The temperature of the sea, where it is not affected by currents from a wanner or colder region, necessarily corresponds to the normal temperature of the latitude; but this is true only of the water at and near the surface, for it has been recently proved by the observations made on deep-sea temperature by Carpenter, Wvville Thomson, and others, that-the temperature rapidly diminishes with the depth, particularly in tropical and tem perate regions, till at great depths ice-cold water is everywhere found. Thus, from the extensive obse.-vations -made by H.M.S. Challenger in the North Atlantic it is shown that at the equator, where the surface temperature is about 80°, the decrease with the depth is so rapid, that at 60 fathoms from the surface the temperature is only 61°.5; at 150 fathoms it is 50°; at 700 fathoms, the temperature has fallen to 40'; at about 1600 fathoms, to 36°. Below this it diminishes at a much slower rate, till dt falla nearly to freezing at all great depths which are connected by under-currents with the Antarctic or Arctic seas. The sea-water of the upper 60 or 80 fathoms is affected by the solar heat. Immediately beneath this sun-heated upper stratum, it is remarkable that all the -water in the North Atlantic, as far as lat. 40°, is warmer than that at the same depth under the equator. The mean temperature of the upper 1500 fathoms 'in the North Atlantic is 4'.5 warmer than the same upper stratum at the equator. As regards the temperature of the water at the bottom, at all stations between Bermuda and the equator on the c. side of the Atlantic, the temperature is remarkably uniform at 35°.2; in the bay of Biscay, to n.e. of this line, it is warmer; s.w. of the same line, 1° colder; •vbereas, further s. at the equator, on the western side of the Atlantic, it is 32°.4, or 2',8 colder. This last fact is of very great importance, since, from the circumstance that at the equator the bottom temperature is 32°.4, and that at all stations to n. of it the bottoni temperature is warmer, it follows that the cold watef at' the bottom of the Atlantic as far n. as the Azores and bay of Biscay, equally with that at the equator, is derived from an Antarctic, and not from an Arctic source. This cold Antarctic current entering the North Atlantic is found between 1700 fathoms and the bottom, a total thickness of 700 fathoms. Ice-cold water has also been found at the bottom in the Arabian sea. In land-locked seas, such as the Mediterranean, whose deep water is not in communication with that of the Atlantic, owing to the shallowness of the sea at the straits of Gibraltar. the bottom temperature does not fall so low as that of the ocean. Thus the tempera ture of the Mediterranean at 1508 fathoms is 55°, whereas at this depth in the ocean it is so low as 36°. See lsornEttatat LINF.s. The highest surface-temperature does not correspond with the equator. but owing to the disturbing influence of currents in the fol lowing reolons; Between Sumatra and the Zanzibar coast; e. of the'Philippine islands, to long. 170° e. ; e. of Cuba and Florida; and n.e. of cape St. Roque.
Color and Phobphoreseenee of the color of the ocean, when free from admixture of foreign substances, as animalcules, vegetable organisms, excessive rain, or the tinted waters of swollen rivers, is a pure deep blue, which becomes less marked where the water is of less depth. A " different" color Of sea-water is due to the presence of some foreign substance; e.g., the red, brown, and white patches of the Pacific and Indian oceans to the presence of swarms of animalcules, and the colors of the Bed and Yellow seas to matters of vegetable origin. The Rhone, at its emergence from the lake of Geneva, and the lake itself, exhibit an intensity of blue far surpassing that of any sea. The phosphorescence of the sea is due to the presence of myriads of invertebrate, espe , cially rhizopoda, tunieata, etc. See Lumuszo,:rry or ORGANIC BODIES.
Depth of the Sea.—Till very recently, it might be said that, with the exception of the more frequented strips along the coast, and such other portions as afforded anchor age ground, our knowledge of the depth of the ocean amounted to nothing. It is true that deep-sea soundings had been frequently made, but from the necessary defective ness of the ordinary lead " and inattention to the effect of under-currents in destroy ing the perpendicularity of the line, little dependence could be placed on the results obtained. It is chiefly in the Atlantic that the new and trustworthymethod of sounding
(q.v.) has been _practiced, and the contours of its bottom may row be considered as fairly ascertained. The greatest depth measured by the Challenger (in n. lat. 19° 41', w. long. 65° 7', was 3,875 fathoms, or 23,250 ft. (about 4 4 miles). Over a great extent of the area the depth ranges between 2,000 and 3.000 fathoms... Along the middle runs an irregular ridge, on.which the depth is less than 2,000 fathoms, and n. of 50° a plateau of similar depth extends, with little interruption, from Ireland to Newfoundland, on which the telegraph cables are laid. Nowhere round the British isles is the sea above 400 ft deep: In the. Pacific, several tracks of soundings were obtained during the Challenger expedi tion (1873-75)., From the numerous islands which stud this ocean, one might be led to assume its comparative shallowness; but this is far from being the case, for the islands rise abruptly from the bottom. and very deep soundings have been obtained near their shOres. Over a gnat part of the area, the depth is over 2,000 fathoms; in the deeper parts. it ranges from 3,000 to over 4,000 fathoms. The deepest sounding got was 4,475 fathoms (26,850 ft. or above 5 in.), in U. lat. 11° 24'. e. long. 143° 16', near the Ladrone islands. From the remarkable gentleness of slope of the bedof the Arctic ocean to the n. of Siberia, the line giving only 14 to 15 fathoms at 150 in. from the shore, and from its con figuration on the n. of America, it Is generally concluded to be by far the shallowest of the oceans. Of the depth of the Antarctic ocean, little is known, but it is supposed to be deeper than its antipodal kinsman. Near the Antarctic circle. s.e. from Kerguelen, the Challenger took a few soundings varying from 1300 to 1975 fathoms. From all that hats hitherto been observed, it would seem as if the land-surface under water were the cotin terpart as regards eminences and hollows, chasms, valleys, plateaus, etc of the land surface above.
Motion of the Sea.—The sea is in a state of perpetual restlessness,its motion being either a vertical oscillation, or an actual transference of its waters front one place to another. The first motion, which constitutes waves, is due either to the attraction of the sun and moon on such a mobile body as the sea (see TtnEs), or to the impulsive action of the winds which blow over its surface (see WAvEs); the second arises from the sun, which, directly through its heat. and indirectly by scorching dry winds, produces evaporation to a great extent, of the parts most exposed to its influence, and by its similar action on the atmosphere (see TRADE-WIND), causes a transference of this vapor to remote latitudes, where it descends as rain, and, destroying the equilibrium of the sea, gives rise to cur rents. The nature of these currents is described under GULF STREAM, and the chief currents of each ocean are found under its own bead. This constant motion of the sea is of great service in tending to equalize the temperature of different parts of the globe; it also produces remarkable changes in the form of coasts, eating into rocks, converting low-lying lands into shoals and sand-banks, or carrying away the earthy materials. and them in some distant region. The erosive action of the sea is generally almost imperceptible during several years, but in course of two or three cen turies the magnitude of the changes effected by it is almost incredible.
On the economic value of the sea as a purifier, and as a commercial highway, it is to dilate. For some of the peculiar phenomena of the sea, see ICEBERGS, AURORA BOREALIS, WHIRLPOOLS, the five great oceans (q.v.), CORAL, etc.
The term sea is also applied in a more limited though, indefinite sense to an offshoot of one of the oceans, as to the Black, Baltic, Okhotsk seas, to any portion of an ocean which from its position or configuration is considered deserving of a special name, and to the two great inland salt lakes of central Asia, the Caspian and Aral seas.