MAMUN (c. 786-833)  , originally ABDALLAH, surnamed ALMA'MON (" in whom men trust "), the seventh of the Abbasid caliphs of Bagdad, was born about A.D . 786, and was the second son of Harun al-Rashid . By Harun's will he was successor- designate to his brother Amin, during whose reign he was to be governor of the eastern part of the empire . On Harun's death (8og) Amin succeeded and Mamun acquiesced . Irritated, how-ever, by the treatment he received from Amin, and supported by a portion of the army, Mamun speedily rebelled . A five years' struggle between the two brothers ended in the death of Amin and the proclamation of Mamun as caliph at Bagdad (Sept . 813) . Various factions and revolts, which disturbed the first years of his reign, were readily quelled by his prudent and energetic measures . But a much more serious rebellion, stirred up by his countenancing the heretical sect of Ali and adopting their colours, soon after threatened his throne . His crown was actually on the head of his uncle Ibrahim b . Mandi (surnamed Mobarek) for a short time (Barbier de Meynard, in Journal Asiatique, March-April 1869) . This inaugurated a period of tranquillity, which Mamun employed in fostering literature and science .

He had already, while governor of

Khorasan, founded a college there, and attracted to it the most eminent men of the day, and Bagdad became the seat of academical instruction . At his own expense he caused to be translated into Arabic many valuable books from the Greek, Persian, Chaldean and Coptic languages; and he was himself an ardent student of mathematics and astronomy . The first Arabic translation of Euclid was dedicated to him in 813 . Mamun founded observatories at Bagdad and Kassiun (near Damascus), and succeeded in determining the inclination of the ecliptic . He also caused a degree of the meridian to be measured on the plain of Shinar; and he constructed astronomical tables, which are said to be wonderfully accurate . In 827 he was converted to the heterodox faith of the Mo'tazilites, who asserted the free-will of man and denied the eternity of the Koran . The later years (829-830) of his reign were distracted by hostilities with the Greek emperor Theophilus, while a series of revolts in different parts of the Arabian empire betokened the decline of the military glory of the caliphs . Spain and part of Africa had already asserted their independence, and Egypt and Syria were now inclined to follow . In 833, after quelling Egypt, at least nominally, Mamun marched into Cilicia to prosecute the war with the Greeks, but died near Tarsus, leaving his crown to a younger brother, Motasim . The death of Mamun ended an important epoch in the history of science and letters and the period of Arabian prosperity which his father's reign had begun . See further under CALIPHATE, sect . C., §§ 5, 6, 7 .