26 Prof. Oliver Lodge on 



This distinction is the cause of some confusion or bewilder- 

 ment, and I verily believe of some incredulity. Prof. Osborne 

 Eeynolds has shown, in his interesting biography of Joule, 

 how great were the difficulties felt by the most eminent men 

 in realising that heat actually disappeared in a heat-engine : 

 that less heat was given to condenser than was received from 

 boiler ; that not only temperature fell but also heat was lost ; 

 and we all know how Carnot's theory was based on the 

 contrary hypothesis. 



But one mode of avoiding what is reasonably felt to be 

 an artificial and puzzling distinction, is to say that in all 

 the engines mentioned transformation of available energy 

 occurs as soon as activity begins, and that that engine is 

 perfect which enables all the energy available to transform 

 itself in the desired way. 



The potential energy of the raised water (so to speak for 

 the moment) , of the compressed air, of the separated chemicals, 

 disappears, and transforms itself into the motion of a turbine, 

 a bullet, or a motor-dynamo ; the potential energy of the hot 

 "working-substance" disappears in like manner, and results 

 in the motion of fly-wheel belts and shafting*. 



It is inconvenient to speak of the energy of heat as kinetic. 

 It is much of it kinetic, just as the energy of compressed air 

 and everything else may be kinetic ; but to us here and now its 

 available portion is not kinetic, but potential. When we can 

 deal with molecules it can be regarded how we please — 

 alternately kinetic and potential, probably, like the energy of 

 a vibrating fork, or on the average half-and-half, like waves ; — 

 but till then the kinetic energy of individual atoms is useless, it 

 is the average energy of a group which alone is useful, and 

 this is all that we attempt to utilise in every one of the cases 

 cited. Practically useful kinetic molecular energy exists only 

 when all the molecules are rushing one way, and that is 

 never the case with heat. 



By the energy of a spring we mean its energy over and 

 above its useless energy of average temperature; by the energy 

 of a storage-battery we mean the portion corresponding to the 

 reduced lead and the peroxide ; by the energy of a water- 

 fall we do not mean to include the warmth of the water above 

 absolute zero ; we always mean that portion of its total energy 

 which we aim at utilising, and so speaking we say that a 

 watch, or a dynamo, or a turbine, are efficient machines. By 



* Prof. Fitzgerald lias pointed out to me that this analogy will work 

 well if I use entropy as the analogue of the water in a waterfall, because 

 entropy does really fall in temperature and remain constant in qiiantity 

 while a perfect heat-engine is working. This suggestion I hope to de- 

 velop aloDg with other thermodynamic matters in a future paper. 



