CHAPTER 8
THE SEA
124.The Sea. -On looking at a map of the world or at a globe, one is immediately impressed by the predominance of the sea. The whole area of the globe is about 196,940,000 square miles, 72% of which is water. The larger part of the land is in the northern hemisphere, and of the water in the southern hemisphere. A comparatively small land area extends below 40° south latitude, about the latitude of Philadel-THE OCEAN.
From a photograph taken in mid-Atlantic.
phia in the northern hemisphere. Most of the maps of the world do not represent the southern hemisphere below latitude 60°, which is about the latitude of Petrograd (St. Petersburg) in the northern hemisphere. Thus the equator is usually considerably below the center of the map.
125.Divisions of the Sea. -Although most of the surface waterof the earth is connected, yet for many purposes it is better to put this water area into somewhat arbitrary divisions. We thus speak of theAtlantic, the Pacific, the Indian ocean, each of which may be divided by the equator into a northern and a southern part, and the Arctic and Antarctic oceans which surround either pole. Sometimes a division is made from the parallel 40° south and this great body of water, almost without land boundaries is called the Southern Ocean.
The boundaries of these oceans are irregular in shape, but with the exception of the great Southern Ocean and of the Arctic Ocean, which is really an inclosed sea, they narrow toward the north. They have a number of partially landlocked seas connected with them. In some instances these penetrate far into the land, as in the cases of the Mediterranean Sea and Gulf of Mexico. The surface of the sea is level, unstable, easily moved and always rising and falling in rapid and changeful undulations.
126.Continental Shelf. -Around the border of the continents andof those islands which are near the continents, there extends, in some cases to a distance of two or three hundred miles, a gradually deepen- ing ocean floor. This gradually deepening border is called the continen- tal shelf. When this floor has reached the depth of about 600 feet, the gradual slant suddenly changes into a quick descent to the depths of the ocean, two or three miles.
CONTINENTAL SHELF.
A model showing the sea floor off the coast of Southern California.
Upon this shelf lie the great continental islands, like the British Isles and the East Indies. It is this that furnishes the great fishing banks of the earth, such as the Grand Banks of Newfoundland and those around Iceland and the Lofoten Islands, where fishermen for ages have obtained vast supplies of fish. There is no equal area of the earth where the life is so varied and the struggle for existence so great as on these shallow continental borders.
Here the mud and sand brought down by the rivers is spread out and the sedimentary rocks formed. It is the elevation of this shelf which has formed the low-lying coastal plains which border many of the continents. There is good reason to believe that the deep floors of the sea have never been raised into dry land, and that the vast extent of sedimentary rocks which make up the larger portion of the land has almost all been laid down in regions which were at the time continental shelves.
127.Composition of Ocean Waters.
Experiment 122. -Into a dish of fresh water put a demonstration hydrometer or stick such as was used in Experiment 35. Mark the depth to which it sinks. Place the hydrometer now in sea water and mark the depth to which it sinks. If sea water cannot be obtained, dissolve in a pint of fresh water about 15 g., or half an ounce, of salt. This will give the water about the same amount of dissolved solid material as sea water would have. About how much more of its length does the hydrometer sink in fresh water than in sea water? Will a piece of ice project more out of salt water than it would out of fresh water?
Experiment 123. -If ocean water can be obtained, boil down about a pint of it in an open dish. Taste of the residue which is left. What is the principal constituent of this residue?
There is probably no water on the surface of the earth which is absolutely pure. Water is the greatest solvent known, and it dissolves to a greater or less extent almost all substances with which it comesin contact. When the rivers run into the sea, they carry with them whatever their water has dissolved from the land, and when the sun evaporates the water, and it is borne away again to fall upon the land, the dissolved material is left behind in the ocean.
Thus the sea has for all time been receiving the soluble contributionsfrom the land. It is easy to prove that it contains salt, for we can taste it. It must contain lime, for corals and shell animals depend upon this for the hard parts of their bodies. There must be organic food material in it, or else fixed animals like oysters could not get food. It contains air, for without air fishes could not breathe. These are the principal substances which we need to consider in the study of the ocean water, but the chemist can find many other substances dissolved in it. On account of the materials dissolved, sea water weighs more than fresh water, or has a greater specific gravity. A cubic foot of fresh water weighs about 62.5 lbs. whereas a cubic foot of sea water weighs over 64.25 lbs.
128.Ocean Depths. -The greatest depth thus far found in the ocean is nearly six miles. This was found in the Pacific Ocean near the Ladrone Islands. The greatest depth in the Atlantic Ocean thus far dis- covered is a little over five miles at a point north of Puerto Rico. The average depth of the sea is probably about two and one half miles.