JOST, ISAAK MARKUS (1793-1860), Jewish historical writer, was born on the 22nd of February 1793 at Bernburg, and studied at the universities of Göttingen and Berlin. In Berlin he began to teach, and in 1835 received the appointment of upper master in the Jewish commercial school (called the Philanthropin) at Frankfort-on-the-Main. Here he remained until his death, on the 22nd of November 1860. The work by which he is chiefly known is _Geschichte der Israeliten seit der Zeit der Maccabäer_, in 9 vols. (1820-1829), which was afterwards supplemented by _Neuere Geschichte der Israeliten von 1815-1845_ (1846-1847), and _Geschichte des Judenthums und seiner Sekten_ (1857-1859). He also published an abridgment under the title _Allgemeine Geschichte des israelitischen Volkes_ (1831-1832), and an edition of the Mishna with a German translation and notes (6 vols., 1832-1834). The _Israelitische Annalen_ were edited by him from 1839 to 1841, and he contributed extensively to periodicals. See Zirndorf, _Isaak Markus Jost und seine Freunde_ (Cincinnati, 1886). JOTUNHEIM, or JOTUN FJELDE, a mountainous region of southern Norway, lying between Gudbrandsdal on the east and Jostedalsbrae and the head of the Sogne fjord on the west. Within an area of about 950 sq. m. it contains the highest mountain in the Scandinavian Peninsula--Galdhöpiggen (8399 ft.)--and several others but little inferior. Such are Glittertind or Glitretind (8380), and Memurutind (7966), which face Galdhöpiggen across the northward-sloping Visdal; Knutshulstind (7812) and several other peaks exceeding 7000 ft., to the south, between lakes Gjende and Bygdin, and Skagastölstind (7723) in the west of the region, above the Utladal, the chief summit of the magnificent Horunger. The upper parts of the main valleys are of characteristic form, not ending in lofty mountain-walls but comparatively low and level, and bearing lakes. The name Jotunheim (giants' home) is a modern memorial of the mountain-dwelling giants of Norse fable; the alternative name Jotun Fjelde was the first bestowed on the region, when it was explored in 1820 by the geologist Balthasar Matthias Keilhau (1797-1858). In modern times the region has attracted mountaineers and many visitors accustomed to rough lodging and difficult travelling.