Sadi Carnot
French military man who died young but left a very important book on the nature of heat. His Reflections on the Motive Power of Heat (1824) contained a theorem which says that a maximum efficiency of heat engine can be obtained by a reversible engine, and that efficiency depends only on the temperatures of the hot and the cool sources of the engine. This theorem played an essential role for the subsequent development of thermodynamics (Clapeyron, Kelvin, Rankine, and Clausius, to mention only a few), leading eventually to the discovery and mathematical formulations of the first and the second law of thermodynamics.