80 PHYSICAL GEOGRAPHY.

 

 

 

equatorial and polar regions, which, for the greater
part, takes place along the bottom from the poles
to the equator, and along the surface from the
equator to the poles. Since, however, the pole is
a mere point, this interchange occurs mainly be-
tween the equator and the polar circles.

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Fig. 76, Currents caused by Difference of Temperature.

Thus in Fig. 76, the mountain-like accumula-
tion is shown as having its crest at about the lati-
tude of the polar circle. The arrows show the
direction of the currents. At the equatorial re-
gions, the surface water is warmer and lighter,
and at the polar regions, probably, colder and
lighter.

As a rule, the warm currents are on the surface, and the,

cold currents, from their greater density, are underneath
them. In shallow oceans, however, the cold currents come
to the surface, thus displacing the warm currents and de-
flecting them to deeper parts of the ocean.

Had the earth no rotation on its axis, this in-
terchange would be due north and south, or would
take place directly between the equatorial and
polar regions. On account of the earth’s rota-
tion, however, and a variety of other causes,
these north-and-south directions are consider-
ably changed. The principal of these deflecting
causes are—

(1.) The earth’s rotation ;

(2.) The position of the land masses

(8.) The winds ;

(4.) Differences of density caused by evapora-
tion ;

(5.) Differences of level caused by evapora-
tion.

The changes in direction caused by the earth’s rotation
and the position of the land masses are as follows: as the
waters are in constant motion, the polar waters reach the
equatorial regions with an eastward motion less than that
of. the earth. In the equatorial regions, therefore, the
waters are unable to acquire the earth’s motion toward the
east, and are left behind; that is, the earth, slipping from
under them, causes them to cross the ocean at a, a’, Fig.
77, from east to west, although they are in reality moving
with the earth toward the east.

Reaching the western borders of the oceans, near J, b’,
the continents prevent their going farther west, and de-
fiect them into northern and southern branches, and they
begin to move toward the poles.

» From ¢, to d, and from ec’, to d’, the poleward-moving
waters are deflected toward the east in both hemispheres.

 

 

The waters on reaching ¢, from a, and J, still retain the
eastward motion they acquired while moving with the

 

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Wee ae
Zz Su

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d’
Fig. 77. Deflections of Ocean Currents.

earth. This motion is greater than that of the earth be-
tween c,and d. Betweén these points, therefore, the water
is acted on by two forces, one tending to carry it toward
the poles, and the other tending to carry it eastward.
The resultant of these forces carries the water from ¢, to
d, and from c’, to d’, or toward the north-east in the North-
ern, and toward the south-east in the Southern Hemisphere.

Between d, and e, and d’, and e’, the waters still retain
this excess of eastward motion, and, therefore, move in
the directions shown.

Between e, and a, and e’,and a’, the waters in both hemi-
spheres are deflected toward the west because they are
unable to acquire the earth’s motion toward the east.
Another, and perhaps the main, cause of this westward.
deflection is the depression caused by the westward move-
ment of the equatorial waters at a, and a’.

The action of the winds is to tend to move the surface
waters in the direction in which they are blowing. This
action is by some authorities regarded as the principal
cause of constant currents. :

The difference in the density of the water, caused by
evaporation, leaving the water salter and denser in some
parts, and fresher and lighter in others, probably acts to
some extent as a deflecting cause. For example, the water
evaporated near the equator, and precipitated, for the
greater part, in regions near the borders of the tropics,
renders the regions salter and denser from which it was
evaporated, and fresher and less dense where it is precipi-
tated.

The difference in level caused by the greater evapora-
tion in the equatorial regions north of the equator than
in corresponding latitudes in the Southern Hemisphere

' has been ascribed as one of the causes of the flow of Ant-

arctic waters toward the equator.

222. General Features of Constant Currents.—
The following motions of the surface currents are
common to all the three central oceans:

(1.) A movement of the equatorial waters, a, a,
from east to west ;

(2.) Their deflection into northern and south-
ern branches (6 and c), on reaching the western
borders of the ocean ;

(3.) A movement of the waters beyond the
equator from west to east (d, e);