Popular Science Monthly/Volume 23/August 1883/The Formation of Sea-Waves
|THE FORMATION OF SEA-WAVES.|
ONE of the first things to be observed in a storm is the way the wind acts. It does not blow regularly, but in gusts. At one moment it bends over the branches of the trees; in the next, it has loosened its hold, and let them fly back. We see it swelling out a ship's sails into a full puff; a minute later the sails hang flapping as if they had been struck down.
We can account for these phenomena and explain the intermittence of the wind-puffs by assuming that the molecules of air, displacing each other, excite a vibratory movement, which gives rise to little undulations following one after another at intervals of a few seconds. The resultant of a series of these undulations is a puff of wind which comes on suddenly and is followed by a short lull. A series of puffs constitutes a squall, and an aggregation of squalls forms the atmospheric wave which is called a gale of wind. We should naturally expect to observe the same phases in the formation of sea-waves; and, in fact, if we carefully examine a wave, we shall find that it is covered with very fine ripples, that correspond to the atmospheric vibrations. The ripples give rise to wavelets, which correspond to the undulations of the air, and are seen on the upper part of the waves. The wave proper appears to consist of a series of wavelets. A number of waves constitute a billow; a series of billows gives rise to a heavy sea (paquet de mer); a series of heavy seas produces the great swell or tidal wave of the storm.
From the nautical point of view, the ripples are of no importance, for they are seldom more than a few millimetres in diameter; but from the scientific point of view they may be considered as the origin of the swing of the liquid element, for they engender the wavelets. The last are still of no interest to the sailor, but are important in their relation to works of art, which are disintegrated by their blows, apparently insignificant, but infinitely multiplied. The wavelets are from ten to thirty centimetres in diameter and not very long. A very heavy wind breaks them up and contributes to the formation of a fine dust of salt water or salt spray, which is destructive to vegetation on exposed coasts. The wave proper may, in the English Channel, be about ten feet high, thirty feet or more broad, and eighty feet long; its proportions do not disturb large ships, but it is destructive, in the long run, to port works, and is dangerous to small craft when it breaks. We may estimate that ten waves make a billow. The first of the ten may be relatively small, but the others go on increasing to the last.
The heavy seas are the terror of sailors. They represent an enormous volume of water in motion. A gust of wind can not possibly raise up such a mass, and it can only be the result of the combined efforts of the tempest. A heavy sea may reach a height equal to twenty-five or thirty or more feet. It is massive, and strikes like a battering-ram. On the land it causes great damage, and makes breaches in works of earth and stone; at sea, it can send a trans-atlantic packet to the bottom with a single blow.
The great tidal wave is produced by two causes. On one side, it is the general resultant of the billows and the great seas; and, on the other hand, it is produced by barometrical depressions causing the waters of the ocean to rise. In cyclones, the rise of the water in the center would be neutralized by the centrifugal force, and it is therefore probable that the former cause acts alone. The tidal wave has but small amplitude, but, when inclosed by parallel coasts, it may rise to a height of several metres. It then causes inundations of low shores.
The singular fact has been remarked at Havre that in a storm the swell almost always comes after the tide. The sea rises to its normal high-water mark at the prescribed hour, and then begins to retire as usual; all at once it rises again, to a height generally much greater than before. This high sea continues for a considerable time, sometimes for several hours; and it has been known to last twenty-four hours.
The billows, heavy seas, and tidal waves possess a considerable inertia, and keep up the swell after the tempest has subsided. The real waves fall, while the billows still subsist, but flattened. It is then easy to estimate the distance between them. On the 26th of March, 1882, I counted eight in a space of four thousand metres.
The most serious event that can take place at sea is a change of wind, such as nearly always occurs in cyclones. The phenomena we have described being well established and sure to continue for a considerable time by virtue of their inertia, when the wind veers around so as to reproduce them in another direction, the new waves cross the old ones, and a chopped sea, dangerous to navigation, is the result.
If any one interested in scientific matters comes to the shore to study the formation of waves, he will experience some disappointment, for the configuration of the coasts, the eddies, and the currents, modify the phenomena in a thousand ways. They are, however, always apparent, even in the calmest weather, and vary only in their amplitude. To observe them it is enough to take notice of the level of the water against a post, a jetty, or other structure. Changes of level are produced there quite similar to the pulsations of the sea; and the extent of these pulsations gives quite exact data respecting everything that we have mentioned.
