Europe, Asia, and America, which remarkably conform to the parallel of 72°, and having an area roughly estimated at 4,000,000 square miles. It penetrates northern Europe by the White Sea and Sea of Kara, and northern Asia by the Gulf of Obi and a few small inlets, and northward from these shores seems interrupted by comparatively few islands. The northern shores of America, however, present so many islands and ice-locked inlets, that it has, up to the present moment, been impossible to determine whether land or water continues northward and surrounds the pole. The Antarctic Ocean, on the other hand, is open on all sides to the Pacific, Atlantic, and Indian Oceans, which thus insensibly merge themselves into the great Southern Ocean. So far as navigators have ventured to approach the southern pole, various islands and shores have been observed which would favour the idea of a circumpolar continent; but whether land, sea, or an ice-bound archipelago occupies the immediate region of the pole, is likely long to remain an undetermined problem. Altogether, the Antarctic is a cold, boisterous, and unapproachable its ice extending 10° nearer the equator than that of the Arctic, and offering few of those inducements that have stimulated repeated research in the Northern Ocean.

ocean

109. Such are the areas occupied by the waters of the oceanareas and subdivisions which are not only necessary to intelligible description, but which are marked in reality by different conditions and characteristics in nature. Position on the globe decides their temperature; area and configuration determine their tides and currents; and the sum of these physical conditions regulates the nature and distribution of their plants and animals. Any change, therefore, either in position, area, or configuration, would be attended by a corresponding change of conditions, and any such alteration would affect all the consequences, physical and vital, that depend on external conditions. As they exist, the North and South Atlantic, situated under different latitudes, enjoy different temperatures; while the north, by virtue of its greater irregularity of form, and numerous ramifica. tions into the land, exhibits a much more varied display of vegetable and animal life. The same also holds good of the North and South Pacific, with these important modifications, that the North Pacific, compared with the North Atlantic, is almost excluded from Arctic influences, while the numerous islands of the South Pacific occasion conditions, physical and vital, peculiarly its own. The Indian Ocean, surrounded on three sides by land, and situated, for the most part, in the torrid zone, presents peculiarities

unknown in other subdivisions; while the Arctic and Antarctic, receiving the minimum of solar heat, are ice-locked for the greater part of the year, and have little in common with the other sections of the ocean. As already stated, it is on these primary relations of position and configuration that the different temperatures, tides and currents of the various oceanic subdivisions depend; and it is entirely owing to these conditions that the specific life of one sea or ocean differs from the life of all other seas and oceans. And yet it must be borne in mind that, though diversely situated and characterised, there is still the most intimate connection and interchange between their waters-the colder ever flowing towards the warmer, and the warmer towards the colder, so that in this respect they constitute in reality one great and indivisible "world of waters."

Composition, Density, Pressure, Depth, &c.

110. This great ocean, in all its areas and ramifications, is characterised by a greater or less degree of saltness-this saltness arising from the presence of certain substances held in chemical solution in its waters. These substances are chloride of sodium (common salt), and sulphates of magnesia and lime, together with minor and varying proportions of salts of potash and ammonia, iodides and bromides of sodium, carbonate of lime, silica, &c.— amounting in all from 3 to 4 grains to the hundred of water. According to M. Regnault, the following is the mean of several analyses of sea-water :

The preceding ingredients may vary in different seas, and according to the locality whence, and the season when, the water is taken, but only to the extent of a fractional percentage-the incessant circulation and intermingling of the ocean's waters by waves, tides, and currents, producing a uniformity, or all but uniformity, in its saline composition. It has been found, however, that the waters of the Southern Ocean are slightly salter

than those of the Northern; that the greatest saltness takes place along the parallels of 22° north and 17° south, or in the courses of the trade-winds, which absorb and carry off an excess of evaporation towards the equator, where it descends in freshening rain-falls; and that from these limits of maximum saltness there is a slight progressive diminution towards either pole.

[In the southern hemisphere, says Captain Maury, there is more sea and less land than in the northern. But the hydrometer indicates that the water in the seas of the former is salter and heavier than the water of seas cis-equatorial; and man's reasoning faculties suggest, in explanation of this, that this difference of saltness, or specific gravity, is owing to the excess of evaporation in the southern half, excess of precipitation in the northern half, of our planet. "When water passes, at 212° Fahr., into steam, it absorbs 1000° of heat, which becomes insensible to the thermometer, or latent; and, conversely, when steam is condensed into water, it gives out 1000° of latent heat, which thus becomes free, and affects both the thermometer and the senses. Hence steam of 212° Fahr. will, in condensing, heat five and a half times its own weight of water from the freezing to the boiling point."-M'Culloch. Now there is in the southern a very much larger water-surface exposed to the sun than there is in the northern hemisphere, and this excess of heat is employed in lifting up vapour (and rendering salter) that broad surface, in transporting it across the torrid zone, and conveying it to extra-tropical northern latitudes, where the vapour is condensed to replenish our fountains, and where this southern heat is set free to mitigate the severity of northern climates.]

