Ormsby Mackniglit Mitchel, an American astronomer, born in Union co., Ky., Aug. 28, 1810, died at Beaufort, S. C, Oct. 30, 1862. At 12 years of age, with a good knowledge of Latin and Greek and the elements of mathematics, he became clerk in a store in Miami, O., and afterward removed to Lebanon, Warren co., where he had been educated. Being appointed a cadet, he earned the money that took him to West Point, which place he reached almost penniless in June, 1825. After graduating in 1829, he was acting assistant professor of mathematics for two years. From 1832 to 1834 he was a counsellor at law in Cincinnati; from 1834 to 1844 professor of mathematics, philosophy, and astronomy in Cincinnati college; in 1836 and 1837 chief engineer of the Little Miami railroad. An observatory having been erected at Cincinnati chiefly through his exertions, he became its director; and in 1859 he took the direction of the Dudley observatory at Albany, retaining his connection with that at Cincinnati. In August, 1861, he was commissioned brigadier general of volunteers, and assigned to the department of the Ohio. After the occupation of Bowling Green and Nashville, he made a forced march into Alabama, fought a battle near Bridgeport at the close of April, 1862, and seized the railroad between Corinth and Chattanooga, for which he was promoted to the rank of major general.

In July he was relieved of his command in the west, and in September was placed in command of the department of the South, where he was preparing for an active campaign when he died of yellow fever. He was a popular lecturer on astronomy, and scarcely less distinguished for his mechanical skill, by which he perfected a variety of astronomical apparatus, among the most important of which was that for recording right ascensions and declinations to within 1/1000 of a second of time. Among his discoveries are the exact period of rotation of Mars, and the companion of Antares or Cor Scorpii. He also remeasured W. Struve's double stars S. of the equator, which resulted in several interesting discoveries. He published " Planetary and Stellar Worlds," a collection of lectures, a treatise on algebra, and a " Popular Astronomv." In 1846-'8 he published "The Sidereal Messenger," a periodical.