Analizing the steam engines Carnot became aware that exists a "inefficiency" changing the heat in mechanical work an was, thus, the discoverer of the second law of thermodynamics.
Formulating the concept of a steam engine efficiency, Carnot established the basics concepts for any transformation.
In the year 1824 he published his " Reflexions sur la Puissance Motrice du Feu," in which he made a first attempt to express the principles involved in the application of heat to the production of mechanical effect.
Carnot had accepted the "caloric theory" in conformity with it the heat was a non-material fluid which can pass from a body to another.
As a water wheel worked by the water descent on the paddles, so the steam engine worked by the heat descent from a hot region (boiler) to a cooler region (condenser).
As all the water returned to the river also the "caloric" (Carnot thinked) keeps after to have worked in the steam engine.
Fathers of thermodynamics: