These masses of ice render navigation very dangerous; and the ice fields especially have caused the loss of many whaling-vessels. These extensive masses are frequently put into a rotatory movement by a cause which has never been discovered. When thus whirled about, their outer edges acquire a velocity of several miles per hour. A. field thus in motion,' coming in contact with another at rest, or with one that has a contrary direction of movement, produces a dreadful shock. The strongest ship is a mere atom between two such masses of matter in motion, and many vessels have thus been destroyed. The ice-fields are particularly dangerous in foggy weather, as their motions cannot then be distinctly observed. Icebergs are much less dangerous, partly on account of the small space which they occupy when compared with ice-fields, and partly because they are easily distinguished at a distance in the night by their natural brightness, and in foggy weather by a peculiar blackness of the atmosphere. As, however, they occur far from land and often in unexpected situations, sailors when crossing the Atlantic between and N. lat., or even farther to the south, must always be on the watch for them in the night time. Occasionally the whale-fishers derive some advantage from them. As they sink deep into the sea, they are very little affected by the wind, 'and they furnish secure mooring to a ship in strong adverse winds, or when it is required for other purposes. But mooring to lofty icebergs is attended with considerable danger. Being sometimes very nicely balanced, they are apt to lose their equilibrium ; and vessels have often been staved and sometimes wrecked by the fall of their icy mooring, while boats have been overwhelmed even at a considerable distance by the swell occasioned. by such a catastrophe. Water is sometimes procured by whaling-vessels from the deep pools of water that are formed in the summer season on the depressions in icebergs, or from the streams which run down their sides.
• On approaching a field or any compact aggregation of ice, the ice blink is seen whenever the horizon is tolerably free from clouds, and 'sometimes even under a thick sky. It consists of a stratum of lucid whiteness, which appears over the ice in that part of the atmosphere which joins the horizon. A clear sky presents a beautiful and perfect map of the ice, 20 or 30 miles beyond the limit of direct vision, but less distant in proportion as the atmosphere is more dense and obscure. Each kind of ice has a different blink. Field-ice has the most lucid blink, accompanied with a tinge of yellow ; that of packed ice is more purely white; and ice newly formed upon the sea has a grayish hue.
According to the experiments of Scoresby, the specific gravity of the ice, when compared with that of sea-water occurring in the Green land Sea, at the temperature of was ascertained to be from to That part of the ice, therefore, which is above the surface appears to be, to that below the surface, in the proportion of I to between 8 and 9. For every solid foot of ice which is seen in a masa floating in the sea, there be 8 or 9 feet below. Hence it some times happens that large icebergs, when they are carried into shallow water, take ground, and remain stationary for one or two years, until so much of their volume has been wasted by the action of the sun and of the atmosphere, that they begin to float again.
it excited some surprise when it was discovered that the ice floating about in the sea consisted of fresh water. It is true that it generally contains a very small portion of salt, but it is probable that this small portion of salt is derived from the salt water contained in the pores of the ice. lf, says Scoresby, in confirmation this opinion, the newest and moat porous ice be removed into the air, allowed to drain for some time in a temperature of 32' and upwards, and then be washed in fresh water, it will be found to be nearly quite free from salt, and the water produced from it may be drunk. According to the Russian explorer, Baron Wrangell, whenever the surface of the ice on the north coast of Siberia is clear of snow,. the salt niay be found deposited in crystals; and in the neighbourhood of the payola", or interior open seas of the Arctic regions already mentioned, the layer of salt is often of considerable thickness. All this is in conformity with the fact, first definitely ascertained by Dr. Faraday, that in the process of freezing the foreign bodies contained in water are separated from it ; agreeably to a principle which appears to be manifested in the crystallisation of most if not all fluids, of the separation of heteroge neous matter. Ou account of the salt contained in it, sea-water does not, like pure water, freeze at the temperature of but in the Greenland Sea, where its specific gravity is P0263, it begins only to freeze at 28r.
