Wave (oceanography)

“ As they pass up the narrowing spaces the waves become compressed—that is, an equal volume of moving water has less horizontal room for its passage, and is forced to rise higher. We see a tolerably good illustration of the same principle when we observe a wind-made wave enter a small recess of the shore, the sides of which converge in the direction of the motion. With the diminished room, the wave gains in height. It thus comes about that the tide throughout the Atlantic basin is much higher than in the southern ocean. ”

“ This rise and fall of the waters along the coast, accompanied frequently by strong currents, are produced directly by the arrival in the shoal water of a pulsation of the ocean, which becomes a true onward-moving gravity wave as it nears the land. In the open sea the amplitude of the tidal undulations is but two or three feet, and their rate of travel in general 700 to 800 miles per hour. ”

“ 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. ”

“ The motion of the tide-wave is not a progressive motion, but a mere undulation, and is to be carefully distinguished from the currents to which it gives rise. If the ocean completely covered the earth, the sun and moon being in the equator, the tide-wave would travel at the same rate as the earth revolves on its axis. ”

“ Progress of the Tide-wave over the Ocean and in the British Seas.—Sufficient tidal data would give the state of the tide at every part of the world at the same instant of time, and if the tide wave is a progressive one, like such wave as we may observe travelling along a canal, we should be able to picture mentally the motion of the tide-wave over the ocean and the successive changes in the height of water at any one place. ”

“ A tidal wave will traverse the open sea in a straight line, but as it passes along the coast the progress of the line nearest the shore is retarded while the centre part continues at the same velocity, so that on plan the wave assumes a convex shape and the branch waves reaching the shore form an acute angle with the coast line. ”

“ WE have all stood many times by the seashore, watching the waves, crested with white foam, roll in and break upon the rocks or beach. Every one has more than once cast a stone upon still water in a lake or pond, and noticed the expanding rings of ripples; and some have voyaged over stormy seas, whereon great ships are tossed by mighty billows with no more seeming effort than the rocking of a cradle. In all these things we have been spectators of a wave-motion, as it is called, taking place upon a water surface. ”

“ We may conceive the moon's tide-generating force as making a wave in the canal and continually outstripping the wave it generates, for the moon travels along the equator at the rate of about 1000 miles an hour, and the sea is less than 13 miles deep. The resultant oscillation of the ocean must therefore be the summation of a series of partial waves generated at each instant by the moon and always falling behind her, and the aggregate wave, being the same at each instant, must travel 1000 m. ”

“ To raise a tidal wave the sun and moon require not merely an ocean of wide extent to act upon, but an ocean so placed that there is a great diversity in their pull on various parts of it; for it is the difference between the pull exerted on various parts, and not the pull itself, which creates the tidal wave. Now the Polar sea has not the required extent, and is not in the proper position, for this diversity of pull to exist in sufficient degree to produce a tidal wave which could be recognized. ”

“ As the derivative wave, consisting of the large mass of water set in motion by the comparatively small rise and fall of the primary wave, is propagated through the branch oceans, it is affected by many circumstances, such as the continual variation in width between the opposite shores, the alterations in the depth of the channels, and the irregularity of the coast line. ”

“ At last, when the great ocean-wave approaches the beach, and the depth of water is much diminished, the velocity of so vast a mass sweeping along the bottom, though greatly accelerated, becomes inadequate to fulfil the conditions of the oscillation, and it has no resource but to curl into a high and toppling wave. ”

“ Some observers have claimed to see them over one hundred feet high, but from twenty to fifty feet is about the average of observations on the Atlantic. The breadth of a wave is calculated as fifteen times its height. Thus, a wave four feet high is sixty feet broad. The inclination of the sides of the waves varies however with the force of the wind, and with the strength of the secondary vibrations in the water, which may interfere with the primary ones. The speed of the waves is only apparent like the motion in a length of cloth shaken up and down. ”

“ No wave has ever been raised on their bosom, no keel has ever ploughed their surface, which has not sent an influence and a change into every ocean, and modified every wave, that has rolled in upon the farthest shores. ”

“ The front side of this great wave takes an almost vertical position, resembling an advancing wall of water, and works great havoc with boats and shipping which have had the misfortune to be left in its path. To understand more completely how a “bore” is formed, the reader must be reminded of the cause of all tidal phenomena. Any one who lives by the sea or an estuary knows well that the sea-level rises and falls twice every 24 hours, and that the average interval of time between high water and high water is nearly 121⁄2 hours. ”

