76 PHYSICAL GEOGRAPHY.

 

gators have occasionally reported higher waves, but the
accuracy of their measurements is, perhaps, to be doubted.
In the open sea, with a moderate wind, the height of
ordinary waves is about 6 feet.

The distance between two successive crests varies from
10 to 20 times their height. Waves 4 feet high have
their successive crests 40 feet apart; those 33 feet high,
about 500 feet apart.

208. No Progressive Motion of Water in
Waves.—In wave motion, the water seems to be
moving in the direction in which the wave is ad-
vancing, but this is only apparent; light cbjects,
floating on the water, rise and fall, but do not
move forward with the wave. In shallow water,
however, the water really advances. The for-
ward motion of the wave is retarded, so that the
waves following reach it, thus increasing its
height. The motion at the bottom is lessened,
and the top curls over and breaks, producing
what are called breakers.

On gently sloping shores, the water which runs down
the beach, after it has been thrown upon it by the breakers,

forms, at a little distance from the shore, the dreaded
“undertow” of our bathing-resorts.

Force of the Waves.—When high, and moving
in the direction of the wind, the waves dash
against any obstacle, such as a line of coast, with
great force, and may thus cut it away and change
the coast-line. This action occurs only on ex-
posed, shelving coasts. The wave-motion is, in
general, very feeble at 40 feet below the surface.
The eroding action of the ocean waves is, there-
fore, far inferior to that of the continental waters.

209. Tides are the periodical risings and fall-
ings of the water, caused by the attraction of the
sun and moon. The alternate risings and fallings
succeed each other with great regularity, about
every six hours. Unlike waves, in which the
motion is confined practically to the surface
waters only, tides affect the waters of the ocean
from top to bottom.

The rising of the water is called flood tide; the
falling, ebb tide. When the waters reach their
highest and lowest points, they remain stationary
for a few minutes. These points are called, re-
spectively, high and low water. Corresponding
high or low water, at any place, occurs fifty-two
minutes later each successive day.

210. Theory of the Tides.—If the earth were
uniformly covered with a layer of water, the pas-
sage of the moon over any place, as at a, Fig. 72,
would cause the water to lose its globular form,
become. bulged at a, and 0, and flattened at ¢,
and d. In other words, the water would become

. greatest.

deeper at a, and 6, at the parts of the earth near-
est and farthest from the moon, and shallower in

“ ss,

ee
g
ao

Twieaee ss

Fig. 72. Lunar Tide,

all places 90° or at right angles to these points,
such, for example, as at c, and d.

This deepening and shallowing of the water is
caused by the attraction of the moon. As the
moon passes over a, the water is drawn toward
the moon, thus deepening the water directly under
the moon, and shallowing it at ¢, and d.

The cause of the deepening of the water at b,
on the side farthest from the moon, is as follows:
the solid earth being, as a whole, nearer the moon
than the water at 6, but farther from it than that
at a, must take a position which will be nearly
midway between a, and 8, leaving a protuberance
at 6, nearly equal to that at a.

The protuberances a, and b, mark the position
of high tides. At all points of the earth 90° from
the protuberances, as at c, and d, the depression is
These mark the position of low tides.

High tides, then, occur at those points of the
earth’s surface which are cut by a straight line,
which passes through the centre of the earth and
that of the attracting body, as the sun or moon.
Low tides are found at right angles to these
points.

Had the earth no rotation, the tidal waves, so
formed, would slowly follow the moon in its mo-
tion around the earth. But, by the rotation of
the earth, different parts of its surface are rapidly
brought under the moon, and the tidal waves,

consequently, move rapidly from one part of the
. ocean to’another.

Had the moon no motion around the earth, there would
be two high tides and two low tides every 24 hours.
While, however, the earth is making one complete rota-
tion, the moon, in its motion around the earth, has
changed its position, and the earth rotates for 52 minutes
longer before the same point again comes directly under
the moon.

Since the uniformity of the water surface is
broken by the elevations of the land, the progress
of the tidal wave is greatly affected by the size,
shape, and depth of the oceanic basin, and the