but I maintain strongly that it is the only one which is 

 clear and accurate and devoid of vagueness. 



The equation ?ngh = ^mv 2 is commonly interpreted to mean 

 that the energy of the stone merely changes its form during 

 the fall, no work being done unless the motion is resisted. 

 But I deny that energy can ever change its form without 



