Page:Reflections on the Motive Power of Heat.djvu/38

16 possible, in order to secure a commensurate range of temperature; (2) The cooling must be carried to the lowest point on the scale that may be found practicable; (3) The passage of the fluid from the upper to the lower limit of temperature must be produced by expansion;” i.e., “it is necessary that the cooling of the gas shall occur spontaneously by its rarefaction;” which is simply his method of stating the now universally understood principle that, for highest efficiency, the expansion must be adiabatic, from a maximum to a minimum temperature. He goes on to explain these principles, and then says that the advantage of high-pressure engines lies “essentiellement dans la faculté de rendre utile une plus grande chute de calorique.” This principle, as a practical system of operation, had already, as he tells us, been enunciated by M. Clément, and had been practised, as we well know, since the days of its originator, Watt; but Carnot saw clearly the thermodynamic principle which underlies it, and as clearly states it, for the first time.

He sees clearly, too, the reasons for the attempts of Hornblower and of Woolf, premature as they proved and as he also sees, in the introduction of the compound engine, and even suggests that this idea might be still further developed by the use