Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/311

Rh charged into the basin, over that which passes off by evapo ration from its surface, is indicated by its low salinity, which, however, varies considerably in its different parts and at different seasons of the year. The temperature of the Baltic is remarkable for its range, which is rather that of a terrestrial than of a marine area this being doubtless owing in great degree to the fact that its shallowness and the low salinity of its water allow a large part of its surface to be frozen during the winter. Nearly the whole of the Gulf of Bothnia, with the land enclosing it on both sides, lies between the January isotherms of 10 D and 20 the former crossing it near its head, and the latter near its junction with the Baltic proper ; and the whole of the Baltic proper, with the land enclosing it on the east, south, and west, lies between the January isotherms of 20 and 30. On the other hand, the July isotherm of GO 3, which crosses England near the parallel of 54, passes across the Gulf of Bothnia near the Walgruud Islands, almost 9 further north ; and the whole of the Baltic proper, with the Gulf of Finland and the southern part of the Gulf of Bothnia, lies between the July isotherms of 60 and 65. Thus the range between the mean summer and mean winter temperatures, which is only about 20 in the British Islands, is about 40 over the Baltic area. The mean annual temperature of the Gulf of Bothnia ranges between 30 at its northern extremity and 40 at its southern, while that of the Baltic ranges from 40 at its northern boundary to about 46 at its southern.

Formation and Transportation of Ice.—The greater part of the Gulfs of Bothnia and Finland is usually frozen over during the winter, the formation of ice beginning at the head and extending downwards. Masses of ice, conveyed by the currents into the Baltic proper, freeze together as the winter advances, and form vast fields, generally extending on the east side as far south as the islands of Dago and Oesel, and on the west to the south of Stockholm. It happens sometimes, though rarely, that large portions of the Baltic proper are continuously frozen over ; but naviga tion is usually interrupted by the blocking up of its bays and harbours with ice, from the latter part of December to the beginning of April. The freezing of the Gulfs of Bothnia and Finland begins earlier and ends later.

The curious phenomenon of the formation of bottom-ice, and its rise to the surface, is more frequently seen in the Baltic and the Cattegat than in the open ocean, chiefly, it seems probable, on account of the shallowness of these seas. It has been particularly observed by Prof. Xilsson in the Cattegat, off Kullen Point, near the southern extremity of Sweden ; but according to Chydenius it is very common in various parts of the Baltic, having been especially noticed by the fishermen off the Aland Islands. In calm winter weather, water of from 4 to 8 feet deep is often covered in a very short time with small plates of ice, mostly circular in form, varying in diameter from 1 to 5 inches, and having a uniform thickness which never exceeds two lines. These plates can be seen coming up from below, rising edgeways towards the surface, often with such force as to lift them selves three or four inches out of the water. When they come up in great numbers they are often piled one upon another, and are then usually soon broken, by the action either of waves or of currents, into small pieces, which unite again by regelation so as to form irregular cakes of ice ; and these, as soon as the water becomes tolerably still, cohere into a continuous rough sheet. But it sometimes happens that if the plates come up more sparsely, and the weather is very still and cold, they remain unbroken, and the diameter of each increases, sometimes to two feet or even more. When the fishermen notice these ice-plates coming up from below in large quantities, they at once make for land, as they know that they might otherwise be soon com pletely ice-bound. The same thing appears to happen in polar seas in the shallow water near land. Chydenius, who was a member of the Swedish Spitzbergen expedition in 1857, states that on one occasion the surface of the sea, which was previously quite clear of ice, became so covered in the course of half an hour, that it was with difficulty that a boat could be forced through it ; and this although the temperature of the air during the day had not been lower than 4 C., and no wind or stream had brought the ice together. It does not seem very clear in what way this formation of bottom-ice is to be accounted for. Bottom-ice has often been noticed in fresh-water lakes and streams ; and large plates have been seen to rise to the surface, sometimes with force enough to bring up stones of considerable size, in one instance a heavy iron chain. In these cases it would seem that the motion of the bottom-water over roughened surfaces contributes to its congelation. And in the shallow water near the sea-shore, stones and sea-weeds may be seen covered with ice, like the hoar-frost on trees, before any ice forms on the surface. It is to be remembered that sea- water increases in density down to its freezing point, so that the water cooled at the surface will always go down, the deepest stratum being thus the coldest. And thus, although no lower temperature can be carried down by the water than that to which it has been subjected at the surface, the water that does not freeze at (say) 2 5 C. when lying upon water, changes into ice when it comes in con tact with the irregular solid bottom, perhaps on account of the more ready dissipation, under the latter circum stances, of the heat set free in the act of congelation. When ice forms over the shallow bottoms which border parts of the Gulfs of Bothnia and Finland, large blocks of stone are frequently frozen into it; and these, being lifted when the water rises in the early summer, are often trans ported by currents to considerable distances, finally subsid ing again to the bottom when the ice melts. In this manner a deposit of rocky fragments, some of them 6 or 8 feet across, is being formed at the bottom of the Baltic outlets ; as is known from the fact, that sunken ships which have been visited by divers in the Sound and in Copenhagen roads have been found covered with such blocks within no very long period. It not unfrequently happens, moreover, that sheets of ice with included boulders are driven up on the coast during storms, and are thus carried some way inland, being sometimes packed to a height of even 50 feet. A case was described by Von Baer in which a block of granite, whose weight was estimated at between 400 and 500 tons, was thus carried by the ico during the winters of 1837-8; and Forchhammer mentions that the Sound being suddenly frozen over during an intense frost in February 1844, sheets of ice driven by a storm were heaped upon the shore of the bay of Taarbeijk, and frozen into one mass so as to form a mound more than 1 G feet high, which threw down the walls of several houses, and left behind it ridges of sand and pebbles when it thawed. It is apparently, moreover, by similar agencies, that the fringe of rocky islands of all dimensions called the Skdr, which lies at a little distance from the shore of many parts of the Baltic, is being gradually modified. Boats and small vessels can sail in smooth water within this skiir, even when the sea outside is strongly agitated ; but the navigation is- intricate, and the danger from sunken rocks to those not thoroughly acquainted with it is very con siderable. The diminution which has been noticed from time to time in the depth of the channels, and the appear ance above water of what were formerly regarded as sunken rocks or reefs, have been regarded as concurring with other evidence to prove that a general rise of land is now going on over this area. But it seems probable, from what has 