SURFACE OF THE EARTH.
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.
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.