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 2¼ 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.
Again, waves are formed by a force acting horizontally; but in the
case of the tide wave, that force acts uniformly from the surface to
the lowest depths of the ocean, and the breadth of the wave is that
curved surface which, commencing at low water, passes over the summit
of the tide down to the next low water--this is a wave of the first
order. In waves of the second order, the force raising them acts only
on the surface, and there the effect is greatest (as in the wind
waves)--where one assists in giving to the water oscillating motion
which maintains the next, and gradually puts the whole surface in
commotion; but at a short distance down that effect entirely
disappears.
If the earth presented a uniform globe, with a belt of sea of great
and uniform depth encircling it round the equator, the tide wave would
be perfectly regular and uniform. Its velocity, where the water was
deep and free to follow the two luminaries, would be 1,000 miles an
hour, and the height of tide inconsiderable. But even the Atlantic is
not broad enough for the formation of a powerful tide wave. The
continents, the variation in the direction of the coast line, the
different depths of the ocean, the narrowness of channels, all
interfere to modify it. At first it is affected with only a slight
current motion toward the west--a motion which only acquires strength
when the wave is heaped up, as it were, by obstacles to its progress,
as happens to it over the shallow parts of the sea, on the coasts, in
gulfs, and in the mouths of rivers. Thus the first wave advancing
meets in its course with resistance on the two sides of a narrow
channel, it is forced to rise by the pressure of the following waves,
whose motion is not at all retarded, or certainly less so than that of
the first wave. Thus an actual current of water is produced in straits
and narrow channels; and it is always important to distinguish between
the tide wave, as bringing high water, and the tidal stream--between
the rise and fall of the tide and the flow and ebb.
In the open ocean, and at a distance from the land, the tide wave is
imperceptible, and the rise and fall of the water is small. Among the
islands of the Pacific four to six feet is the usual spring rise. But
the range is considerably affected by local causes, as by the shoaling
of the water and the narrowing of the channel, or by the channel
opening to the free entrance of the tide wave. In such cases the range
of tide is 40 to 50 feet or more, and the tidal stream is one of great
velocity. It may under such circumstances even present the peculiar
phenomenon called the _bore_--a wave that comes rolling in with the
first of flood, and, with a foaming crest, rushes onward, threatening
destruction to shipping, and sweeping away all impediments lying in
its course.
It is certain that in the open ocean the _great tide wave_ could not
be recognized as a wave, since it is merely a temporary alteration of
the sea level.
_Waves_ which have their origin in the action of the wind striking the
surface of the water commence as a series of small and slow
undulations or wavelets--a mere ripple. As the strength, and
consequently the pressure, of the wind increases, waves are formed;
and a numerical relation exists between the length of a wave, its
velocity of progress, and the depth of the water in which it travels.
The _height_ of a wave is measured from trough to crest; and though
waves as seen from the deck of a small vessel appear to be "enormous"
and "overwhelming," their height, in an ordinary gale, in deep water,
does not exceed 15 to 20 feet. In a very heavy gale of some days'
continuance they will, of course, be much higher.
Scoresby has observed them 30 ft. high in the North Atlantic; and Ross
measured waves of 22 ft. in the South Atlantic. Wilkes records 32 ft.
in the Pacific. But the highest waves have been reported off the Cape
of Good Hope and Cape Horn, where they have been observed, on rare
occasions, from 30 to 40 ft high; and 36 ft. has been given as the
admeasurement in the Bay of Biscay, under very exceptional
circumstances. In the voyage round the world the Venus and Bonite
record a maximum of 27 ft., while the Novara found the maximum to be
35 ft. But waves of 12 to 14 ft. in shallow seas are often more trying
than those of larger dimensions in deeper water. It is generally
assumed that a distance from crest to crest of 150 to 350 ft. in the
storm wave gives a velocity (in the change of form) of from 17 to 28
miles per hour. But what is required in the computation of the
velocity is the period of passage between two crests. Thus a distance
of 500 to 600 ft. between two crests, and a period of 10 to 11
seconds, indicates a velocity of 34 miles per hour.
The following table, by Sir G.B. Airy (late Astronomer Royal), shows
the velocities with which waves of given lengths travel in water of
certain depth:
Depth of | Length of the Wave in Feet.[1]
the Water| | | | | | |
in Feet. | 10 | 100 | 1,000 | 10,000 | 100,000 |1,000,000 |10,000,000
---------+-----+------+-------+--------+---------+----------+----------
|
| Corresponding Velocity of Wave per Hour in Nautical Miles.
|
1 | 3.2 | 3.4 | 3.4 | 3.4 | 3.4 | 3.4 | 3.4
10 | 4.3 | 10.1 | 10.7 | 10.8 | 10.8 | 10.8 | 10.8
100 | 4.3 | 13.5 | 32.0 | 34.0 | 34.0 | 34.0 | 34.0
1,000 | 4.3 | 13.5 | 42.9 | 101.8 | 107.5 | 107.5 | 107.5
10,000 | 4.3 | 13.5 | 42.9 | 135.7 | 320.3 | 340.0 | 340.3
100,000 | 4.3 | 13.5 | 42.9 | 135.7 | 429.3 | 1013.0 | 1075.3
---------+-----+------+-------+--------+---------+----------+----------
[Footnote 1: As an example, this table shows that waves 1,000 feet
in length travel 43 nautical miles per hour in water 1,000 feet
deep. The length is measured from crest to crest.]