SYLLABUS. 83

The north equatorial current in this ocean is indistinct—

(1.) Because the ocean has no outlet to the north;

(2.) Powerful seasonal winds, called the monsoons, move
the waters alternately in different directions, as huge drift
eurrents.

Sargasso Seas.—Near the centre of the ellip-
tical movement in each of the central oceans,
masses of seaweed have collected where the water

is least disturbed. These are called sargasso seas,

226. Utility of Currents:

(1.) They moderate the extremes of climate by
carrying the warm equatorial waters to the poles,
and the cold polar waters to the equator;

(2.) They increase materially the speed of ves-
sels sailing in certain directions ;

(3.) They transport large quantities of timber
to high northern latitudes.

OSE III

SYLLABUS.

2020400

Ocean water contains about three and one-third pounds
of various saline ingredients, in every one hundred. ‘Chlo-
ride of sodium; sulphates and carbonates of lime, mag-
nesia, and potassa; and various chlorides, bromides, and
iodides, are the principal saline ingredients.

The salt of the ocean is derived either from the
washings of the land, or is dissolved out from the por-
tions of the crust which are continually covered by its
waters.

The ocean is salter in those parts where the evaporation
exceeds the rainfall. Seas like the Mediterranean, which
are connected with the ocean by narrow channels, and in
which the evaporation is greater than the rainfall, are
salter than the ocean. Others, like, the Baltic, in which
the rainfall exceeds the evaporation, are fresher than the
ocean.

Most of the bed of the ocean is covered with a layer of
dense water, at about the temperature of its maximum
density.

The Pacific and Atlantic Oceans occupy about three-
fourths of the entire water-area of the earth.

South of the southern extremities of South America,
Africa, and Australia, the meridians of Cape Horn, Cape
Agulhas, and South Cape in Tasmania, are assumed as
the eastern boundaries of the Pacific, Atlantic, and Indian
Oceans.

The articulation of land and water assumes four distinct
forms: Inland Seas, Border Seas, Gulfs and Bays, and Fiords.
Inland Seas characterize the Atlantic; Border Seas, the Pa-
cific; Guifs and Bays, the Indian Ocean; and Fiords, the
Atlantic and Pacific.

The telegraphic plateau lies between Ireland and New-
foundland. Its average depth is about two miles.

The bottom of the ocean is not as much diversified as
tne surface of the land. Its plateaus and plains are be-
lieved to be much broader than are those of the land. The
profound valleys of the ocean are called deeps, its shallow
parts, rises.

The greatest depth of the ocean that has as. yet been
accurately sounded is about 5% miles. ae is probably deeper
than this in some places.

Over extended areas, the floor of the ocean is uniformly
covered with a deposit of fine calcareous mud or ooze,
formed of the hard parts of the bodies of minute animal-
cule.

The movements of the oceanic waters may be arranged |

under the three heads: waves, tides, and currents.

 

The height and velocity pf a wave depend upon the
force of the wind and the depth of the oceanic basin.

In ordinary wave motion, the water rises and falls, but
does not move forward.

Tides are the periodical risings and fallings of the water,
caused by the attraction of the sun and moon.

The rising of the water is called flood tide; the falling,
ebb tide.

If the earth were uniformly covered with a layer
of water, two high tides would occur simultaneously;
one on the side of the earth directly under the sun
or moon, the other on the side farthest from the sun
or moon.

The tidal wave crosses the ocean from east to west, fol-
lowing the moon in the opposite direction to that in which
the earth passes under it while rotating. Its progress is
considerably retarded by the projections of the continents,
and the shape of the oceanic beds. Had the moon no real
motion around the earth, there would be two high and
two low tides every twenty-four hours, or the high and

~ low tides would be exactly six hours apart.

Spring Tides are caused by the combined attractions of
the sun and moon on the same portions of the earth. Neap
tides by their opposite attractions.

The parent tidal wave is considered as originating in
the great water-area of the Pacific on the south.

Co-tidal lines are lines connecting places which have
high tides at the same time.

When the progress of the tidal wave is retarded by the
shelving coast of a continent, what the tide loses in speed,
it gains in height. The highest tides, therefore, occur
where the co-tidal lines are crowded together.

Bores, Races, and Whirlpools are tidal phenomena.

Oceanic currents are either temporary, periodical, or
constant.

The heat of the sun and the rotation of the earth are
the main causes of constant oceanic currents.

The following peculiarities characterize the constant
currents in the three central oceans:

(1.) A flow in the equatorial regions from the east to
the west;

(2.) A flow in extra-tropical regions from the west to
the east;

(3.) A division of the eastwardly flowing extra-tropical
waters in mid-ocean into two branches; one of which
flows toward the poles, and the other toward the equator,
where it merges into the equatorial currents.