88 Dr. Rankine on the Phrase " Potential Energy/ 3 



the first curve is no exception, since the times at which the 

 numbers were counted could not show this increase. 



Other slight differences in the details of the different curves 

 are doubtless due to the differences in the intervals, and in the 

 periods during which the numbers were counted. 



XIII. On the Phrase " Potential Energy/ 3 and on the Defini- 

 tions of Physical Quantities. By W. J. Macquorn Rankine, 

 C.E., LL.D., F.R.SS. Lond. $ Edinb., £fc* 



1. TN the course of an essay by Sir John Herschel " On the 

 -i- Origin of Force," which appeared some time ago in the 

 Fortnightly Review, and has lately been republished in a volume 

 entitled " Familiar Lectures on Scientific Subjects," the opinion 

 is expressed that the phrase " potential energy" is " unfortunate, 

 inasmuch as it goes to substitute a truism for the announce- 

 ment of a great dynamical fact" (Familiar Lectures, p. 469). 



2. There is here no question as to the reality of the class of 

 relations amongst bodies to which that phrase is applied, nor as 

 to any matter of fact concerning those relations, but as to the 

 convenient and appropriate use of language. This is a sort of 

 question in the decision of which authority has much weight ; 

 and when an objection to the appropriateness of a term is made 

 by an author who is not less eminent as a philosopher than as a 

 man of science, and whose skill in the art of expressing scientific 

 truth in clear language is almost unparalleled, it becomes the 

 duty of those who use that term to examine carefully their 

 grounds for doing so. 



3. As the phrase " potential energy," now so generally used 

 by writers on physical subjects, was first proposed by myself, in 

 a paper "On the General Law of the Transformation of Energy "f* 

 read to the Philosophical Society of Glasgow on the 5th of Janu- 

 ary, 1853, 1 feel that the remark of Sir John Herschel makes it 

 incumbent upon me to explain the reasons which led me, after 

 much consideration, to adopt that phrase for the purpose of de- 

 noting all those relations amongst bodies, or the parts of bodies, 

 which consist in a power of doing work dependent on mutual 

 configurations. 



4. The kind of quantity now in question forms part of the 

 subject of the thirty -ninth proposition of Newton's Principia ; 



* Communicated by the Author, having been read to the Philosophical 

 Society of Glasgow. 



t Viz. that the effect of the presence of a quantity of actual energy, in 

 causing transformation of energy between the actual and the potential 

 forms, is the sum of the effects of all the parts of that quantity. 



