Encyclopaedia Britannica > Volume 3, Athens-BOI

B A L
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° 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 60°, which
crosses England near the parallel of 54°, passes across the
Gulf of Bothnia near the Walgrund 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. Nilsson 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-
TIC 295
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 mslrequently
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 A on Baer
in which a block of granite, whose weight was estimated at
between 400 and 500 tons, wTas thus carried by the ice
during the winters of 1837-8; and Forchhammer mentions
that the Sound being suddenly frozen oyer 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
16 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 w7ater within this skar,
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