the Foundations of Dynamics. 27 



the energy of a hot body we ought to mean that portion of 

 its energy which it has over and above its useless energy of 

 average temperature, the portion which it is willing and able to 

 part with, the only portion we hope to use or aim at using ; and 

 so speaking we might call a heat-engine an efficient machine. 



We get a notion of low efficiency in the one case, of high 

 efficiency in the others — we confuse ourselves sometimes with 

 statements about the dissipation of energy — all because we 

 perversely attend to the energy of average temperature in the 

 one case but not in any of the others. We have done it 

 naturally enough, because in that one case our attention was 

 specially directed to the subject of heat ; but it may be a help 

 to realise that all the cases are essentially similar and on the 

 same footing. 



True a steam-engine and boiler is not an efficient ar- 

 rangement, only about 8 or 9 per cent, at the best, but that 

 is because of the great unnecessary drop of temperature 

 between furnace and boiler ; starting with the temperature 

 of the toiler, and ignoring all energy below the temperature 

 of the condenser, it may be efficient enough, 80 or 90 per 

 cent. I suppose. It is better not to pretend to be able to 

 use average molecular energy until we have learnt how to 

 do it. 



The portion of heat which can at a single operation be con- 

 verted into work is very nearly the same as that which leaks 

 away when the body is allowed to cool ; just as it is the 

 potential portion which disappears from a standing Leyden 

 jar, or a running-down weight, or a rusting spring, or a leaky 

 reservoir. The difference between the cases is that whereas 

 the capacity of Leyden jars and tanks is constant, capacity for 

 heat is apt to vary with the other conditions of a body ; hence 

 intrinsic energy is not solely a function of temperature, but 

 subordinately of volume also. This fact necessitates caution 

 before the above statement can be regarded as complete. 



What a body will freely yield throughout a cycle of opera- 

 tions, that can be utilised. We cannot advantageously gain 

 energy by pumping it. Whatever must be pumped is unavail- 

 able. We require an artesian well or automatic supply of 

 energy if we are to get work out of it. 



A further statement also is necessary if we are to concern 

 ourselves with practical modes of utilising the theoretically 

 available energy ; some such statement as the following : — 

 Available energy can only be continually utilised by means of 

 reversible operations. Unless the working-substance is re- 

 stored to its original condition the process cannot continually 

 go on ; and any operation which is not reversible involves 

 dissipation of energy or needless waste of availability. 