111. Though communicating freely (by currents and countercurrents) with the ocean, the majority of inland seas are less salt than the ocean, in consequence of the influx of rivers into their circumscribed areas; but some, like the Red Sea, receiving no rivers, and subjected to active evaporation, have their saltness slightly in excess. As a general rule, inland seas receiving numerous rivers, and from their situation subjected to little evaporation, will be fresher than the ocean (e.g., the Baltic); while others also receiving rivers, but subjected to a more active evaporation (as the Mediterranean), will have their saltness somewhat in excess. Though the saltness of the sea be pretty uniform at great depths, still at the surface, owing to the admixture of rain, river, and iceberg water, it is not quite so salt; and this freshness will increase, of course, according to proximity to the mouths of the entering rivers. It has also been ascertained that water from the surface contains less air than that from depths, and the difference may equal one-hundredth of the volume of water.

112. Another noticeable property of salt water is, that it is less sensitive, if we may so speak, to cold than fresh water-the latter freezing, as is well known, at 32°, while sea-water is not converted into ice till the thermometer sinks to 28° Fahr. It is likewise less vaporisable than fresh water-that is, a given extent of salt-water surface

gives off less vapour during the same time, and under the same conditions, than an equal extent of fresh-water surface. Such composition and properties are no doubt all-essential in the economy of nature. Shell-fish, crustacea, coral-zoophytes, and other creatures, derive the calcareous matter of their structures from the salts of the ocean; fishes breathe the aërated waters of the sheltered and undisturbed depths; and both plants and animals obtain conditions of existence which absolutely pure water would fail to supply. By its lower freezing-point a larger amount of surface is ever kept open and accessible; and by its slower evaporation a less amount of moisture is borne from its greater expanse to the comparatively smaller surface of the land. As sea-water freezes (the ice being fresh) the surface portions become salter and salter, and then, through their greater specific gravity, sink downward, while the lighter and warmer portions arise to supply their place—a circulation which at once limits the surface-cold and maintains the equilibrium of density.

113. It will be seen that no notice has been taken in the preceding paragraphs of ingredients-sand, mud, and organic debris -that may be mechanically suspended in the waters of the ocean. These are purely local and accidental, depending on river-floods, tidal-currents, waves, storms, and other commotions. When the commotion subsides, the waters regain their transparency; and altogether, unless along wasting shores, in tidal estuaries and river embouchures, there is really very little matter mechanically suspended in the waters of the ocean. On the other hand, the ingredients held in chemical solution are all but constant and universal. The water that evaporates from the ocean is all but absolutely pure; it falls on the land in mist and rain and snow; percolates the soil and rocks; and returns again to the ocean, carrying with it the saline substances it has dissolved from the rocky strata. The ocean is thus the great equalised repository of all that is borne from the continents; and there they would accumulate, were it not for the beautiful counterpoise that is ever kept up by the requirements of plants and animals, as well as by the intervention of new chemical arrangements among its multifarious sediments. So far as Geology can determine by a study of the marine life (shell-fish, corals, foraminifera, &c.) of former ages, the composition of the ocean seems to have been much the same as it is now; and thus, in all our reasonings, we may regard its saline contents as having long arrived at a state of equilibrium and fixity. Even if there were a slight excess at any one period, that excess would be merely temporary, as those incessant mutations of sea and land, involving the formation of new limestones, magnesian

limestones, rock-salts, and the like, are ever taking up the surplus, and restoring the equilibrium.

114. The mean specific gravity of sea-water, as compared with absolutely pure water at 62° Fahr., is found to be 1.0275—an amount that corresponds to a percentage of 3.505 of saline ingredients. The salter, therefore, that water is, the greater its gravity; and hence the fresh water of rivers, of melting icebergs, &c., will float for many miles on the surface of the sea before the two fluids be thoroughly diffused and commingled. It is owing to this that potable water has been skimmed from the surface several miles from the mouths of large and rapid rivers; and it is also for this reason of unequal densities that currents are established in different parts of the ocean-the heavier ever seeking to establish its equilibrium. As already mentioned, water acquires its minimum volume, or greatest density, at a temperature of 40°, and becomes lighter and lighter either as it rises above or falls below this temperature. Owing to this property a perpetual interchange or circulation is kept up among the waters of the ocean, horizontally from colder to warmer, and from warmer to colder regions; and vertically from surface to depths, and from depths to surface.

115. Again, water being slightly compressible, it follows that at great depths in the ocean the water will be denser than at the surface, and consequently what takes place near the shore will be impossible at extreme depths. According to experiment, water at the depth of 1000 feet is compressed th of its bulk; and at this ratio the pressure at the depth of one mile would be equivalent to 160 atmospheres, or 2320 lb. on the square inch; while at the depth of 4000 fathoms, or about 4 miles, it would amount to 750 atmospheres! It is owing to this enormous pressure that corked bottles sunk to great depths have their corks always forced in; and that pieces of oak-wood carried down to similar depths have their fibres and pores so compressed as to be afterwards incapable of floating on the surface. At vast depths, therefore, it is generally supposed that vegetable and animal life, as known to us, could not possibly exist; and though some recent soundings in the north seas at the depth of 1260 fathoms would seem to oppose this opinion, yet the paucity and uncertainty of these trials leave the question still in doubt; and we may, in the mean time, adhere to the general belief that the extreme depressions of the ocean, like the extreme elevations of the land, are barren and lifeless solitudes.

116. Touching the depth of the ocean, it has been already observed, that as the dry land rises variously and irregularly above the level of the ocean, so the bottom of the ocean sinks variously