OCEAN CURRENTS. 79
a er A ee oe LP Be Smee eee

wave from the Pacific, meets and moves along
with a wave started in this ocean by the moon,
and advances in the direction indicated by the
co-tidal lines entering the Atlantic Ocean about
12 hours afterward.

216, The Atlantic Ocean.—The tidal wave from
the Indian joins two other waves, one formed by
the moon in this ocean, and the other a deflected
wave that has backed into the Atlantic from the
Pacific. The tidal wave thus formed advances
rapidly up the deep valley of the Atlantic, reach-
ing Newfoundland 12 hours afterward, or 48 hours
after it started in the Pacific. It then advances
rather less rapidly toward the north-east, reach-
ing the Loffoden Islands 12 hours afterward, or
60 hours after leaving its starting-place in the
Pacific.

217. Tides in Inland Seas and Lakes are very
small and, consequently, difficult to detect. In
the Mediterranean Sea the tides on the coasts
average about 18 inches. The tide in Lake
Michigan is about 1% inches.

218. Height of Tidal Wave.—Ocean tides are
lowest in mid-ocean, where they range from two
to three feet. Off the coasts of the continents,
especially when forced up narrow, shelving bays,
deep gulfs, or broad river mouths, they attain
great heights. The cause of these unusual heights
is evident. When the progress of the tidal wave
is retarded, either by the contraction of the chan-
nel or by other causes, the following part of the
wave overtakes the advanced part, and thus, what
the wave loses in speed it gains in height, from the
heaping up of the advancing waters. Where the
co-tidal lines, therefore, are crowded together on the
chart, high tides are likely to occur; for example,

-the Arabian Sea-and Bay of Bengal, the North
and South China Seas, the eastern coasts of Pata-
gonia, the Bay’ of Fundy, the English Channel,
and the Irish Sea, have very high tides.

Near the heads of the Persian Gulf and China
Seas, the tides sometimes rise about 36 feet. At
the mouth of the Severn, the spring tides rise
from 45 to 48 feet; on the southern coast of the
English Channel, 50 feet; and in the Bay of
Fundy, near the head, the spring tides, aided by
favoring winds, sometimes reach 70 feet, and, oc-
casionally, even 100 feet.

A strong wind, blowing in the direction in which the
tidal wave is advancirig, causes an increase in the height
of the tide.

A low barometer is attended by a higher tide than
usual; a high barometer, by a lower tide.

 

 

219, Other Tidal Phenomena.

The Bore or Hager.—On entering the estuary of a
river, the volume of whose discharge is considerable, the
onward progress of the tidal wave is checked; but, piling
up its waters, the incoming tide at last overcomes the re-
sistance of the stream, and advances rapidly, in several
huge waves. The tides of the Hoogly, the Elbe, the
Weser, and the Amazon, are examples. In the latter

river, the wave is said to rise from 30 to 50 feet.

Races and Whirlpools.—When considerable differ-
ences of level are caused by the tides, in parts of the ocean.
separated by narrow channels, the waters, in their effort
to regain their equilibrium, move with great velocity, pro-
ducing what are called races. At times, several races meet
each other obliquely, thus producing whirlpools. Near the
Channel Islands, and off the northern coasts of Scotland,
races are numerous. The Maélstrom, off the coasts of Nor-
way, isan instance of a whirlpool, though the motion of
the waters is not exactly a whirling one. The main phe-
nomenon is a rapid motion of the waters, alternately back-
ward and forward, caused by the conflict of tidal currents
off the Loffoden Islands.

Ete ee ee
CHAPTER Iii.

Ocean Currents.

220. Constant Ocean Currents—Besides tidal
currents, the waters of the ocean are disturbed
to great depths, by currents, moving with consid-
erable regularity to and from the equatorial and
polar regions, and thus producing a constant in-
terchange of their waters. These movements are
called constant currents, and, unlike waves, con-
sist in a real, onward movement of the water.

Constant currents resemble rivers, but are im-
mensely broader and deeper. As a rule, their
temperature differs considerably from that of the
waters through which they flow. They are not
confined to the surface, but exist as well at great
depths, when they are called under or counter cur-
rents, and flow in a direction opposite to that of the
surface currents.

221, The Principal Cause of Constant Ocean
Currents is the difference of density of the water
produced by the differences of temperature be-
tween the equatorial and the polar regions.

As the waters of the polar regions lose their
heat they become denser, and, sinking to the bot-
tom, form a mountain-like accumulation of dense,
cold water, which, as rapidly as formed, spreads
over the floor of the ocean underneath the lighter
waters. The consequent lowering of the level of
the polar waters causes an influx of the surface
waters from the equatorial regions.. In this man-
ner a constant interchange is effected between the