Mean sea level

“ The tides, in short, in the Caribbean Sea are scarcely perceptible, not equalling those in some parts of the Mediterranean, whereas the rise is very high in the Bay of Panama; so that the Pacific is at high tide lifted up several feet above the surface of the Gulf of Mexico, and then at low water let down as far below it.388 But astronomers are agreed that, on mathematical principles, the rise of the tidal wave above the mean level of a particular sea must be greater than the fall below it ”

“ But also the powerful attraction of the land, particularly of great mountain ranges, lifts the sea-surface in some regions hundreds of feet higher than in others. So that a ship, passing from the neighbourhood of a mountainous coast to [p. 128] mid-ocean, may be actually sailing or steaming downhill, actually descending a vast gentle watery slope.
Another question as to Ocean’s level has much exercised the minds of men.
Throughout the world, remains of sea-creatures in countless multitudes are found embedded in the substance of water-built rocks: sometimes the very rocks being made up of them. ”

“ 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. ”

“ There is also a predilection for regarding the rigid land as moving and the mobile sea-level as fixed. In reality, the sea is an extremely adaptive body that settles into the irregular hollows of the lithosphere, and is shifted about with every warping of the latter. Whatever change affects the capacity of its depressions affects also the sea-level. ”

“ But whether or not these causes would produce any important fluctuations of the sea-level is of comparatively little importance to our present inquiry, because the wide extent of marine Tertiary deposits in the northern hemisphere and their occurrence at considerable elevations above the present sea-level, afford the most conclusive proofs that great changes of sea and land have occurred throughout the entire Tertiary period ”

“ If the sea be imagined as rising 100 ft., a new coast-line, with bays and estuaries indented in the valleys, would appear at the new sea-level. If the sea sank once more to its former level, the 100-ft. contour-line with all its irregularities would be represented by the beach mark made by the sea when 100 ft. higher. If instead of receding the sea rose continuously at the rate of 100 ft. per day, a series of levels 100 ft. above one another would be marked daily upon the land until at last the highest mountain peaks appeared as islands less than 100 ft. ”

“ Thus, it appears that the level of the sea is sinking in the more northern latitudes, and rising in the Mediterranean, and it is evident that this cannot happen by the motion of the sea itself. The parts of the ocean all communicating with one another, cannot rise in one place and fall in another; but, in order to maintain a level surface, must rise equally or fall equally over the whole of its extent. ”

“ The cause of this change of level in the water-surface is the attraction exerted by the sun and moon upon the ocean. The earth is, so to speak, clothed with a flexible garment of water, and this garment is pulled out of shape by the attractive force of our luminaries; very roughly speaking, we may say that the ocean-surface is distorted into a shape called an ellipsoid, and that there are therefore two elevations of water which march across the sea-covered regions of the earth as it revolves on its axis. ”

“ 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. ”

“ 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. ”

“ A low-tide elevation is a naturally formed area of land which is surrounded by and above water at low tide but submerged at high tide. Where a low-tide elevation is situated wholly or partly at a distance not exceeding the breadth of the territorial sea from the mainland or an island, the low-water line on that elevation may be used as the baseline for measuring the breadth of the territorial sea. ”

“ The height to which the drift-timber is thrown up on the shores at the western entrance of the Dolphin and Union Straits is, I think, an indication of an occasional great rise in the sea, which, as the tides are in comparison so insignificant, I can ascribe only to the north-west winds driving the waters of an open sea towards the funnel-shaped entrance of the straits. ”

“ To make it quite clear that the surface of the Red Sea is not a sea level, but is an inclined plane, suppose the channel of the Red Sea to have a perfectly smooth and level floor, with no water in it, and a wave ten feet high to enter the Straits of Babelmandeb, and to flow up the channel, like the present surface current, at the rate of twenty miles a day for fifty days, losing daily, by evaporation, half an inch ”

“ Now, although these shore belts and shallow seas are broadly connected with the great ocean body, and are usually regarded as a part of it, they are singularly separated from it, or rather they are singularly separated by it, so far as the life dependent on shallow-water conditions is concerned. To this life, the deep sea is a barrier not quite as effective as the land, but still a barrier. The key to this important fact may be found in a consideration of the vertical distribution of life. ”

“ It is, therefore, evident that about this parallel of latitude there is a belt or circle round the earth, where the mean temperature of the sea obtains throughout its entire depth, forming a boundary, or kind of neutral ground, between the two great thermic basins of the ocean. To the north of this circle the sea has become warmer than its mean temperature, by reason of the sun's heat, which it has absorbed, elevating its temperature at various depths in different latitudes. So that the line of mean temperature of 39°.5, in latitude 45° S., has descended to the depth of 600 fathoms ”

“ When, through adjustment of the relations between continental masses and masses beneath the oceans, the internal stresses of the globe have been balanced, the average elevation of lands above sea level is a maximum. ”

