90 Dr. Rankine on the Phrase " Potential Energy" 



years*. I had also the satisfaction of receiving a very strong 

 expression of approval from the late Professor Baden Powell. 



10. Until some years afterwards I was not aware of the fact 

 that the idea of a phrase equivalent to " potential energy " in its 

 purely mechanical sense had been anticipated by Carnot, who, in 

 an essay on machines in general, employed the term " force vive 

 virtuelle," of which " potential energy '? might be supposed to 

 be almost a literal translation. That coincidence shows how na- 

 turally the phrase "potential energy," or something equivalent, 

 occurs to one in search of words appropriate to denote that power 

 of performing work which is due to configuration and not to 

 activity. 



11. Having explained the reasons which led me to propose 

 the use of the phrase " potential energy/' I have next to make 

 some observations on the objection made by Sir John Herschel 

 to that phrase, that " it goes to substitute a truism for a great 

 dynamical fact." 



12. It must be admitted that the use of the term " potential 

 energy" tends to make the statement of the law of the conser- 

 vation of energy wear to a certain extent the appearance of a 

 truism. It seems to me, however, that such must always be the 

 effect of denoting physical relations by words that are specially 

 adapted to express the properties of those relations, or, what 

 amounts virtually to the same thing, of drawing up precise and 

 complete definitions of physical terms. Let A and B denote 

 certain conceivable relations, and let them be precisely and com- 

 pletely defined; then from the definitions follows the proposition 

 that A and B are related to each other in a certain way ; and 

 that proposition wears the appearance of a truism, and is virtu- 

 ally comprehended in the definitions. But it is not a bare truism ; 

 for when with the definitions are conjoined the two facts ascer- 

 tained by experiment and observation — that there are relations 

 amongst real bodies corresponding to the definition of A, and 

 that there are also relations amongst real bodies corresponding 

 to the definition of B — the proposition as to relation between A 

 and B becomes not a bare truism, but a physical fact. In the 

 present case, for example, (< actual energy " and a potential 

 energy " are defined in such a way that from the definitions it 

 follows that what a body or a system of bodies gains in one form 

 of energy through mutual actions it loses in the other form — in 

 other words, that the sum of actual and potential energies is 

 " conserved ; " and this sounds like a truism ; but when it is 

 proved by experiment and observation that there are relations 

 amongst real bodies agreeing with the definitions of " actual 



* Sir William Thomson and Professor Tait have lately substituted the 

 word "■ kinetic " for " actual." 



