In the Sound there are superior and inferior cur rents. , These were first observed by some English men, who, being in a boat in the middle of the chan nel, found that they drifted towards the Cattegat ; but, having let down a loaded bucket to the depth, of four or five fathoms, the boat became stationary; and when the bucket was sunk deeper, the boat drift ed against the superficial current. The general cur rents of the Baltic are strong, and are evidently oc casioned by the vast number of rivers and streams that pour their waters into it, many of which, espe cially towards the north, rise thrice in the course of the year. At the north extremity of the island of Bornholm, a violent agitation of the waters, or kind of whirlpool, called by the Swedes Malt-quern, or the grinding-mill, is occasioned by the current rush ing over a circular cluster of sunken rocks. The waves of the Baltic are short and broken, in conse quence of sudden changes of wind, irregular depths, ond strong currents.
The waters of this sea are not nearly so salt as those of the ocean ; and when the wind blows strong from the north, they become so fresh as to be fit for drinking or cooking meat. The degree of their salt ness varies in different parts, and in the same parts according to the season, or wind. According to Bergman, in his Physical Geography, near the south coast of Norway, at the entrance of the Scagerac, the waters contain from Ath to +th part of their weight of salt : in the Cattegat Ath ; in the Baltic Ath ; and in tlie Gulf of Bothnia from to Ath. The south-west and west winds augment the saltness, by introdtking the waters of the ocean ; in the summer, it requires 300 tons of the water of the Gulf of Bothnia to produce one ton of salt, but in the winter, only 50 tons ; this difference is caused by congelation, and by less fresh water flowing into it.
The analysis of three pounds of water taken up from the British Sea, on the coast of East Frizeland, and the same quantity from near Rostock in the' The following, are the results of some experiments made by Dr Thomson on the specific gravity of the water in the Firth of Forth, the Baltic off Tuna bergh, the Sound, and off the Scaw Point ; and also on the comparative weight of salt, obtained from 1000 grains of each of the waters evaporated.
Specific Weight of • gravity. salt.
land in the Gulf of Bothnia, in the month of July, the temperature of the atmosphere was observed to be 68, while the temperature of the surface of the water was 65 ; and in October, the temperatures were respectively 89 and 46. In the Sound, in the month of August, the temperature of the atmosphere was 70 ; on the surface of the water 68 ; and at three fathoms 66. On the 10th of October 1818, Dr Thomson found the temperature of the Sound to be 54. The Scagerac and Gulf of Norway are open to navigation all the winter, whereas several portions of the Baltic are covered with ice in a very moderate degree of cold; generally the bays and chan nels are encumbered with ice at the latter end of De cember. The waters towards the heads of the Gulfs of Bothnia and Finland are first frozen, and the ice being conveyed by the currents to the south, the masses of it are by the increasing cold united into vast fields, which become stationary on the west towards Stockholm, and in the east towards the Islands of Dagce and CEsel. In the southern parts of the sea, the ice begins to break up in April, but the Gulf of Bothnia and Finland often continue closed till May. The rigour of the climate in the Baltic is supposed to be considerably diminished by the clearing of the forests and the progress of culti vation ; at least more intense and longer continued colds, as well as greater extent and solidity of ice, are recorded during the fourteenth and fifteenth cen turies, than what have happened latterly.
The winds are extremely variable in the Baltic, but they blow most commonly from the east in the spring, and from the west in autumn ; calms are sel dom experienced, except in the middle of summer.
11. There is historical evidence, that the herring fish ery was a branch of national industry in the Sound, as early as the year 1168; * and in 1889, Philip de Mesieres says, that such vast shoals of herrings crowd ed into the Sound, that 40,000 boats, with six to ten men each, were employed in the fishery, besides 900 large vessels, in which the herrings were salted. In the Gulfs of Flensborgh and Slie, besides what are consumed fresh, 1000 tons salted are at present annually exported to Copenhagen and Germany; and, according to the registers of the custom-house at Dalborgh, on the south shore of the Gulf of Limfiord, this city exported yearly, from 1720 to 1780, above 23,000 tons, but from 1754 to 1765, the exportation had fallen to about 8000 tons. In the year 1748, the herring first appeared in shoals in the Gulf of Gottenburgh ; at first they arrived in August and September, but gradually later, and at present not till November or December. In 1752, there were 1000 tons taken ; in 1753; 2000 tons ; in 1761, 100,000 tons ; from 1790 to 1796, nearly 2,000,000 tons were salted, and it was estimated that from 50,000 to 100,000 tons were consumed fresh.