398 Mr. W. J. IVIacquork Rakklne on the Science of Energetics. 



XVII. Efficiency of Engines. 



An engine is a contrivance for transforming energy by means of 

 the periodical repetition of a cycle of variations of the accidents of a 

 substance. 



The efficiency of an engine is the proportion which the energy perma- 

 nently transformed to a useful form by it bears to the whole energy com- 

 municated to the working substance. 



In a perfect engine the cycle of variations is thus : — 



I. The metabatic function is increased, say from ^o to ^i. 



II. The metamorphic function is increased by the amount A (p. 



III. The metabatic function is diminished from &i back to ^q. 



IV. The metamorphic function is diminished by the amount A (p . 



During the second operation, the energy received by the working sub- 

 stance, and transformed from the actual to the potential form is 3^ A<fi. 

 During the fourth operation energy is transformed back, to the amount 

 $Q A <p. So that the energy permanently transformed during each cycle 



is (^j — (Jq) a (p; and the efHciency of the engine ^^ " 



XVIII. Diffusion of Actual Energy ; Iebeversible or Feictional 

 Operations. 



There is a tendency in every substance or system of substances, to the 

 equable diffusion of actual energy ; that is to say, to its transfer between 

 the parts of the substance or system, until the value of the metabatic 

 function becomes uniform. 



This process is not directly reversible ; that is to say, there is no such 

 operation as a direct concentration of actual energy through a tendency 

 of the metabatic function to become unequal in different parts of a sub- 

 stance or system. 



Hence arises the impossibility of using the energy re-converted to the 

 actual form at the lower limit of the metabatic function in an engine. 



There is an analogy in respect of this property of irreversibility, be- 

 tween the diffusion of one kind of actual energy, and certain irreversible 

 transformations of one kind of actual energy to another, called by Pro- 

 fessor William Thomson, "Frictional phenomena," viz., the production 

 of heat by rubbing and agitation, and by electric currents in a homo- 

 geneous substance at a uniform temperature. 



In fact, a conjecture may be hazarded, that immediate diffusion of the 

 actual energy produced in frictional phenomena, is the circumstance 

 which renders them irreversible ; for, suppose a small part of a substance 

 to have its actual energy increased by the exertion of some kind of work 



