the Foundations of Dynamics. 17 



It is easy to express oneself so as to be understood to 

 object to the whole idea of potential energy. It is not to the 

 use of the term that I object, nor even to its misuse, so long 

 as it be done in the temporary interest of some specific 

 problem. It is by no means necessary always to attend to 

 everything : acting mechanism is frequently, with con- 

 venience and brevity, ignored, especially when one is really 

 ignorant of its dynamical nature ; but in laving foundations we 

 should not make these omissions and slurrings. It is the 

 erroneous localization of energy in a fundamental or theoret- 

 ical treatment of the subject that I deprecate, and I believe 

 that everyone will agree that in all such treatment con- 

 venient fictions are better avoided. 



There certainly exists potential energy in the case of a 

 raised stone, but it does not belong to the stone ; it belongs 

 to the medium, whatever it is, which is exerting force on 

 the stone and on the earth, and pressing the two lumps of 

 matter together. It is properly called static, as distinguished 

 from kinetic, because it is the energy of force, not the energy 

 of motion *. As I have frequently pointed out, the two forms 

 of energy correspond to the two factors of the product work 

 or activity. Both factors are necessary for the actual per- 

 formance of work. Until both factors are present, the energy 

 is merely stored. As soon as the missing factor is supplied, it 

 is transferred from the body acting to the body acted upon, 

 e. g., from the gravitation medium to the stone. 



But it will be said, suppose the stone is not allowed to drop 

 freely, but is used as a clock-weight ; it is doing work all 

 the time it slowly falls, hence when raised it must have had 

 energy. But in this case I should deny that the stone is doing 

 anything active ; it is a necessary concomitant, it is a link of 

 communication, it is not really itself doing work. Its presence 

 indeed and that of the earth are necessary to the existence of 

 the stress in the ether [permitting that or some other equivalent 

 hypothesis for the moment] ; but it is the ether stress which 

 is doing the work and driving the wheels of the clock. A 



* I do not mean to assert or deny anything concerning the nature of 

 gravity. It may be that all energy is ultimately of the nature of motion, 

 as we know Lord Kelvin has taught us may be the case with elastic stress 

 for instance ; hut force is the proximate mode of action of a coiled spring, 

 and so it may properly be described as static energy, whatever its ultimate 

 nature. The reasons for affixing to static energy the epithet potential 

 are mainly mathematical and historical. There is much convenience in 

 thus linking it on to the potential function, and there is no particular 

 inconvenience unless the adjective be misunderstood in the sense of 

 " possible." It is indeed possible work or possible activity, but so is every 

 kind of available energy ; it is not possible energy, for that term is equi- 

 valent to the assertion that not yet is it energy at all. 



Phil Mag. 8. 5. Vol. 36. No. 218. July 1893. C 



