1856.] and Transformations of Motive Power. 201 



clear that in this experiment the electric currents, discovered by 

 Faraday to be induced in the copper in virtue of its motion in the 

 neighbourhood of the magnet, generated the heat which became 

 sensible. Joule first raised the question, Is any heat generated by 

 an induced electric current in the locality of the inductive action ? 

 He not only made experiments which established an affirmative 

 answer to that question, but he used the mode of generating heat 

 by mechanical work established by those experiments, as a way of 

 finding the numerical relation between units of heat and units of 

 work, and so first arrived at a determination of the mechanical 

 value of heat. At the same time (1843) he gave another determi- 

 nation founded on the friction of fluids in motion ; and six years 

 later he gave the best determination yet obtained, according to 

 which it appears that 772 foot pounds of work, (that is 772 times 

 the amount of work required to overcome a force equal to the 

 weight of 1 lb. through a space of 1 foot,) is required to generate 

 as much heat as will raise the temperature of a pound of water by 

 one degree. 



The reverse transformation of heat into mechanical work was 

 next considered, and the working of a steam-engine was referred 

 to as an illustration. An original model of Stirling's air-engine 

 was shown in operation, developing motive power from heat sup- 

 plied to it by a spirit lamp, by means of the alternate contractions 

 and expansions of one mass of air. Thermo-electric currents, and 

 common mechanical action produced by them, were referred to as 

 illustrating another very distinct class of means by which the same 

 transformation may be effected. It was pointed out that in each 

 case, while heat is taken in by the material arrangement or machine, 

 from the source of heat, heat is always given out in another locality, 

 which is at a lower temperature than the locality at which heat is 

 taken in. But it was remarked that the quantity of heat given out 

 is not, (as Camot pointed out, it would be if heat were a substance,) 

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

 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 dis- 

 tinction which it leads to between reversible and not reversible 

 transformations of motive power was only mentioned. 



To facilitate farther statements regarding transformations of 

 motive power, certain terms, introduced 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 may be 

 called either " 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 among different pieces of matter, in virtue of their relative 



