As a rule the sea is shallowest near the land, though in a few cases
there is a sudden descent to a great depth at a very short distance from
the coast. Lowlands have usually shallow seas near the coast, and
highlands deep water.
Along the American shores, in the latitude of New York, the depth, for a
distance of more than 100 miles, is less than 600 feet; then suddenly
the bed descends, by a steep slope, to the depth of 6,000 or 9,000 feet.
After a comparatively narrow interval, a second terrace descends to the
main basin, from 15,000 to 18,000 feet deep.
The bottom of the trough of the ocean, in general, is equally varied
with that of the land surface of the globe, forming mountains, hills,
valleys, tablelands, etc. In many parts these marine mountains reach
above the surface and form islands. On the table land extending across
the Atlantic between Newfoundland and Ireland is laid the
submarine-telegraph cable which connects the two hemispheres.
THE DEPTH OF THE OCEANS. The average depth of the Pacific Ocean has been
estimated at between 15,000 and 18,000 feet, which is slightly greater
than that of the Atlantic. The deeper portions may be learned on
reference to the map. The western portion of the North Pacific in
particular shows some very deep depressions. To the east of Japan lies a
long deep trough which in one part has furnished the sounding of nearly
five and one-half miles. This abyss is often called the Tuscarora Deep.
South of the Ladrone Islands, in the Caroline Archipelago, there is also
a deep abyss where an English ship, the _Challenger_, obtained a
sounding of nearly 27,000 feet. In the Pacific soundings of over 30,000
feet have been made.
The Indian Ocean has an average depth of about 12,000 feet, and the
deepest soundings have been taken on the eastern side. It is interesting
to observe that the deepest sounding, about five and three-quarter
miles, in the South Pacific somewhat exceeds the height of the highest
mountain. Mount Everest has a height of 29,000 feet above the sea level.
And it must also be noted that the mean height of the land, 1,000 feet,
is only about one-twelfth the mean depth of the whole ocean, 12,000
feet. (See colored map showing comparative surfaces of land areas and
ocean depths.)
INLAND AND BORDER WATERS. These enclosed basins belong to the structure
of the continents, rather than to the oceans. All are shallow in
comparison with the great basins with which they are connected, as is
apparent from the depths given below.
The Gulf of Mexico is from 5,000 to 7,000 feet in depth. The deepest
part of the Caribbean Sea, on a line connecting Porto Rico and Costa
Rica, averages 7,000 feet, and near the latter it reaches a depth of
14,000; but the ocean, immediately outside of the Lesser Antilles,
is more than 18,000 feet deep.
The Mediterranean is divided into two basins, by a rocky isthmus,
from 50 to 500 feet below the surface, lying between Sicily and Cape
Bon, in Africa. The western basin is over 9,000 feet in depth, and
comparatively uniform; while the eastern is more irregular, varying
from 6,000 near the center, to 13,000 feet, south of the Ionian
Islands. The Red Sea has an irregular bottom, with an average depth
of 3,000 feet, but in some places it reaches 6,000.
The Baltic Sea, being a simple depression in the great European
plain, is but a few hundred feet deep. In the North Sea, the depth
averages 300 feet, and rarely exceeds 600. The continent is here
prolonged in the form of a submarine plain, whose highest portions
form the British Isles.
The Border Seas of Asia, lying within the chain of continental
islands, are only a few hundred feet in depth, while immediately
without those islands, abrupt slopes descend to the great depths of
the Pacific basin.
Smaller inlets are also of frequent occurrence, especially in
districts where mountain ranges approach the borders of the ocean.
Such are the _lochs_ of Scotland, the _voes_ of the Shetland
Islands, and the _fiords_ of Norway and Greenland. The term _lagoon_
is usually applied to lake-like inlets.
SALT AND OTHER INGREDIENTS OF SEA-WATER. The waters of the ocean are
salt, holding in solution various saline matters. The saline ingredients
amount to rather more than thirty-five grains in a thousand grains of
sea-water. The most abundant of these is chloride of sodium or common
salt, which in general forms about a third of the whole. Besides this,
sea-water contains some magnesia, lime, potash, and traces of iodine and
bromine.
The following table exhibits the exact percentage composition of
sea-water.
One hundred parts by weight of sea-water contain:
Water 96.470
Sodium Chloride 2.700
Magnesium Chloride .360
Potassium Chloride .070
Magnesium Sulphate .230
Calcium Sulphate .140
Calcium Carbonate .003
Magnesium Bromide .002
Traces of Iodides, Silica, etc., estimated .025
-------
100.000
HOW THE SEA GETS ITS COLOR. The color of sea-water is due to the
character of the skies and clouds above, and to vegetable and animal
objects growing and living in it. The luminosity or phosphorescence of
the ocean is due to the decay of animal and vegetable substances, but in
some cases it arises from the presence of myriads of living animals,
which, like the glow-worm and fire-fly of the land and air, have the
power of emitting light.
OCEAN TEMPERATURE. The water of the ocean appears generally to agree
with that of the climate in which it is situated. In warm latitudes the
temperature of the deep sea diminishes with the depth below the surface
until a certain depth is reached, below which it appears to retain an
equable temperature, this being about 40 degrees Fahrenheit. In the
Polar Seas, where the temperature of the surface is lower than 40
degrees the heat increases downward until it reaches that point. In
latitude 70° the temperature of the ocean is considered to be the same
at all depths.
[Illustration: =HOW TIDES ARE FORMED BY ACTION OF THE MOON=
The moon pulls the waters of the earth into a great double wave heaping
it up on the side nearest to the moon and on the opposite side. As the
earth rotates, this double wave moves round the earth, and the crests
and troughs alternately produce high and low tide. Thus there are two
high and two low tides daily, at intervals of about twelve hours, or
half a Sun or day.]