required to make an observation of any kind Is 

 known as a signal wave, or more often, simply as 

 a signal. (4) 



SIGNIFICANT WAVE . A statistical term denoting 

 waves with the average height and period of the 1/3 

 highest waves of a given wave group. The composi- 

 tion of the higher waves depends upon the extent 

 to which the lower waves are considered. Experi- 

 ence so far indicates that a careful observer who 

 attempts to establish the character of the higher 

 waves will record values which approximately fit 

 the definition. A wave of SIGNIFICANT WAVE PERIOD 

 and SIGNIFICANT WAVE HEIGHT. (11) 



SIGNIFICANT WAVE HEIGHT (CHARACTERISTIC WAVE 

 HEIGHT) . The average height of the one-third high- 

 est waves of a given wave group. Note that the 

 composition of the highest waves depends upon the 

 extent to which the lower waves are considered. In 

 wave record analysis, the average height of the 

 highest 1/3 of a selected number of waves, this 

 number being determined by dividing the time of 

 record by the significant period. (11) 



SIGNIFICANT WAVE PERIOD . An arbitrary period gen- 

 erally taken as the period of the 1/3 highest waves 

 within a given group. Note that the composition of 

 the highest waves depends upon the extent to which 

 the lower waves are considered. In wave record 

 analysis, this is determined as the average period 

 of the most frequently recurring of the larger, well- 

 defined waves In the record under study. (11) 



SIKUSSAK . Very old SEA ICE trapped in FIORDS. 

 Sikussak resembles GLACIER ICE since snowfall and 

 snow drifts contribute to its formation. (25) 



SILICIOUS . Containing more than 30 per cent of 

 silicon-based material. Diatom and radiolarian 

 oozes are siliclous. (27) 



SILL . A RIDGE or RISE separating partially closed 

 BASINS from one another or from the adjacent sea 

 floor. (26) 



SILL DEPTH . The greatest depth over a SILL. (26) 



SILT . A clastic deposit of an inorganic granular 

 material with median diameters approximately be- 

 tween 0.005 mm and 0.05 mm. It is thus between 

 sand and clay in size. The Corps of Engineers 

 defines silt as soil having a liquid limit of 27 

 or less, and a plasticity limit (based on -40 frac- 

 tion) of 6 or greater. (27) 



SIMPLE HARMONIC MOTION . A simple harmonic motion 

 is a motion such that the displacement is a sinu- 

 soidal function of time. (2) 



SIMPLE SOUND SOURCE . A simple sound source is a 

 source that radiates sound uniformly in all direc- 

 tions under free-field conditions. (2) 



SINGLE SHOT UNDERWATER CAMERA . 

 CAMERAS . 



See UNDERWATER 



SINKING CENTER . Subartlc region where strongly 

 saline surface water of tropical origin sinks 

 through the colder, but less saline underlying 

 layers . 



SINS (SHIPS INTERNAL NAVIGATION SYSTEM) . A precise 

 dead reckoning method which establishes the naviga- 

 tion coordinates through measurements made by its 

 gyroscopes and accelerometers. SINS uses two ac- 

 celerometers, one oriented north-south and the 

 other east-west, to determine ship travel over the 

 earth. The effects of gravity accelerations and 

 ships roll and pitch are eliminated by use of three 

 gyroscopes. By integration, present latitude and 

 longitude, velocity and heading can be determined. 

 (34) 



SIO. Scrlpps Institution of Oceanography. 



SIROCCO . A warm wind of the Mediterranean area, 

 either a foehn or a hot southerly wind in advance 

 of a low pressure area moving from the Sahara or 

 Arabian deserts. Called leveche in Spain. (17) 



SECFLT . Sixth Fleet (Atlantic) (USN) . 



SKIN DIVING . Diving without the use of SCUBA or 

 artificial breathing apparatus. 



SLACK WATER . The state of a tidal current when its 

 velocity Is near zero, especially the moment when a 

 reversing current changes direction and Its velocity 

 Is zero. The term is also applied to the entire 

 period of low velocity near the time of turning of 

 the current when it is too weak to be of any prac- 

 tical Importance in navigation. The relation of 

 the time of slack water to the tidal phases varies 

 in different localities. In some places slack 

 water occurs near the times of high and low water, 

 while in other localities the slack water may occur 

 midway between high and low water. (14) 



SLEEPER . See ETESIANS. 



SLICK . A smooth area of water, as one caused by 

 the sweep of a vessel's stem during a turn, or by 

 a film of oil. (17) 



SLIP . 1. A berthing space between two piers. Also 

 called DOCK. 



2. The difference between the distance a 

 propeller would travel longitudinally in one revolu- 

 tion if operating In a solid and the distance it 

 travels through a fluid. (17) 



SLOPE . The degree of inclination to the horizon- 

 tal. Usually expressed as a ratio, such as 1:23, 

 indicating one unit rise in 25 units of horizontal 

 distance; or in a decimal fraction (0.04); degrees 

 (2 18'); or per cent (47.). It is sometimes ex- 

 pressed by such adjectives as steep, moderate, 

 gentle, mild, flat, etc. Also called GRADIENT. 

 (27) 



SLOPE AMPLIFICATION . Amplification, In an FM 

 sonar receiver amplifier, which Increases uniformly 

 with Increasing frequency. (5) 



SLOUGH . 1. A small muddy marshland or tidal 

 waterway which usually connects other tidal areas. 



2. A tide-land or bottom-land creek. (11) 



SLUDGE FLOE . See FLOE. 



SLUSH . 1. Partly melted, wet snow. 



2. Partly frozen sea water, when Its con- 

 sistency is soupy. Called snow slush or snezhura 

 when formed from snow that has fallen into water 

 that is at a temperature below that of the snow. 

 (17) 



SMALL FLOE . See FLOE. 



SMITH-MCINTYRE MUD SAMPLER . With this device 



the digging and hoisting mechanisms are separated. 



This fact Improves overall performance even under 



SMITH-MclNTYRE(GRAB) 



108 



