Prof. Thomson on the Dynamical Theory of Heat. 11 



Part I. — Fundamental Principles in the Theory of the Motive 

 Power of Heat. 



7. According to an obvious principle, first introduced, how- 

 ever, into the theory of the motive power of heat by Carnot, 

 mechanical effect produced in any process cannot be said to have 

 been derived from a purely thermal source, unless at the end of 

 the process all the materials used are in precisely the same phy- 

 sical and mechanical circumstances as they were at the beginning. 

 In some conceivable " thermo-dynamic engines," as for mstance 

 Faraday's floating magnet, or Barlow^s " wheel and axle," made 

 to rotate and perform work uniformly by means of a current 

 continuously excited by heat communicated to two metals in 

 contact, or the thermo-electric rotatory apparatus devised by 

 Marsh, which has been actually constructed ; this condition is 

 fulfilled at every instant. On the other hand, in all thermo- 

 dynamic engines, founded on electrical agency, in which discon- 

 tinuous galvanic currents, or pieces of soft u'on in a variable 

 state of magnetization are used, and in all engines founded on 

 the alternate expansions and contractions of media, there are 

 really alterations in the condition of materials ; but, in accord- 

 ance with the principle stated above, these alterations must be 

 strictly periodical. In any such engine, the series of motions 

 performed during a period, at the end of which the materials are 

 restored to precisely the same condition as that in which they 

 existed at the beginning, constitutes what will be called a com- 

 plete cycle of its operations. Whenever in what follows, the 

 work done or the mechanical effect produced by a thermo-dynamic 

 engine is mentioned without qualification, it must be understood 

 that the mechanical effect produced, either in a non-varying 

 engine, or in a complete cycle, or any number of complete cycles 

 of a periodical engine is meant. 



8. The source of heat will always be supposed to be a hot 

 body at a given constant temperature, put in contact with some 

 part of the engine ; and when any part of the engine is to be 

 kept from rising in temperature (which can only be done by 

 drawing ofi' whatever heat is deposited in it), this will be sup- 

 posed to be done by putting a cold body, which will be called 

 the refrigerator, at a given constant tenn)erature in contact with it. 



0. The whole theory of the motive power of heat is founded 

 on the two following propositions, duo respectively to Joule, and 

 to Carnot and Clausius. 



Prop. I. (Joule). — When equal quantities of mechanical effect 

 arc produced by any means whatever from purely thermal sources, 

 or lost in purely thermal effects, equal quantities of heat are put 

 out of existence or arc generated. 



