Although the assertion that salt water never freezes, has been
contradicted by repeated experience, it is still certain that it
requires a much greater degree of cold to produce its congelation, than
fresh water. It is, therefore, one of the greatest blessings which we
derive from this element, that when we find all the stores of nature
locked up to us on the land, the sea is, with few exceptions, ever open
to our necessities. It is well known that at particular seasons, the
mouth of the river St. Lawrence, the entrance into the Baltic sea, &c.,
are so much frozen over as to be impassable by ships; while the vast
mountains and fields of ice in the polar regions, have for ages past
been insurmountable obstructions to the daring researches of modern
navigators. These exceptions, however, will appear of comparatively
trifling importance to navigation, when the number of ports which are,
in almost every region, open at all seasons of the year, are considered;
and this facility of intercourse would certainly not have been afforded,
if sea-water had admitted of as easy a congelation as that of water not
impregnated with salt.
On the origin of ice in the frozen seas, different opinions have been
entertained. The authority of Capt. Cook and Lord Mulgrave, has been
cited by Bishop Watson, to show that good fresh water may be procured
from ice found in those seas; but he observes that, notwithstanding the
testimonies of these very able navigators, it may still be doubted
whether the ice from which the water was obtained, had been formed in
the sea, and, consequently, whether sea-water itself would, when frozen,
yield fresh water. He thinks it probable that the ice had either been
formed at the mouths of large fresh-water rivers, and had thence, by
tides or torrents been drifted into the sea, or that it had been broken
by its own weight, from the immense cliffs of ice and frozen snow which,
in countries where there are few rivers, are found in high latitudes to
project a great way into the sea. An early navigator, Fotherbye, in the
relation of his voyage toward the south pole, in 1614, considers snow to
be the original cause of the ice found at sea, he himself having
observed it to lie an inch thick on the surface; and Captain Cook, from
his own observation in the South sea, was disposed to think that the
vast floats of ice he met with in the spring, were formed from the
congelation of snow. It is certain that the snow which falls upon the
surface of the sea, being in a solid state, and, bulk for bulk, lighter
than sea-water, will not readily combine with it, but may, by a due
degree of cold in the atmosphere, be speedily converted into a layer of
ice. The upper layer of this first surface of ice being elevated above
the surface of the sea, will receive all the fresh water which falls
from the atmosphere in the form of snow, sleet, rain or dew, by the
successive congelation of which, the largest fields of ice may at length
be formed.
It is a matter of little consequence to a navigator, whence the ice
which supplies him with fresh water is produced. Leaving, therefore,
these hypotheses relative to the formation of ice in frozen seas, it
should be observed that the question, whether congealed sea-water will,
when thawed, yield fresh water, has been satisfactorily decided by
experiments made with every suitable attention. A quantity of sea-water
having been taken up off the English coast, was exposed to a freezing
atmosphere, and afforded an ice perfectly free from any taste of salt;
and it has likewise been found, that not only sea-water, but water
containing double the proportion of salt commonly found in our
sea-water, and more than is contained in the sea-water of any climate,
may be frozen by the cold prevailing in our atmosphere.
[Illustration: ICEBERGS, OR ICE-ISLANDS.]