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Page 29
* * * * *
WAVES.
In the first days of August, two startling announcements reached us
from the United States. They were as follows:
(1.) "The commander of the Cunarder Umbria reports that at 3 o'clock
on July 27, about 1,500 miles from Sandy Hook, the vessel was struck
by a tidal wave 50 ft. high, which swept the decks, carried away a
portion of the bridge and the forward hatch, and flooded the cabins
and steerage."
(2.) "The captain of the Wilson line steamer Martello reports that at
half-past 8 on the evening of July 25, when in lat. 49� 3' N., long.
31� W., an enormous wave struck the vessel, completely submerging the
decks."
In view of these reports, and inasmuch as questions were asked on the
subject in Parliament, though it is quite possible that, as regards
the "tidal" character of the waves, there may be something of
newspaper _gobemoucherie_ in the announcements, we offer a few remarks
on _waves_ in general, which may be useful to some of our readers.
_Tidal phenomena_ present themselves under two aspects: as alternate
elevations and depressions of the sea and as recurrent inflows and
outflows of streams. Careful writers, however, use the word _tide_ in
strict reference to the _changes of elevation_ in the water, while
they distinguish the recurrent streams as _tidal currents_. Hence,
also, _rise_ and _fall_ appertain to the tide, while _flood_ and _ebb_
refer to the tidal current.
The _cause of the tides_ is the combined action of the sun and moon.
The relative effects of these two bodies on the oceanic waters are
directly as their mass and inversely as the square of their distance;
but the moon, though small in comparison with the sun, is so much
nearer to the earth that she exerts the greater influence in the
production of the great _tide wave_. Thus the mean force of the moon,
as compared with that of the sun, is as 2� to 1.
The attractive force of the moon is most strongly felt by those parts
of the ocean over which she is vertical, and they are, consequently,
drawn toward her. In the same manner, the influence of the luminary
being less powerfully exerted on the waters furthest from her than on
the earth itself, they must remain behind. By these means, at the two
opposite sides of the earth, in the direction of the straight line
between the centers of the earth and moon, the waters are
simultaneously raised above their mean level; and the moon, in her
progressive westerly motion, as she comes to each meridian in
succession, causes two uprisings of the water--two high tides--the one
when she passes the meridian above, the other when she crosses it
below; and this is done, not by drawing after her the water first
raised, but by raising continually that under her at the time; this is
the _tide wave_. In a similar manner (from causes already referred to)
the sun produces two tides of much smaller dimensions, and the joint
effect of the action of the two luminaries is this, that instead of
four separate tides resulting from their separate influence, the _sun
merely alters the form of the wave raised by the moon_; or, in other
words, the greater of the two waves (which is due to the moon) is
modified in its height by the smaller (sun's) wave. When the summit of
the two happens to coincide, the summit of the combined wave will be
at the highest. When the hollow of the smaller wave coincides with the
summit of the larger, the summit of the combined wave will be at the
lowest.
It is necessary to have a clear and distinct conception of the
difference between the _motion_ of a _wave_ and that of a _current_.
In the current there is a transfer of water; in the wave the transfer
is no more than would be brought about by a particle of water
impinging on another where that particle has a motion perpendicular to
the surface, and a rising and falling results. The onward movement of
the wave itself is always perceptible enough. That the water is not
moving with the same velocity is also evident from watching the
progress of any light body floating on its surface. This fact may be
practically illustrated in the case of a ship at sea, sailing before
the wind in the same direction as the waves are moving. When the crest
of a wave is near the stern, drop a piece of wood on it. Almost
instantly the wave will be seen shooting ahead of the vessel, while
the wood is scarcely removed from the position where it fell on the
water. The wave has moved onward, preserving its identity as a wave,
the water of which it is formed being constantly changed; and thus the
motion of the wave is one thing, that of the water in which the waves
are formed is quite another thing.
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