CARNOT, NICOLAS LEONHARD SADI (1796-1832)', French physicist, elder son of L. N. M. Carnot, was born at Paris. He was admitted to the Ecole Polytechnique in 1812, and late in he left with a commission in the Engineers and with pros pects of rapid advancement in his profession. But Waterloo and the Restoration led to a second and final proscription of his father ; and though not himself cashiered, Sadi was purposely told off for the merest drudgeries of his service. In 1819 he pre sented himself at the examination for admission to the staff corps (etat-major) and obtained a lieutenancy. He then studied mathematics, chemistry, natural history, technology and even political economy. He was an enthusiast in music and other fine arts, and practised all sorts of athletic sports, including swimming and fencing. He became captain in the Engineers in 1827, but left the service altogether in the following year. He died of cholera in Paris on Aug. 24, 183 2. He was an original and profound thinker. The only work he published was his Re flexions sur la puissance motrice du feu et sur les machines propres a developper cette puissance (1824). This contains but a fragment of his scientific discoveries, but it is sufficient to put him in the very foremost rank, though its full value was not recognized until pointed out by Lord Kelvin in 1848 and 1849. Fortunately his manuscripts had been preserved, and extracts were appended to a reprint of his Puissance motrice by his brother, L. H. Carnot, in 1878. These show that he had not only realized for himself the true nature of heat, but had noted down for trial many of the best modern methods of finding its mechanical equivalent, such as those of J. P. Joule with the perforated piston and with the fric tion of water and mercury. Lord Kelvin's experiment with a cur rent of gas forced through a porous plug is also given. "Carnot's principle," that the efficiency of a reversible engine depends on the temperatures between which it works, is fundamental in the theory of thermodynamics (q.v.).