THE PROGRESS OF PHYSICS. 145 



him in an almost perfect form, were the idea of a 

 Cycle of Operations and the further idea of a Re- 

 versible Cycle. In order to reason upon the working 

 of a heat-engine (suppose it for simplicity a steam- 

 engine) you must imagine a set of operations, such 

 that at the end of the series you bring the steam or 

 water back to the exact state in which you had it at 

 starting. That is what Carnot calls a cycle of opera- 

 tions, and of it Carnot says, then, and only then, i.e., 

 at the conclusion of the cycle, are you entitled to 

 reason upon the relation between the work which you 

 have acquired, and the heat which you have spent 

 in acquiring it." * 



" The other grand point with reference to Carnot 

 is this, that he started the notion of a Reversible En- 

 gine, reversible not in the ordinary technical sense 

 of working its parts backwards, not in the mere 

 sense of backing, but reversible in the sense that, 

 instead of using heat and getting work from it, 

 you can drive your engine through your cycle the 

 other way round, and by taking in work, pump back 

 heat (as it were) from the condenser to the boiler 

 again a reversing of the whole process, not a mere 

 reversing of the direction in which the engine is 

 driving. Now, Carnot introduced that notion, and 

 he showed by perfectly conclusive reasoning that if 

 you can obtain a reversible engine, it is the perfect 

 engine; i.e., that it is impossible to get an engine 

 more perfect than a reversible one." f 



Although he began with a firm belief in the caloric 

 theory, Carnot ended to all intents and purposes as 

 an adherent to the modern dynamical view, and that 

 he had grasped the principle of conservation is evi- 

 dent from his conclusion : " Motive power is in 

 * P. G. Tait, loc. cit., p. 97. t P. G. Tait, loc. cit., p. 98. 



