Swell (oceanography)

“ Not infrequently these occasional waves are great enough to overwhelm persons who are upon the rocks next the shore. These greater surges are probably to be accounted for by the fact that in the open sea waves produced by winds blowing in different directions may run on with their diverse courses and varied intervals until they come near the shore. Running in together, it very well happens that two of the surges belonging to different sets may combine their forces, thus doubling the swell. ”

“ When the tide begins to flood his range is six hundred yards, but as it runs in on the beach it is shortened to half that or less. At either distance it requires close calculation to get a good aim, or to overcome the effect of the ocean swell and movement. ”

“ In the Bristol Channel, on the coast of England, the spring tides rise to 40 feet, and swell to 50 in the English Channel at St. Malo on the coast of France.”
To this cause is also due in some degree those great oceanic currents which form another striking fact in the history of the sea; but they are mainly due to temperature, wind, and the rotation of the earth.
The drops that make up a body of water are the most restless things in the world ”

“ Unequal evaporation and precipitation likewise disturb the level of the sea and occasion movement. In the open sea the movements generated by differences of level, like those generated by differences of density, are chiefly slow, creeping movements, but movements which never cease. In bays and gulfs, on the other hand, the surface of the water may be so raised, either as the result of wind, river discharge, or heavy precipitation, as to give rise to strong outward currents. ”

“ Thus, between tide-waves and currents, changes are going on continually both in the volume and the physical constitution of the sea-water. And this, which might at first on a hasty glance look like casual and unimportant variation, when we come to consider it attentively, is found to be an arrangement fraught with wisdom and beneficence; for it insures continual variations of temperature, it attracts the purifying storms, restores the elastic spring of the atmosphere, and provides for weary man a healthy tonic breeze when he rambles along the beach or scales the face of the cliff. ”

“ Had not the sea been salt, the torrid zone would have been hotter and the frigid colder for lack of aqueous circulation; had the sea not been salt, intertropical seas would have been at a constant temperature higher than blood heat, and the polar oceans would have been sealed up in everlasting fetters of ice, while certain parts of the earth would have been deluged with rain. Had the seas been of fresh water, the amount of evaporation, the quantity of rain, the volume and size of our rivers, would all have been different from what they are ”

“ But is it probable that, throughout all the geological periods, during all the changes which have taken place in the distribution of land and water surface over the earth, the winds, which in the general channels of circulation pass over the Dead Sea, have alone been unchanged? Throughout all ages, periods, and formations, is it probable that the winds have brought us just as much moisture to that sea as they now bring, and have just taken up as much water from it as they now carry off? ”

“ Now is it probable that, in passing from their summer to their winter temperature, the sea-waters to the right of the Gulf Stream should change their specific gravity exactly as much in the aggregate as do the waters in the whole ocean to the left of it? If not, the difference must be compensated by some means. Sparks are not more prone to fly upward, nor water to seek its level, than Nature is sure with her efforts to restore equilibrium in both sea and air whenever, wherever, and by whatever it be disturbed. ”

“ Every page of the geological record proves to us that the relative levels of land and sea, and the position of the ocean and of continents and islands, has been always varying, and we may feel sure that some portions of the submarine area are now rising and others sinking. The force of tidal and other currents and of the waves during storms is sufficient to prevent the emergence of many lands, even though they may be undergoing continual upheaval. ”

“ A proper conception of the bathymetry of Polar seas is necessary for an adequate discussion of physical problems connected with the temperature, salinity, specific gravity and circulation, and the effect of wind, air-temperature and other phenomena that affect these seas. The physical problems of ice-covered seas are much more complicated than in seas where there is no ice, because, as we have seen previously, when the surface of the sea is being frozen over, the salt in that part of the water which is changed into ice is thrown out and must therefore make the neighbouring water more saline ”

“ 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. ”

“ Influence of the trade-winds upon the specific gravity of sea water.—It is the trade-winds, then, which prevent the thermal and specific gravity curves from conforming with each other in intertropical seas. The water they suck up is fresh water, and the salt it contained, being left behind, is just sufficient to counter-balance, by its weight, the effect of thermal dilatation upon the specific gravity of sea water between the parallels of 34° north and south. As we go from 34° to the equator, the water grows warm and expands. ”

“ Old sailors tell us of such dead calms of long continuance here, of such heavy and constant rains, that they have scooped up fresh water from the sea to drink. The conditions to which this air is exposed here under the equator are probably not such as to cause it to precipitate all the moisture that it has taken up in its long sweep across the waters. Let us see what becomes of the rest; for Nature, in her economy, permits nothing to be taken away from the earth which is not to be restored to it again in some form, and at some time or other. ”

“ There is still another way in which heat and cold must occasion great movements in the ocean, a cause to which, perhaps, currents are principally due. Whenever the temperature of the surface of the sea is lowered, condensation takes place, and the superficial water, having its specific gravity increased, falls to the bottom, upon which lighter water rises immediately and occupies its place. ”

