HYDROGRAPHY. The sea, including the Pacific, Atlantic, Indian, Arctic, and Antarctic oceans, with many great gulfs and bays, covers 72 per cent. of the earth's surface. Of these the Pacific is far the largest, comprising much more than half the water surface of the globe. The average depth of the sea is about two and a half miles, or 13,200 feet. The greatest depth yet measured in the Atlantic, at a point north of the West Indies, is 1660 fathoms. This depth is con siderably exceeded in the Pacific, where, east of the Kermadec Islands, a sounding of 5147 fathoms, or 30,882 feet, has been7 obtained. See OCEAN; ATLANTIC OCEAN; etc.
The water of the ocean is strongly saline, being supplied constantly by streams whose waters contain saline material in solution in greater or less amount. Even if the amount be trifling, since there is no outlet save evapora tion, its degree of salinity is merely a question of time. The salinity of certain land-locked seas, situated in hot regions, such as the Red Sea and the Mediterranean, is greater than that of the open ocean, owing to excessive evaporation from their surfaces.
The temperature of the surface water ranges from the freezing-point in Arctic regions to 90° in landlocked seas, in the tropics, such as the Red and Caribbean seas. The annual range of temperature at the surface is small, except in localities where currents change their positions with the seasons. At moderate depths there is no change of temperature throughout the year, and at great depths the temperature in all parts of the sea is very nearly the same, being but little above the freezing-point.
The surface waters of the sea are disturbed by waves and tides, and moved about by currents and drifts. Waves are set in motion by the wind, but have little movement of translation, consisting mainly of vertical oscillations. They are rarely one-fourth of a mile in length from crest to crest, and fifty feet in height. The tides (q.v.) are oscillations in the sea surface, occur ring twice a day, one of them following the pas sage of the moon across the meridian, the other nearly twelve hours later. They are slight in the open sea, being not more that two or three feet, but upon the seacoast they are commonly much higher, and at the heads of funnel-shaped bays are, in many cases, very high. The tides are due to the difference in the force of attrac tion, mainly of the moon, upon the surface of the earth and its centre, owing to their difference in distance from it. Drifts are surface water transported by the wind. The movement is com monly very slow, and changes in direction with the wind. When driven by constant winds, such drifts do, in some cases, develop into currents. (See OCEAN CURRENTS.) The great ocean cur rents, such as the equatorial currents, the Gulf Stream (q.v.), and the Japan current, thus origi nate. The constant trade winds, blowing from the northeast and the southeast diagonally to ward the equator, induce great drifts in these directions. These, meeting near the equator, flow westward across the oceans. See HYnnoa BAPIEY.