Baltic Sea (anc. Pelagus Scythicum or Mare Suevicum; Ger. Ostsee, eastern sea), an inland sea of N. Europe, nearly enclosed by Sweden, Russia, Germany, and Denmark, and communicating with the Cattegat and the North sea by the Sound and the Great and Little Belt. Its extremes of latitude are Wismar, in Mecklenburg, 53° 53' N., and Tornea, on the gulf of Bothnia, 65° 51' N. Its greatest length between these points is 900 m. Its width varies from 200 to 75 m. Its area, including the gulfs of Bothnia, Riga, and Finland, is estimated at about 150,000 sq. m. This is exclusive of the Cattegat and the Skager Rack, for which a further addition of 18,000 to 19,000 sq. m. must be made. - The direction in which the Baltic penetrates inland is extremely tortuous. From its straits it runs first E. to Memel, about 300 m., then N. as far as the latitude of Stockholm, 59° 21', a further distance of 260 m. It is to these portions that the term Baltic sea is in its limited sense restricted; for at this point it separates into two great gulfs. Of these the gulf of Finland runs nearly due E. between Finland and Esthonia, while the gulf of Bothnia runs a little E. of N. between Finland and Sweden. The gulf of Finland is 250 m. long, with a mean breadth of 60 to 70 m.

That of Bothnia is about 400 m. long, with 120 m. of average width, although at its narrowest part it is not above 40 m. wide. Another important inlet is the gulf of Riga or Livonia, S. of the gulf of Finland, and extending between Livonia and Courland, 70 m. from E. to W., and about 90 m. from N. to S. - The Baltic is shallow. The greatest depth, between Gothland and Windau, was found in 1871 to be 720 ft. At a depth of 600 to 700 ft., at the latter end of July, the temperature was 33° to 36.5° F. No marine plants were found in this cold area, and only a few an-nelida. Life was very abundant to the depth of about 300 ft., while plants were seldom found at a depth of more than 30 ft. The entrance to the sea is crowded with islands and shoals, and as the Baltic itself has no regular tides, the varying currents, depending upon prevailing winds and changing temperature, add to the difficulties of the navigator. The western portions of the sea have a depth of not more than 16 fathoms. Toward the east it deepens, and midway between Memel and Oeland there is found from 60 to 100 fathoms water. The gulf of Finland suddenly shoals from 50 to from 4 to 16 fathoms.

The gulf of Bothnia has no greater average depth, but its navigation is less obstructed by shoals and sand banks. - The basin of the Baltic is difficult to determine accurately, as, with the exception of the mountains of Sweden and Norway on the north and northwest, all its other borders stretch away in vast plains, occupying a large part of Europe. This great district is exceedingly well watered; upward of 200 rivers flow into the Baltic; the lakes in its neighborhood, with many of which it is connected by rivers, are almost innumerable; and altogether this sea receives the drainage of nearly one fifth of Europe. The most peculiar part of this basin is in its S. W. corner. Here, although the nearest mountains are those of the Hartz, yet the basin of the Baltic is not above 20 or 25 m. wide. The Elbe, which runs within 50 m. of the Baltic, flows into the North sea; so also the Eider, which rises close to its shores. These and their tributaries belong to another system; yet so flat is the country that the different waters continually unite, and a canal 3 m. long has served to connect the Baltic with the Elbe, by joining a small affluent of the latter with the Steck-nitz and Trave, between Lubeck and Lauen-burg. The Baltic receives, among others, the waters of the lakes of Ladoga, Onega, and Mselar, and of the rivers Dtina, Niemen, Vistula, and Oder. The rivers which flow from the south and southeast are the longest.

The great amount of mud and sand carried down into the sea has considerably changed its soundings in various parts, filling up the mouths of many of the rivers and harbors, and generally raising the bed of the entire sea, creating many small islets and shoals, and rendering navigation, particularly along the Danish shores, difficult and dangerous. - Being a close sea, with its entrance protected from the approach of the tidal wave, the Baltic has no tides. There is, however, observed at irregular periods a rise in the water, equal sometimes to 3 1/2 ft. This occurs at all seasons of the year, but chiefly in autumn or winter, or at a time of heavy rain, or during lowering weather. The water maintains its height for days, and sometimes weeks, and often overflows its usual limits. Dr. Schulten, a Swede, in 1804, by a series of close observations, ascertained that this rise was occasioned, not by heavy rains, winds, melting snow, or ice, to all of which it had been ascribed, but by the unequal pressure of the atmosphere upon different portions of the surface of the sea; the greatest height of the water corresponding to the greatest depression of the barometrical column, and the greatest variation of the barometer in that region, 2 1/2 inches, corresponding to a rise and fall of 84 inches in the water.

The waters of the Baltic are much less salt than those of the North sea or the Atlantic ocean; the relative proportion may be stated as about 1/54 to 1/28 in the North sea. The entire sea is every year more or less encumbered with ice, and its straits are usually impassable from December to April. Severe frosts have made the sea several times passable on the ice in its widest parts, between Denmark and Prussia, especially in the 14th and 15th centuries. In 1809 a Russian army crossed the gulf of Bothnia on the ice. - There seems to be no doubt that the Baltic is decreasing. The innumerable lakes which lie between it and the White sea are but the remnants of what was once a continuous sea. This is proven by the existence of similar animals in those lakes, although these are no longer salt. A gradual drainage is no doubt lessening the volume of all the bodies of water still left in the basin of the Baltic. It is in the south that such changes have been most remarked in modern times. Ltibeck, which when originally built was a seaport town, is now 12 m. from the shore. The isle of Rugen is nearly joined to the German shore, and annually extends its bounds, while the names of its various parts show that not long since that which is now one large island was a cluster of small islets.

Olaf Dalin, a Swedish mathematician, calculated the rise of the shore at one inch per annum, and this is probably not too high. - The Baltic is extremely rich in fish of various kinds. Seals are found in considerable numbers, and are chased for their oil and skins. Whales are sometimes seen. Along the shores of East Prussia and the isle of Rugen quantities of amber are collected. The countries surrounding the Baltic are all rich in useful natural products, and its waters are therefore crowded with the ships of all nations. - The ancients were but slightly acquainted with the Baltic. The origin of the name Baltic is not certainly known, some etymologists deriving it from the Danish belt, a girdle; some from the Lithuanian balta, white, in allusion to the great quantity of snow which annually falls in its neighborhood. Others have referred it to the Balti, the family name of the kings of the Visigoths. The name, however, is old, and appears to have been first used by Adam of Bremen, who described the sea in the 11th century.

The most important ports on the Baltic and its various arms are St. Petersburg, Riga. Memel, Konigsberg, Dantzic, Stralsund, Ltibeck, Copenhagen, Carlscrona, and Stockholm.