Waves may be classified as direct waves and waves of transmission. The former, with which the surface of the sea is frequently agitated, are those which the wind raises directly. Waves of the second class may be produced in the calmest weather; their origin is frequently quite distant from the places where they are observed; and they reach those places by transmission. A well-known physical experiment will suggest an explanation of the phenomenon of an agitated and raging sea when there is no wind. If we have a long line of billiard-balls arranged in contact one with another, and give a quick blow to the first one, the last one will roll away. The shock is transmitted from the first ball to the last one, without the intermediate ones suffering any appreciable motion. Marine disturbances caused by direct waves, tides, earthquakes, etc., may in the same way be transmitted through molecules of still water without agitating them. If the liquid space is free, the vibrations are gradually extinguished; if they meet an obstacle, there is a shock. If the obstacle is a shore, they form a tidal-wave and raise large billows, while a few miles away from the shore the sea is quiet. When the obstacle is a shoal or a contrary vibration, heavy waves are raised on the surface of the sea; they seem to start from the bottom, and put ships in great danger. The waves produced in both cases are waves of transmission, as also are those which beat on reefs in pleasant weather, and those which prolong the swell after storms. The transmission may be effected at very great distances—several hundred miles, for example. It presents a kind of analogy with earthquake-shocks which pass over dead points, and make themselves felt only in places where there are faults or differences in the density of the terrestrial strata.
At Havre, on stormy days, the perturbations of the open sea, transmitted to the shoals of the roadstead, cause oscillations of the water within it, and produce what the sailors call the levée, which, in bad weather, prevents the transatlantic steamers from entering the port. Crossing the entrance of the port, with its breakwaters, the levée penetrates into the outer harbor and spreads out there, attaining two or three metres in amplitude. It enters the Bassin du Roi through a sluice sixteen metres in width, and thence is propagated through a sluice thirteen metres wide to the Bassin du Commerce, where, involved in the ins and outs of the quays, it does not reach more than thirty or fifty centimetres in amplitude. This remarkable phenomenon of the levée, passing into a chain of basins, appears analogous to that of the vibrations of a tense cord divided into sections by a series of frets in contact with it. When we draw a bow over one of the sections of the cord, the others will also vibrate, while a dead point or node will be formed at each place of contact. In the phenomenon under consideration, the entrance of the port and each sluice give rise to a node.
When a mass of water in motion meets an obstacle, it accumulates against it by virtue of its inertia; the water rises, then falls back. This is called the surf, and may be observed along all coasts. It is produced at sea after every tide. The most curious effect induced by it is the back-flow in rivers. The Seine, for instance, flows rapidly at low water; but, as the tide rises, a liquid obstacle several metres high is piled up in less than two hours against the mouth of the river. The water of the Seine then stops, rises, and falls back as surf, while the surf in its turn acts as an obstacle to the current of the river above it. The phenomenon is repeated, and again, and so on, steadily going higher up the river, so that in effect a strong wave ascends the stream. The phenomenon may be easily reproduced on a small scale; every time we suddenly stop the rapid current of a brook or any stream of water, we may see a back-water ascend it.
The amplitude of the movement of waves remains to be spoken of. It appears to be proportional for direct waves to the force of the wind. On the other hand, since each ripple, wavelet, or wave, occupies a given space, and since, as I have already said, a certain number of these are necessary to give rise to a billow or a heavy sea, it is evident that, with a given wind, a billow of a particular dimension can be formed only if a sheet of water extends over a certain area of surface. This is precisely what takes place, and the dimensions of the billows produced by a given wind appear to be proportional to the extent of the sea in which they are formed. Thus the largest surface of water to be found on the earth is in the latitude of the Cape of Good Hope; and the strongest waves are met in the neighborhood of this cape. In the Mediterranean the wave is short.
In all ages men have sought for means of calming the agitation of the waves, which is so prejudicial to shipping. The best means hitherto employed has been that of breakwaters, the operation of which is too well known to need description. It may be added that floating wave-breaks, such as would be constituted by a large number of spars or planks left to drift, afford a perfect amelioration of the agitation of the waves. Hitherto engineers have applied their efforts only against the larger waves. Why not attack the evil in its origin? Why not take up the ripples and the wavelets, and oppose them with floating ripple-breaks? Such breaks might be made with twigs, saw-dust, or soot, etc., and experiment has proved that they will be efficacious. The needles of ice which form in cold weather on the surface of the water are excellent natural wavelet-breaks. Generally, every cause hindering the formation of wavelets appeases the agitation of the waves. Thus a rain, every drop of which breaks a ripple, calms the sea to a certain extent. Sailors know this. Billows have been fought against with ordinary wave-breaks. Wavelets may be destroyed by employing light bodies, ripples with dust, microscopic ripples with an infinitely fine powder. A liquid will serve the end admirably. Oil is the best of all agents for the purpose. It has the property (to which capillarity is probably not foreign) of spreading over the water as a pile of billiard balls spreads over a well-polished marble table. Its molecules form as many floating microscopic pebbles, in the intervals between-which the ripples break, as the billows break upon the shingle of the coasts. Oil thus acts as a lubricant, attenuating the friction of the wind. Capillary phenomena, due to the minuteness of the intervals between the oleaginous molecules, intervene to divide up and draw off the surface of the water and completely neutralize the force of the wind. All of these causes together may give us the reason of the efficacy of oil in destroying waves.
It should be understood that all the means of restraining the agitation of waves here indicated are good only against direct waves due to the formation of ripples. They have but slight influence on waves of transmission, which are due to other causes. Oil may appease the billows, but the swell will continue.—Translated for the Popular Science Monthly from La Nature.