SECONDARY CUERENT PATTERN 



3. The sixtieth part of a minute of time. (35) 



secondary current pattern — A short-period vari- 

 ation of the prevailing current pattern. 



secondary electron — An electron ejected from 

 an atom, molecule, or surface as a result of a 

 collision with a charged particle or photon. 

 (70) 



secondary production — The organic matter pro- 

 duced by herbivores of the zooplankton in a 

 given area or volume in a given time. The sec- 

 ond trophic level. 



secondary tide station — A place at which tide 

 observations are made over a short period. 

 (68) See subordinate station. 



secular change — An increase or decrease of in- 

 tensity and/or change of direction of the total 

 magnetic field over a period of many years. 



sediment — Particulate organic and inorganic 

 matter which accumulates in a loose unconsoli- 

 dated form. It may be chemically precipitated 

 from solution, secreted by organisms, or trans- 

 ported by air, ice, wind, or water and deposited. 



sedimentary basin — A depression, often marine, 

 in which sediments are deposited. The deposits 

 are usually thickest in the center and thinner 

 toward the edges. (2) 



sedimentary rocks — Rocks formed by the ac- 

 cumulation of sediment in water (aqueous de- 

 posits) or from air (eolian deposits). The sedi- 

 ment may consist of rock fragments or particles 

 of various sizes (conglomerate, sandstone, 

 shale) ; of the remains or products of animals or 

 plants (certain limestones and coal) ; of the 

 product of chemical action or of evaporation 

 (salt, gypsum, etc.) ; or of mixtures of these 

 materials. (2) 



sedimentation — The process of breakup and 

 separation of particles from the parent rock, 

 their transportation, deposition, and consolida- 

 tion into another rock. 



sedimentation method — A technique used in the 

 quantitative estimation and identification of 

 phytoplankton organisms. The plankton to be 

 measured is concentrated by settling in glass 

 cylinders and the organisms are counted or iden- 

 tified by using an inverted microscope. 



sedimentation unit — That thickness of material 

 which are deposited under essentially constant 

 physical conditions. (2) 



sedimentology — The study of sedimentary rocks 

 and the processes by which they were formed. 

 (2) 



segmented worm — See annelid. 



seiche — 1. A standing wave oscillation of an 

 enclosed or semienclosed water body that con- 

 tinues, pendulum fashion, after the cessation of 

 the originating force, which may have been 

 either seismic, atmospheric, or wave induced. 



2. An oscillation of a fluid body in response to 

 a disturbing force having the same frequency as 

 the natural frequency of the fluid systein. Tides 

 are now considered to be seiches induced primar- 



ily by the periodic forces caused by the sun and 

 moon. 



seme — A type of net used to catch fish by encircle- 

 ment, usually by active closure of the two ends 

 but also including closure or pursing of the bot- 

 tom (purse seine). 



seismic activity — See seismicity. 



seismicity — The phenomenon of earth movements. 



seismic profile — The data resulting from a single 

 series of observations made at one geographic 

 location with a linear arrangement of seis- 

 mometers. (4) 



seismic profiler — A continuous deep sea reflection 

 system used to study the structure beneath the 

 ocean floor to depths of 10,000 feet or more. The 

 reflections are recorded on a drum whose rotation 

 is actuated by the initial explosion. (4) 



seismic reflection — The measurements, and rec- 

 ording in wave form, of the travel time of acous- 

 tic energy reflected back to detectors from rock 

 or sediment layers which have different elastic 

 wave velocities. 



seismic sea wave — See tsunami. 



seismograph — An instrument used to measure and 

 record earthquake vibrations and other earth 

 tremors. ( 5 ) 



selachian — See shark. 



selective filter — (or colored -filter). An optical 

 filter which changes, by absorption, the spectral 

 distribution of the energy passing through it. 



(8) 

 self noise — See own ship's noise, 

 semidaily — See semidiurnal. 



semidiurnal — (or semidaily). Having a period 

 or cycle of approximately half a lunar day 

 (12.42 solar hours). The tides and tidal cur- 

 rents are semidiurnal when two flood and two 

 ebb periods occur each lunar day. 



semidiurnal constituent — A tidal constituent 

 that has two maximums and two minimums each 

 constituent day; its symbol is usually distin- 

 guished by the subscript 2, as M2, S^, N2, etc. 



semidiurnal current — The type of tidal current 

 having two flood and two ebb periods of nearly 

 the same duration during a tidal day; usually 

 associated with a semidiurnal tide. 



semidiurnal tide — The type of tide having two 

 high waters and two low waters each tidal day, 

 with small inequalities between successive high 

 and successive low water heights and durations. 

 {See figure for types of tide.) 



sensible heat — The portion of energy exchanged 

 between ocean and atmosphere which is utilized 

 in changing the temperature of the medium into 

 which it penetrates. 



sensor — A technical means, usually electronic, to 

 extend man's natural senses by means of energy 

 emitted or reflected. The energy may be nu- 

 clear, electromagnetic, including the visible and 

 invisible portions of the spectrum, chemical, 

 biological, thermal, or mechanical. 



144 