“ When this circulation of ascending and descending currents has gone on for a certain time in high latitudes, the inferior parts of the sea are made to consist of colder or heavier fluid than the corresponding depths of the ocean between the tropics. If there be a free communication, if no chain of submarine mountains divide the polar from the equatorial basins, a horizontal movement will arise by the flowing of colder water from the poles to the equator, and there will then be a reflux of warmer superficial water from the equator to the poles. ”

“ As no sea can be supposed absolutely motionless, the presence of a shoal, by casting up to the surface water which, but for it, would have continued to sweep along at a greatly lower level with the general body of the current, must bring the temperature of the surface water into nearer correspondence with that below. In low latitudes the surface water is hotter than that below; and accordingly it is a general remark, that the temperature sinks as the water shoals, or even in passing over banks whose depth is very considerable. ”

“ In the Red Sea, where a similar seasonal change occurs, the depth to which the surface layer warms up is about 275 fathoms. The great difference in salinity between the surface and the deep water excludes the possibility of effective convection in the seas of northern Europe, and in the open ocean the currents which are felt everywhere, and especially those with a vertical component, must exercise a very disturbing influence on convection. ”

“ In inland seas, where the tides are insensible, or on those parts of the borders of the ocean where they are feeble, it is scarcely possible to prevent a harbor at a river's mouth from silting up; for a bar of sand or mud is formed at points where the velocity of the turbid river is checked by the sea, or where the river and a marine current neutralize each other's force. For the current, as we have seen, may, like the river, hold in suspension a large quantity of sediment, or, co-operating with the waves, may cause the progressive motion of a shingle beach in one direction. ”

“ Wherever a shoal of sand exists in the offing, at a distance beyond where the ebbing of the tide recedes to its greatest extent, denominated low water mark, there the innermost shallow will probably be: another shoal immediately forms, the base commencing at low water mark, and a gradual rise takes place towards the cliffs, terminating at or beyond the extent of the flowing of the tide denominated high water mark. Here, then, the shoal will be more efficient; the tidal wave and current will be checked and broken against the ascending bank. ”

“ The dominion of heat has passed away; the excess of water has been precipitated from the atmosphere, and now covers the earth as a universal ocean. The crust has folded itself into long ridges, the bed of the waters has subsided into its place, and the sea for the first time begins to rave against the shores of the newly elevated land, while the rain, washing the bare surfaces of rocky ridges, carries its contribution of the slowly wasting rocks back into the waters whence they were raised, forming, with the material worn from the crust by the surf, the first oceanic sediments. ”

“ In considering the effect of the natural forces still in operation during the many thousands of years during which they have been at work, it is necessary to bear in mind that the level of the sea has all along remained the same, except so far as it is varied by the rise and fall of the tide, and by the exceptional exaggerations of tidal movement caused by the wind. ”

“ The in-shore tidal waters having to encounter the shelving shores, islands, sunken rocks, and projecting points of land, which lie in their course up the Firth , acquire lateral as well as perpendicular motion, and being thus checked in their progress, are brought sooner to a maximum state than the off-shore stream, which flows in deeper water, and comparatively free of obstruction, till it reaches its ultimate limits; though it gradually diminishes in breadth, till the stream of the new tide gaining strength becomes general ”

“ A stream of colder water also flows from the Black Sea into the Mediterranean. It must happen in many other parts of the world that large quantities of water raised from one tract of the ocean by solar heat, are carried to some other where the vapor is condensed and falls in the shape of rain, and this, in flowing back again to restore equilibrium, will cause sensible currents. ”

“ The time of greatest flow of an artesian well will occur at low tide in the ocean. This might be accounted for from the fact that when the tide is at its height the moon is also pulling upon the crust of the earth, which would tend to take the pressure off the sand rock which lies one or two thousand feet below the surface and through which the flow of water comes, and thus slacken the flow. When the moon is in position for low tide, the crust of the earth would settle back and thus produce a greater pressure upon the water-bearing rock. ”

“ For if there is one thing considered constant in this changing world of ours, it is the level of the sea. All our heights and depths are compared to and computed from it, so unchanging do we esteem it; since though great tides may come and go the level of the sea itself seems never to change, so delicately balanced are the combined processes of evaporation and condensation which deplete and replenish it. And for this hitherto unchanging level to rise suddenly for almost three feet, in as many days, was a phenomenon of intense interest and mystery. ”

“ It follows, therefore, that for spring tides the range and rise are synonymous terms, but at neap tides the range is the total height between high and low water, while the rise is the difference between high water of the neap tide and the mean low water level of spring tides. Neither the total time occupied by the flood and ebb tides nor the rate of the rise and fall are equal, except in the open sea, where there are fewer disturbing conditions. In restricted areas of water the ebb lasts longer than the flood. ”

“ As the height of the confined waters increases, the range of their journey over the bottom as the wave comes and goes rapidly increases. When they have an elevation of ten feet they can probably stir the finer mud on the ocean floor, and in shallow water move yet heavier particles. In the embayments of the land, where a great body of water journeys like an alternating river into extensive basins, the tidal action becomes intense; the current may be able to sweep along large stones quite as effectively as a mountain torrent. ”