Portrait (by Boilly, 1813) of Nicolas Leonard Sadi Carnot (1796-1832), French physicist and founder of the science of thermodynamics. Carnot was interested in the amount of work that could be obtained from a heat engine. In Carnot's time the efficiency of the steam engine was only about 5%, meaning that 95% of the heat energy of the burning fuel was wasted. He was able to demonstrate theoretically that the maximum possible efficency depended on the difference in temperature between the hottest (T) and coolest (t) parts of the engine: efficiency = (T-t)/t. This important result later led to Clausius's formulation of the second law of thermodynamics.