“ Heat and cold, rain and sunshine, the winds and the streams, all assisted by the forces of gravitation, are unceasingly wasting away the high places on the land, and as perpetually filling up the low.
But in contemplating the leveling agencies that are at work upon the solid portions of the crust of our planet which are at the bottom of the sea, one is led, at first thought, almost to the conclusion that these leveling agents are powerless there. ”

“ All recent observations, he said, have shown that the vast expanse of water which has its centre in the southern hemisphere is the one great ocean of the world, and the Atlantic with the Arctic Sea, and the North Pacific, are merely its northward-extending gulfs: any physical phenomena affecting obviously one portion of its area must be regarded as one-of an interdependent system of phenomena affecting the ocean as a whole. ”

“ 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 ”

“ The unit used for elevations is also stated on the face of the chart, as also the plane to which elevations are referred. On the United States charts this is generally mean high water and on British charts the high water of ordinary spring tides. Rocks and islets usually have figures shown beside them, either in brackets or underscored, which indicate the height above high water. Rocks which are bare at low water sometimes have a note "dries" or "bares" so many feet, indicating their height above low tide, although they are covered at high tide. ”

“ Then my eyes focused on a huge world map displayed on the table, and I put my finger on the very spot where our just-determined longitude and latitude intersected.
Like the continents, the sea has its rivers. These are exclusive currents that can be identified by their temperature and color, the most remarkable being the one called the Gulf Stream. Science has defined the global paths of five chief currents: one in the north Atlantic, a second in the south Atlantic, a third in the north Pacific, a fourth in the south Pacific, and a fifth in the southern Indian Ocean. ”

“ The height of the tide varies in the different regions of the globe, according to local circumstances. The eastern coast of Asia and the western coast of Europe are exposed to extremely high tides; while in the South Sea Islands, where they are very regular, they scarcely reach the height of twenty inches. ”

“ Seas, often traversed by currents of warmer or colder water, (oceanic rivers) , have an effect very different from that of continental masses, whether unbroken or articulated, or of islands, which latter may be regarded as shallows in the aerial ocean, and which, notwithstanding their small dimensions, exert, often to a great distance, a notable influence on the climate of the sea. In continental masses we must distinguish between sandy deserts devoid of vegetation, savannahs or grassy plains, and forest-covered districts. ”

“ You must understand, then, that all tide takes its rise in the free and open ocean under the action of the moon. No ordinary-sized sea like the North Sea, or even the Mediterranean, is big enough for more than a just appreciable tide to be generated in it. The Pacific, the Atlantic, and the Southern Oceans are the great tidal reservoirs, and in them the tides of the earth are generated as low flat humps of gigantic area, though only a few feet high, oscillating up and down in the period of approximately twelve hours. ”

“ So Chancellor Kent says: 'The 'high seas' mean the waters of the ocean without the boundary of any county, and they are within the exclusive jurisdiction of the admiralty, up to high-water mark when the tide is full. The 'open ocean, which washes the seacoast,' is used in contradistinction to arms of the sea inclosed within the fauces terrae, or narrow headlands and promontories ”

“ In the western half, reaching up from the equator to the Gulf Stream, both of the North Atlantic and North Pacific, the water is warmer, parallel for parallel, than it is in the eastern half. On the west side, where the water is warm, the flow is to the north; on the east side, where the temperature is lower, the flow is to the south—making good the remark (§ 80) that, when the waters of the sea meet in currents, the tendency of the warm is to seek cooler latitudes ”

“ Nevertheless we are by no means entitled to infer, that the denuding force of the great ocean is a geological cause of small efficacy, or inferior to that of rivers. Its chief influence is exerted at moderate depths below the surface, on all those areas which are slowly rising, or are attempting, as it were, to rise above the sea. From data hitherto obtained respecting subterranean movements, we can scarcely speculate on an average rate of upheaval of more than two or three feet in a century. ”

“ First, the height of the waves appears far less impressive, when seen from on board an Atlantic steamer, than from any ordinary vessel; for the deck in the case of these steamers is so high, that the spectator, as it were, looks down upon them. Any one who has ever ascended a moun [Pg 132] tain knows very well what the effect is upon the apparent height of all smaller hills, when they are seen from an elevation that is far higher than they. In fact, a country that is really quite hilly is made to appear almost level, by being surveyed from any one summit that rises above the other elevations. ”

“ When we travel out upon the ocean, and get beyond the influence of the land upon the winds, we find ourselves in a field particularly favorable for studying the general laws of atmospherical circulation. Here, beyond the reach of the great equatorial and polar currents of the sea, there are no unduly heated surfaces, no mountain ranges, or other obstructions to the circulation of the atmosphere — nothing to disturb it in its natural courses. The sea, therefore, is the field for observing the operations of the general laws which govern the movements of the great aerial ocean. ”