The production of motion in the steam engine always occurs in circumstances which it is necessary to recognize, namely when the equilibrium of caloric is restored, or (to express this differently) when caloric passes from the body at one temperature to another body at a lower temperature.

'Réflexions sur la Puissance Motrice du Feu et sur les Machines Propres a Développer cette Puissance' (1824). Trans. Robert Fox, Reflexions on the Motive Power of Fire (1986)