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Page 30
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.
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