In a general point of view, the _surface_ of the earth may properly
be considered as one vast ocean, in which are placed a great number
of islands, whose sizes are various. Two of these islands are called
CONTINENTS;—because it has hitherto been impossible to sail round them,
owing to the frozen state of the waters of the Arctic seas. That which
has been for the longer space of time inhabited by civilized nations,
is called the _eastern_, or _old continent_, and contains Asia, Europe,
and Africa. The other comprises America, and is named the _western_,
or _new continent_. New Holland, a large tract of land situated in
the midst of the most extensive mass of waters, is by some called a
continent; but it is more proper to consider it as the largest island
in existence. An island differs from a continent only in size, and the
name _continent_ is given to certain extensive portions of land, for
the sake of convenience in geographical descriptions.
The bottom of the basin of the ocean has irregularities, similar to
those seen on the surface of the land; and, if the waters were dried
up, would present to view, mountains, valleys, and plains. Islands are
therefore, the flattened tops of mountains, situated in the midst of
the sea. Immense masses of rocks are found in many places under the
water, and often rise so near the surface, that they are dangerous
to navigation. Near Marseilles, there is a quarry under the sea,
from which marble is obtained. These facts prove that the sea is not
bottomless, as some people suppose, although there are some places so
deep that no bottom has yet been found. It has been supposed, by many
geographers, that the depth of the abysses of the ocean, are equal to
the heights of mountains on land, and therefore that the ocean is in no
place more than _thirty thousand feet deep_. The greatest depth that
has ever been measured without finding bottom, is _four thousand six
hundred and eighty feet_.
The chief characteristic of the ocean, is the saltness of its waters,
which renders them disagreeable to the taste, and unfit for the use of
man. Thus mariners are often, although surrounded by water, in danger
of perishing from thirst, when their supply of fresh water has been
exhausted, while they are yet at a great distance from land. The cause
of this saltness is not yet ascertained. It is supposed that by the
constant agitation of the waters the ocean preserves its purity.
The Arctic ocean is constantly frozen over, and immense pieces of
ice become detached, and being driven about in the ocean, are often
dangerous to navigators. Some of them are more than a mile long, and
rise to the height of one hundred and fifty feet; others rise but a few
feet above the waves, and form floating fields of ice of great extent.
These fields are often visited by the natives of Greenland, in their
canoes, while pursuing their seal fisheries. Ships are sometimes, while
sailing in these dangerous waters, surrounded and crushed between these
masses of ice.
[Illustration: FLOATING ICE-FIELD AND ISLANDS.]
The form which the shores of the land presents is very irregular;
in consequence of which, the ocean extends towards the interior of
countries, and thus forms inland seas, gulfs, &c. which are in reality
parts of the ocean, detached, but not separated from it. The different
names, as sea, bay, &c. are given for the sake of convenience.
The level of the sea is nearly the same in all parts of the globe,
except in some inland seas, &c. For this reason all the measurements of
the heights of places and mountains are calculated from this level.