1911 Encyclopædia Britannica/Carnot, Sadi Nicolas Léonhard
CARNOT, SADI NICOLAS LÉONHARD (1796–1832), French physicist, elder son of L. N. M. Carnot, was born at Paris on the 1st of June 1796. He was admitted to the École Polytechnique in 1812, and late in 1814 he left with a commission in the Engineers and with prospects 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. Disgusted with an employment which afforded him neither leisure for original work nor opportunities for acquiring scientific instruction, he presented himself in 1819 at the examination for admission to the staff corps (état-major) and obtained a lieutenancy. He then devoted himself with astonishing ardour to mathematics, chemistry, natural history, technology and even political economy. He was an enthusiast in music and other fine arts; and he habitually practised as an amusement, while deeply studying in theory, 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. His naturally feeble constitution, further weakened by excessive study, broke down finally in 1832. An attack of scarlatina led to brain fever, and he had scarcely recovered when he fell a victim to cholera, of which he died in Paris on the 24th of August 1832. He was one of the most original and profound thinkers who have ever devoted themselves to science. The only work he published was his Réflexions sur la puissance motrice du feu et sur les machines propres à développer cette puissance (Paris, 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 friction of water and mercury. Lord Kelvin’s experiment with a current of gas forced through a porous plug is also given. “Carnot’s principle” is fundamental in the theory of thermodynamics (q.v.).