SEA RAINBOW . See MARINE RAINBOW. 



SEARCHLIGHT -TYPE SONAR . Echo-ranging system in 

 which the same narrow beam pattern is used for 

 transmission and reception. (6) 



SEAS . Scientific Exploration for the Atlantic 

 Shelf. 



SEA-SALT NUCLEUS . A condensation nucleus of a high- 

 ly hygroscopic nature produced by partial or com- 

 plete desiccation of particles of sea spray or of 

 sea-water droplets derived from breaking bubbles. 



That such nuclei are important In condensation 

 processes over the oceans and near coasts is fairly 

 well established; but it has not yet been demon- 

 strated that sea-salt particles are the chief 

 source of nuclei in condensation over continental 

 interiors. (12) 



SEASCO . Southeast Asia Science Cooperation Office. 



SEASHORE. (1) Law - All the ground between the or- 

 dinary high-water and low-water mark. 



(2) The SHORE of the SEA or OCEAN. (27) 



SEA SLICK . An area of sea surface, variable in 

 size and markedly different in appearance (color 

 and oiliness) usually caused by plankton blooms. 

 (15) 



SEASONAL CURRENT . See OCEAN CURRENT. 



SEA SQUIRT . See TUNICATES . 



SEA STATE . Numerical or written description of 

 ocean surface roughness. For more precise usage 

 sea state may be defined as the average height of 

 the highest one-third of the waves observed in a 

 wave train referred to a numerical code which 

 covers an increasing range of such heights as in- 

 dicated by the table below: 



Code Height (feet) 



SEATO . South East Asia Treaty Organization. 



SEA URCHINS . Echinoderms with long movable (2-15 

 cm long) spines radiating out from a round body 

 about 10 cm in diameter. 



SEA WALL . See JETTY. 



SEA WATER . The water of the seas, distinguished 

 from fresh water by its appreciable salinity. 



The distinction between the usage of salt 

 water and sea water is not very sharply drawn. 

 Commonly, sea water is used as the antithesis of 

 specific types of fresh water, as river water, 

 lake water, rain water, etc., whereas salt water 

 is merely the antithesis of fresh water in general. 

 (24) 



SEA -WATER THERMOMETER . A thermometer designed for 

 use in measuring the temperature of sea water. One 

 form of this instrument consists of a mercury-in- 

 glass thermometer protected by a perforated metal 

 case. This is used to measure the temperature of 

 a sample of sea water. Another form consists of a 

 mercury- in-glass thermometer surrounded by a metal 

 case which forms a well around the bulb of the ther- 

 mometer. When the thermometer is raised from the 

 water, a sample is retained in the well for tem- 

 perature measurement. (24) 



SECANT FUNCTION GENERATOR . A device for use with 

 a longitude-latitude Indicator to make allowance 

 for meridian convergence toward the poles. Infor- 

 mation of meridian curvature at the given latitude 

 is stored in a relay system through a conducting 

 grid on a rotating disk. As the initial position 

 changes to another latitude, the disk is rotated 

 proportionately by motor action reorienting the 

 grid conductors within the relay system. The re- 

 sulting new voltage, corresponding to the new 

 meridian curvature, is relayed to the longitude 

 indicator. Developed by Naval Air Development Cen- 

 ter for use mainly in high-speed aircraft. (30) 



SECCHI DISK . A circular plate, usually of wood, 

 having a diameter of 30 centimeters. It is covered 

 with a flat white paint. A ring attached at the 

 center of the upper surface allows a graduated line 

 to be secured. Opposite the ring on the under sur- 

 face is attached a lead weight so that the disc 

 will sink rapidly and vertically. This weight 

 should be at least 5 pounds and may be increased 

 when conditions of drift and current warrant. 

 Secchi discs used on Navy surveys are provided with 

 5- and 7 1/2-pound weights. The line attached to 

 the Secchi disc should be marked off in 1-meter 

 intervals to at least 50 meters. 



The Secchi disc is designed to measure trans- 

 parency and is dependent upon the available il- 

 lumination which varies with the time of day, cloud 

 formation, and amount of cloud cover. (35) 



SECONDARY LOBES . See LOBES. 



SEDIMENT . 1. Any material carried in suspension 

 by water, which will ultimately settle to the 

 bottom. 



2. Fine water-borne matter deposited 

 or accumulated in beds. (27) 



SEDIMENTO-EUSTATISM . See EUSTATISM. 



SEER . Submarine Explosive Echo Ranging. 



SEICHE . A stationary wave oscillation with a 

 period varying from a few minutes to an hour or 

 more, but somewhat less than the tidal periods. 

 They are usually attributed to strong winds or 

 changes in barometric pressure and are found both 

 in enclosed bodies of water and superimposed upon 

 the tide waves of the open ocean. The period of 

 a seiche in an enclosed body of water is usu ally 

 represented by the formula. Period = 2L/ •! gd 

 in which L is the length and d the average depth 

 of the body of water and g is the acceleration of 

 gravity. (14) 



SEIS . Submarine Emergency Identification Signal. 



SEISMICITY . The phenomenon of earth movements. 

 (27) 



SELEROPROTEIN. See SPONGIN. 



SELF NOISE . See OWN-SHIP'S NOISE. 



S EMID lURNAL . Having a period or cycle of approxi- 

 mately one-half of a tidal day. The predominating 

 type of tide throughout the world is semidiurnal, 

 with two high waters and two low waters each tidal 

 day. The tidal current is said to be semidiurnal 

 when there are two flood and two ebb periods each 

 day. A semidiurnal constituent has two maxima and 

 two minima each constituent day, and its symbol is 

 usually distinguished by the subscript 2. (14) 



SENSIBLE HEAT . Heat resulting in a temperature 

 change but not a change in state. (33) 



SENSITIVITY LEVEL; (RESPONSE LEVEL. SENSITIVITY) 

 (RESPONSE) . The sensitivity (or response) level of 

 a transducer, in decibels, is 20 times the logarithm 

 to the base 10 of the ratio of the amplitude sen- 



105 



