THE ORIGIN OF MOTIVE POWER. 425 



pointed out it would be if heat were a substance) 

 the same as the quantity of heat taken in, but, 

 as Joule insisted, is less than the quantity taken 

 in by an amount mechanically equivalent to the 

 motive power developed. The modification of 

 Carnot's theory to adapt it to this truth was 

 alluded to ; and the great distinction which it leads 

 to between reversible and not reversible transfor- 

 mations of motive power was only mentioned. 



To facilitate farther statements regarding trans- 

 formations of motive power, certain terms, intro- 

 duced to designate various forms under which it 

 is manifested, were explained. Any piece of matter 

 or any group of bodies, however connected, which 

 either is in motion, or can get into motion without 

 external assistance, has what is called mechanical 

 energy. The energy of motion 1 may be called 

 cither u dynamical energy," or " actual energy." 

 The energy of a material system at rest, in virtue 

 of which it can get into motion, is called " potential 

 energy," or, generally, motive power possessed 



1 Shortly after the date of this Lecture, I gave the name ' ' kinetic 

 energy," which is now in general use. It is substituted for "actual " 

 and for ' ' dynamical " in the remainder of the text. (K. Nov. 21 , 1893). 