There is not any portion of the surface of the sea which is not subjected to some kind of motion, and this circumstance must tend greatly to preserve its purity. The water in some parts of the sea is always propelled in the same direction by the current& [ATLANTIC OCEAN; PACIFIC OCEAN, in 0E00. Div.] Nearly the whole sea is four times in the day subject to a change in its level by the movements of the tides. The motion produced by the winds, and known by the name of waves, is much less regular. Each wave presents a gently ascending surface to the windward, and a perpendicular descent lee ward. The elevation of the waves varies according to the strength of the wind. A rather heavy gale raises them from six to eight feet above the common sea-level ; but in very strong gales they attain an elevation of thirty feet. This motion of the surface of the sea is not perceptible to a great depth. In the strongest gales it is supposed not
to extend beyond 72 feet below the surface,'.and at a depth of 90 feet the sea is perfectly stilL The form and even the size of the waves vary according to the depth and the extent of the sea. In shallow water, where the lower part of the waves approaches the bottom, and meets with resistance, the waves are abrupt and irregular, and this is also the case in confined seas ; whilst on the open ocean they are wide and long, and rise and fall with great regularity. When the waves run to a low shore, the slope of the ground breaks their force, and they terminate in a tranquil manner; but when they are impelled against an elevated rocky coast, being repelled by the rock, they produce what is called a surf. This violent rising of the sea on a rocky coast some times attains an elevation of 100 feet above the sea-leveL The surf is always dangerous to pass, except in boats of a peculiar construction. The waves do not subside simultaneously with the wind. The momen tum of the water preserves the sea in its agitated state for many hours. The air being little agitated, or not at all, is unable to depress the undulations of the sea, and therefore the waves during a calm after a gale rise higher, and their most elevated part forms a more acute angle than during the gale. Such a state of the sea is called a hollow sea. [Wave.] The term Ocean (a Greek word, Oceanus, nolavos) is applied in a general and somewhat indefinite manner to distinguish the greater areas of sea. The word first occurs in Homer, who uses it to designate the river or stream which, according to his ideas, surrounded the sur face of the earth like a circle. The Greek geographers, however, knew that the ocean was a wide expanse of water, which surrounded the land, and the term ocean was used by them in this sense. They supposed that it penetrated deep into the mass of the continent by four great bays or seas : these were, on the south the Arabian Sea and the Persian Gulf ; on the west the Mediterranean ; and ou the north an imaginary strait which connected the Northern Ocean with the Caspian Sea. (Strata), p. 121 ; Pomp. Mela, i. 1.) [GEocBRrne.) The actual surface of the globe may be reckoned at about 197,000,000 of square British statute miles ; of which 145,000,000 are covered by the waters of the ocean—now using this term in its widest and most emphatic sense, as meaning tho entire collection of seas— while the area of tho land is about 52,000,000 of miles; the proportion of land to sea being thus as about 1 to 3, the land occupying one fourth, and the sea three-fourths of the entire surface of our planet. The latter, however, is so unequally distributed that there is three times more land in the northern than in the southern hemisphere. The torrid zone is principally occupied by sea, and only one-twenty seventh part of the Laud on one side of the earth has land opposite to it on the other. Sir John Herschel has recorded, that "one result of maritime discovery on the great scale, is, so to speak, massire enough to call for mention as an astronomical feature" of our planet. " When the continents and seas are laid down on a globe (and since the dis covery of Australia, and the recent addition to our Antarctic know ledge of Victoria Land by Sir J. C. Ross, wo are sure that no very extensive tracts of land remain unknown) we find that it is possible so to divide the globe into two hemispheres, that one shall contain nearly all Me land ; the other being almost entirely sea. It is a fact, not a little interesting to Englishmen, and, combined with our insular station in that great highway of nations, the Atlantic, not a little explanatory of our commercial eminence, that London occupies nearly the centre of the terrestrial hemisphere." He adds that "more exactly Falmouth," the most westerly port of Great Britain, occupies this centre. " The central point of the hemisphere which contains the maximum of land," he continues, "falls very nearly indeed upon this port. The land in the opposite hemisphere, with the exception of the tapering extremity of South America and the slender peninsula of Malacca, is wholly insular, and were it not for New Holland would be quite insignificant in amount." Outlines of Astronomy,' par. (284). Another illustrative view of the distribution of land and water has been given by Professor Ansted. In the space between the Equator and the Antarctic Circle (consisting of the southern half of the Torrid Zone and the whole of the South Temperate Zone) which occupies 90,000,000 of square miles, newly 77,000,000 (almost seven-eighths of this space) are water, while in the North Temperate Zone the quantity of land is nearly equal to that of water. As the great depressions on the solid surface, occupied by the principal part of the water on the globe, are connected together by comparatively narrow passages, and are therefore really united, forming one wide and continuous expanse of sea, the emphatic application of the term ocean, in the singular, is as obviously correct in modern science, as it was in its original use by the Greeks. But notwithstanding this unity, the different parts of the ocean are known by distinct names, the most important being the Pacific, the Atlantic, Indian, Arctic, and Antarctic oceans. There are also some internal seas of considerable extent, as the Mediterranean, the Baltic, and others, which are almost entirely inclosed by land, and, like the ocean, are filled with saltwater; besides the isolated Caspian and Sea of Aral, and also the great gulfsand penetrating bays of North America, together with others, better known, hut far less extensive, in Europe.