"The criterion of what may be called a perfect thermo-dynamic engine is thus stated: A perfect thermo-dynamic engine is such, that, whatever amount of mechanical effect it can derive from a given thermal agency, if an equal amount be spent in working it backwards, an equal reverse thermal effect will be produced. Any two perfect engines, however different in their constructions, or in the physical media employed, must derive the same equivalent of mechanical effect from a given thermal agency. Carnot describes a steam-engine and an air-engine, each of which satisfies the criterion laid down above (the construction being however in each case, practically impossible); and he shews how, with certain physical data, with reference to steam in one case, and with reference to air or any gas in the other, the equivalent of mechanical effect, derivable from a given thermal agency, may be calculated."