Sea

Sea, the general name for the hydrosphere or water-shell resting in the hollows of the globe and covering about 72 per cent. of its surface, or about 58 per cent. of the northern, and about 83 per cent of the southern hemisphere. In the hemisphere of which New Zealand is the centre two-thirds of the entire ocean-surface is situated, only 8 per cent. being land. Most of the hydrosphere is a connected whole, the Caspian being the only considerable isolated area of sea or truly inland sea, though there are many neatly land-locked or mediterranean seas. It is usual to divide the hydrosphere into four oceans, the Atlantic, Pacific, Indian, and Sonthern, the Arctic being considered as part of the first-named. The Atlantic Ocean thus considered has an area of 33,000,000 square miles, and receives the rivers of half the land area of the globe. It has as more or less enclosed portions the Arctic, Kara, White, Norwegian, North, Baltic, Black, AEgean, Adriatic, Mediterranean, and Caribbean Seas, the Gulf of Mexico, and Hudson's Bay. The Pacific Ocean, the largest in area, covers 55,000,000 square miles, an area equal to the entire land-surface of the globe, and has as enclosed portions -the Behring, Okhotsk, Japan, Yellow, China, Celebes, and Arafura Seas, and the Gulf of California. The Indian Ocean covers 17,000,000 square miles, and has the Red and Bengal Seas and the Persian Gulf as partially enclosed areas. The Southern Ocean, extending from 40° south to the ice of the Antarctic land, covers about 30,000,000 square miles.

Sea-water contains on an average 3.5 per cent. by weight of saline matter, and is, therefore, about 26 per cent. more dense than pure water. The saline matter consists of over 77 per cent. of common salt (sodium-chloride), nearly 11 per cent. of magnesium-chloride, an equal percentage of sulphates of magnesium, ca1cium, and potassium, and very minute traces of carbonates of calcium and magnesinm and of silica. These salts give to seawater a bitter as well as a salt taste. Whilst the Baltic is exceptionally fresh, the Mediterranean and Red Seas are the regions of saltest water. It has been calculated that the salts in the ocean would cover its surface 170 feet deep. Sea-water is aerated by the action of waves at the surface, and the dissolved gases being circulated by convection-currents, respiration is rendered possible for marine organisms at all depths. Sea water freezes at 28° F., most of the salts separating out in the, process, thus yielding nearly fresh ice with more saline water below. The Arctic Sea is mostly frozen over every winter, the floe-ice being from 2 to 10 feet thick.; but Sir George Nares, finding floes over 150 feet thick, estimated that they might be five hundred years old, and named that part of the Arctic the Palaeocrystic Sea. In the tropical zone the surface-water has an annual temperature exceeding 80° F., but in the Red Sea 90° and 100° have been recorded. At 300 to 400 fathoms below the surface a temperature of 40° is common in all latitudes, whilst at greater depths it is only in polar regions that the temperatnre falls below 30°.

Dr. John Murray considers half the globe to be covered with water over 10,000 feet deep. This he terms the abysmal area. About 22 per cent. of the surface, covered by more shajlow water, he terms the transitional area, the remainder being the permanently continental, or land, area. The average depth of the sea is 2,100 fathoms (12,600 feet). the deepest abyss, that known from the United States exploring vessel as the Tuscarora Deep, between 20° and 50° N, lat. in the Pacific, almost reaching 4,700 fathoms. The movements of the sea include tides (q.v.), waves (q.v.) due to wind, currents (q.v.) of surface-water, due mainly to the constant winds, and circulation by convection-currents, produced by concentration by heat in the tropics, and by freezing in polar seas, by dilution with fresh-water, and possibly by olher causes affecting temperature. The sea equalises temperatures, keeps up breezes and monsoons, supplies the atmosphere with its water-vapour, is constantly tending by its waves and shingle to wear down its coasts to a plane. and is the receptacle, not only for terrigene deposits formed from the wear and tear of the land, but also for pelagic deposits, or oozes, formed in deep water, far from land, by the slow accumulation of minnte organisms, decomposed pnmice, and meteoric dust. The terrigene deposits, besides gravels, and sands, consist of muds, including coral mud, volcanic mud, and widely-distributed blue mud, coloured by iron sulphide and ferrous oxide, and green mud, coloured by glauconite (q.v.). The pelagic deposits include the oozes known, from their prevailing organisms and colours, as the pteropod ooze, the Globigerina or white ooze, and the straw-coloured or radiolarian and diatomaceous oozes, of which the two former are mainly calcareous. and the two latter mainly siliceous, together with a very ubiquitous red clay, which covers half the floor of the Pacific. This consists of the residue of dissolved Globigerina ooze, of waterlogged pumice, and of meteoric and volcanic dust and contains manganic nodules, crystals of zeolites (q.v.), and numerous slowly-encrusted shark's teeth,