Baltic Sea is the great gulf or inland sea bordered by Denmark, Germany, Russia, and Sweden, and communicating with the Kattegat and North Sea by the Sound and the Great and Little Belts. Its length is from 850 to 900 miles; breadth, from 100 to 200; and area, including the Gulfs of Bothnia and Finland, 184,496 sq. m., of which 12,753 are occupied by islands. Its mean depth is 44 fathoms, and the greatest ascertained depth, between Gotland and Courland, 140. Its shallowness and narrowness, its numerous islands and reefs, the shoal coasts of Prussia on the one side, and the rocky coasts of Sweden on the other, and above all, the numerous and sudden changes of wind accompanied by violent storms, make the navigation of the Baltic very dangerous. The group of the Aland Islands divides the south part of the sea from the north part or Gulf of Bothnia (q.v.). The Gulf of Finland (q.v.), branching off eastwards into Russia, separates Finland from Estonia. A third gulf is that of Riga or Livonia. The Kurisch and other Haffs (q.v.) are not gulfs, but fresh-water lakes at the mouths of rivers.
The water of the Baltic is colder and clearer than that of the ocean, and contains only a fourth of the proportion of salt found in the Atlantic. Ice hinders the navigation of the Baltic from three to five months yearly. Rarely, as in 1658 and 1809, the whole surface is frozen over. Tides, as in all inland seas, are little perceptible—at Copenhagen, about a foot; yet the water rises and falls at times, though from other causes, chiefly from the varying quantity of water in the rivers at different seasons. Upwards of 250 rivers flow into this sea, which, through them and its lakes, drains rather less than one-fifth of all Europe, its drainage area being estimated by Dr W. B. Carpenter as 717,000 sq. m. The chief of these rivers are the Oder, Vistula, Niemen, Dwina, Narva, Neva; the waters of Lake Maeler, and those of Wetter and other lakes reach the sea through the river Motala. The principal islands are Zealand, Fünen, Bornholm, Samsoe, and Laaland, belonging to Denmark; the Swedish islands Gotland, Oland, and Hveen (in the Sound); the Aland Islands, belonging to Russia; and Rügen, to Prussia. Timber, hides, tallow, and grain are the chief exports from the countries bordering on the Baltic. The number of vessels that pass the Sound to or from the Baltic annually is very large. See SOUND.
The Eider Canal, connecting the Baltic near Kiel with the North Sea at Tønning, facilitates the grain trade in mild winters; and the two seas are also connected by the Gotha Canal, which joins the lakes of South Sweden. These are navigable for boats of light draught only; but miles long, 28 feet deep, 66 yards wide at the surface, and 24 at the bottom; and as the voyage round from the Elbe to Kiel represents nearly 600 miles of dangerous sailing, the waterway will be of great value to the German navy. It cost some £8,000,000, and the yearly maintenance is stated at £50,000. The most important harbours in the Baltic are: in Denmark, Copenhagen; in Germany, Kiel, Lübeck, Stralsund, Stettin, Danzig, Königsberg, and Memel; in Russia, Riga, Narva, Kronstadt, and Sveaborg; and in Sweden, Stockholm and Karlskrona.—The shores of the Baltic in Prussia and Courland have been long noted for the amber cast ashore by the waves in stormy weather. Another important phenomenon connected with the Baltic is a slow vertical movement of its coasts, downwards in the south of Sweden, but farther north upwards, being there supposed to be at the rate of 3 feet in a century. Its area is held to be gradually decreasing. The Germanic nations call this sea Ostsee, or East Sea; the name Baltic first appears in the 11th century, in a work by Adam of Bremen.