78 PHYSICAL GEOGRAPHY.

 

apart, or in quadrature, each produces a tide on
the portion of the earth directly under it, dimin-
ishing somewhat that produced by the other body.
High tide, then, occurs under the moon, while the
high tide caused by the sun, becomes, by compari-
son, a low tide. Such tides are called neap tides.
During their prevalence, the flood is not. very
high, nor the ebb very low. They occur twice
during each revolution of the moon, but are low-
est about the time of the June and December
solstices.

The average relative height of the spring tide to that
of the neap tide is about as 7 to 4.

212. Birthplace of the Tidal Wave——Although
a tidal wave is formed in all parts of the ocean
where the moon is overhead, yet the “ Cradle of
the Tides” may properly be located in the great
southern area of the Pacific Ocean. Here the
combined attraction of the sun and moon origin-

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

ate a wave, which would travel around the earth
due east and west, with its crests north and south;
but, meeting the channels of the oceans, it is
forced up them toward the north. ts progress is
accelerated in the deep basins, and retarded in the
shallow ones. On striking the coasts of the con-
tinents, deflected or secondary waves move off in
different directions, thus producing great com-
plexity in the form of the parent, wave.

213. Co-Tidal Lines——The progress of the tidal
wave, in each of the oceans, is best understood by
tracing on a map, lines connecting all places
which receive the tidal wave at the same time.
These are called co-tidal lines. The distance be-
tween two consecutive lines represents the time, in
hours, required for the progress of the tidal wave.
In parts of the ocean where the wave travels rap-
idly the co-tidal lines are far apart; when its prog-
ress is retarded, they are crowded together.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The figures on the lines
show the time of flood tide;
the lines show the path of
the tidal wave.

 

 

 

 

 

 

 

Fig. 75. Qo-Tidal Chart,

Since it is only possible to. take the height of the tide
on the coasts of islands and of the continents, the tracks
of the co-tidal lines must be to a.considerable extent con-
jectural.

214. The Pacific Ocean.—Twice every day a
tidal wave starts in the south-eastern part of the
Pacific Ocean, west of South America, somewhere
between the two heavy lines marked x1 on the

chart. It advances rapidly toward the north-
west in the deep valley of this ocean;. reaching~
Kamtchatka in about 6 hours. Toward ‘the west
its progress is retarded by the shallower water,
and by the numerous islands, so that it only
reaches New Zealand in about 6 hours and enters
the Indian Ocean in about 12 hours. {

215. The Indian Ocean.—The 12-hour-old tidal

}