“ We see a wave a quarter of a mile off, then a few hundred yards nearer in, then just offshore; we seem to have been watching the same travelling mass of water—there has been no appreciable change in mass or in shape—yet all the while the original beat has taken on a flowing series of liquid bodies, bodies so alike, so much the same, that our eye will individualize them and follow them in—the third wave, we say, or the second wave behind the great wave. ”

“ A tidal wave coming up the Atlantic coast meets the ocean wave from the east, and, entering the bay with their combined force, they raise the water at the head of it to the height of sixty or seventy feet. ”

“ When the waves leave the generating area and continue as free waves, the wavelength and period continue to increase, while the height decreases. The rate of change gradually decreases. The speed (S) of a free wave in deep water is nearly independent of its height or steepness. ”

“ The semi-synchronous oscillation of the Southern Ocean raises the magnitude of oceanic tides somewhat above these directly generated values.
Oceanic tides are true waves, not currents. Coast tides are currents. The momentum of the water, when the tidal wave breaks upon a continent and rushes up channels, raises coast tides to a much greater height—in some places up to 50 or 60 feet, or even more. ”

“ Their chief object has been to show you that the swiftly moving ocean waves, which dash and roll unceasingly against the coast-lines of our island home, are only instances of one form of wave-motion, of which we find other varieties in other media, giving rise to all the entrancing effects of sound and light. In these expositions we have been able to do no more than touch the fringe of a great subject. ”

“ They are very complex, and consist of westward waves and eastward waves of very various periods. He finds, as was to be expected, that if for an ocean of given depth, a free wave very nearly coincides in period with the forced lunar or solar wave, the actual tide is largely augmented. Thus, for example, for an ocean of 29,000 ft. in depth the solar semi-diurnal tide would have a height at the equator 235 times as great as the equilibrium height, and would be inverted so that low water would agree with the high water of the equilibrium theory. ”

“ If two infinitely long sets of deep-sea waves, having slightly different wave-lengths, and therefore slightly different velocities, are superimposed, we obtain a resultant wave-train which exhibits a variation in wave-amplitude along its course periodically. ”

“ It is also well known that while the earth turns from west to east on its axis, the tidal wave moves around the globe from east to west, thus acting like a powerful friction-brake to stop the earth’s rotation. ”

“ If we watch a sea wave rolling in towards the beach, we shall notice that, as it nears the shore, it gets steeper on the shore side, and gradually curls over until it falls and breaks into spray. The reason is because, as the wave gets into the shallow water, the top part of the wave advances more rapidly than the bottom portion. ”

“ A relatively long wind wave, or series of waves, that has traveled out of the generating area. In contrast the term SEA is applied to the waves while still in the generating area. As these waves travel away from the area in which they are formed, the shorter ones die out. The surviving waves exhibit a more regular and longer period with flatter crests. When these waves reach shoal water, they become more prominent in height and of decreased wave length and are then known as ground swell.
swell direction. The direction from which swell is moving.
swept-frequency racon. ”

“ The wave is not noticeable at sea, but becomes apparent along the coasts, particularly in funnelshaped estuaries. In certain river mouths, or estuaries of particular configuration, the incoming wave of high water overtakes the preceding low tide, resulting in a steep, breaking wave which progresses upstream in a surge called a bore. ”

“ Wave-Action.—The powerful effects of the beating of waves upon abrupt shores of seas or lakes are in evidence all over the world, and these effects are so characteristic that they can be recognized even where no sea or lake exists at present. ”

“ If, however, the ball is hanging by a long string we can play at battledore and shuttlecock with it, and it always meets our blows. The latter is the analogue of the tides, for a free wave in our shallow canal Tides Inverted. goes slowly, whilst the moon's tide-generating action goes quickly. Hence when the system is left to settle into steady oscillation it is low-water under and opposite to the moon, whilst the forces are such as to tend to make high-water at those times. ”

“ Our investigations go to show that the roaring waves and the mightiest billows of the ocean repose, not upon hard or troubled beds, but upon cushions of still water; that everywhere at the bottom of the deep sea the solid ribs of the earth are protected, as with a garment, from the abrading action of its currents, and that the cradle of its restless waves is lined by a stratum of water at rest, or so nearly at rest that it can neither wear nor move the lightest bit of drift that once lodges there. ”

“ For, see, since a wave is only a communicated agitation or impetus, and since the water that composes a wave is changing every instant, new water is rising into the wave as fast as the wave travels. You slide down this new water, and yet remain in your old position on the wave, sliding down the still newer water that is rising and forming the wave. You slide precisely as fast as the wave travels. If it travels fifteen miles an hour, you slide fifteen miles an hour. Between you and shore stretches a quarter of mile of water. ”