Oesel (51), a marshy, well-wooded island at the mouth of the Gulf of Riga, in the Baltic, 45 m. long and 25 m. of average breadth; has some low hills and precipitous coasts; Arensburg (4), on the SE. shore, is the only town; Danish from 1559, the island passed to Sweden in 1645 and to Russia in 1721; the wealthier classes are of German descent.