DYNAMICAL THEORY OF HEAT. 263 
power of heat, must be made when the hypothesis of the Dynamical Theory, 
contrary as it is to CarNnor’s fundamental hypothesis, is adopted. 
(2.) To point out the significance in the Dynamical Theory of the numerical 
results deduced from ReEaNavut’s Observations on Steam, and communicated 
about two years ago to the Society, with an account of Carnor’s Theory, by 
the author of the present paper; and to shew that by taking these numbers 
(subject to correction when accurate experimental data regarding the density of 
saturated steam shall have been afforded), in connection with Joutr’s mechanical 
equivalent of a thermal unit, a complete theory of the motive power of heat, 
within the temperature limits of the experimental data, is obtained. 
(3.) To point out some remarkable relations connecting the physical properties 
of all substances, established by reasoning analogous to that of Carnot, but 
- founded in part on the contrary principle of the Dynamical Theory. 
Part I.—FUNDAMENTAL PRINCIPLES IN THE THEORY OF THE MOTIVE 
Power oF HEAT. 
7. According to an obvious principle, first introduced, however, 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 physical 
and mechanical circumstances as they were at the beginning. In some conceiv- 
able “thermo-dynamic engines,” as for instance Farapay’s floating magnet, or 
Bariow’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 Marsu, 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 
discontinuous galvanic currents, or pieces of soft iron 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 mate- 
_ Trials; but, in accordance with the principle stated above, these alterations must 
be strictly periodical. In any such engine, the series of motions performed during 
ba 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 complete cycle of its cperations. 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. 

