and on the Definitions of Physical Quantities. 89 



but it is there represented by the area of a figure or by symbols 

 only, and not designated by a name ; and such is also the case 

 in many subsequent mathematical writings. 



5. The application of the word " force " to that kind of quan- 

 tity is open to the objection, that when "force" is taken in the 

 sense in which Newton defines " vis motrix" the power of per- 

 forming work is not simply force, but force multiplied by space. 

 To make such an application of the word " force," therefore, 

 would have been to designate a product by the name properly 

 belonging to one of its factors, and would have added to the con- 

 fusion which has already arisen from the ambiguous employment 

 of that word. 



6. The word " power," though at first sight it might seem very 

 appropriate, was already used in mechanics in at least three differ- 

 ent senses, viz. : first, the power of an engine — meaning the rate at 

 w r hich it performs work, and being the product of force and space 

 divided by time ; secondly, the power, in the sense of effort or 

 pressure, which drives a machine; and thirdly, " mechanical 

 powers," meaning certain elementary machines. Thus u power " 

 was open to the same sort of objection with "force." 



7. About the beginning of the. present century, the word 

 " energy " had been substituted by Dr. Thomas Young for " vis 

 viva, }> to denote the capacity for performing work due to velocity ; 

 and the application of the same word had at a more recent time 

 been extended by Sir William Thomson to capacity of any sort 

 for performing work. There can be no doubt that the word 

 " energy " is specially suited for that purpose \ for not only does 

 the meaning to be expressed harmonize perfectly with the ety- 

 mology of ivepyet,a, but the word " energy " has never been used 

 in precise scientific writings in a different sense; and thus the 

 risk of ambiguity is avoided. 



8. It appeared to me, therefore, that what remained to be 

 done was to qualify the noun " energy " by appropriate adjec- 

 tives, so as to distinguish between energy of activity and energy 

 of configuration. The well-known pair of antithetical adjectives 

 " actual " and " potential " seemed exactly suited for that pur- 

 pose ; and I accordingly proposed the phrases " actual energy " 

 and " potential energy," in the paper to which I have referred*. 



9. I was encouraged to persevere in the use of those phrases 

 by the fact of their being immediately approved of and adopted 

 by Sir William Thomson — a fact to which I am disposed to as- 

 cribe in a great measure the rapid extension of fe their use in the 

 course of a period so short in the history of science as fourteen 



* Green's " Potential," or " Potential Function," is a quantity homoge- 

 neous with one form of potential energy, but of opposite algebraical sign. 



