are made locally at specified time intervals, an 

 Eulerian system is usually, though by no means 

 always, more convenient. A sequence of synoptic 

 charts is an Eulerian representation of the data. 

 Eulerian coordinates are to be distinguished 

 from LAGRANGIAN COORDINATES. The particular coor- 

 dinate system used to identify points in space 

 (Cartesian, cylindrical, spherical, etc.) is quite 

 independent of whether the representation is 

 Eulerian or Lagrangian. (24) 



EULITTORAL ZONE . See BENTHIC DIVISION. 



EUPHOTIC ZONE . 1. For the purpose of biological 

 investigations it is convenient to consider the sea 

 as divided vertically into three zones with respect 

 to the amount of light that is present. These are: 

 (1) The euphotic zone, which is abundantly supplied 

 with light sufficient for the photosynthetic proces- 

 ses of plants. This zone extends from the surface 

 to 80 or more meters. (2) The disphotic zone, 

 which is only dimly lighted and extends from about 

 80 to 200 or more meters . No effective plant pro- 

 duction can take place in this zone, and the plants 

 found here have mostly sunk from the layer above. 

 (3) The aphotic zone, the lightless region below 

 the disphotic zone. In the deep sea it is a very 

 thick layer in which no plants are produced and 

 the animal life consists only of carnivores and 

 detritus feeders. (13) 



2. The layer of a body of water 

 which receives ample sunlight for the photosynthetic 

 processes of plants. The depth of this layer 

 varies with the water's extinction coefficient, 

 the angle of incidence of the sunlight, length of 

 day, and cloudiness; but it is usually 80 meters 

 or more. The depth of compensation is the lower 

 boundary of the euphotic zone. (24) 



EURYHALINE . See STENOHALINE . 



EURYTHERMIC . See STENOHALINE. 



EUSTATISM . The fluctuations of sea-level due to 

 the changing capacity of the ocean basins or the 

 volume of ocean water. Glacio-eustatism causes 

 variations of sea-level related to the changing 

 volume of glacier ice; sedimento-eustatisra is re- 

 lated to the rise of sea-level due to the filling 

 of the ocean basin with sediment; and tectono- 

 eustatism is due to changes in the capacity of the 

 ocean basins resulting from earth movements, such 

 as basin formation, which, by increasing the 

 capacity of the ocean receptacles, lowers sea- 

 level. (27) 



EVAPORATION AND CONCENTRATION UNIT FOR DILUTE 

 RADIOACTIVE AND NON-RADIOACTIVE SOLUTIONS . See 

 RADIOACTIVITY METER FOR LIQUIDS. 



SWING PISTON CORER . This corer, weighing approxi- 

 mately 1200 lbs. in air and measuring 20 ft. in 

 length (1, 2 or 3 - 20 ft . sections of the coring 

 tube can be coupled together to collect cores up 

 to 60 ft. in length) is the largest used by the 

 U. S. Navy Oceanographic Office. It has no tailfin 

 for stability and employs no core liner--the cores 

 either being extruded after each cast or stored in 

 the coring tube itself. The operation of the trig- 

 ger mechanism and piston is similar to those used 

 on the KULLENBERG PISTON CORER and the counter- 

 weight or trigger weight is constructed similar to 

 that of the PHLEGER CORER. It consists of a main 

 tube with a cast lead drive weight of approximately 

 50 lbs., a short coring tube, plastic liner, core 

 catcher, and cutting edge. (35) 



EXCHANGE COEFFICIENTS (Also called AUSTAUSCH CO- 

 EFFICIENTS . EDDY COEFFICIENTS. INTERCHANGE COEFFI- 

 CIENTS) . Coefficients of eddy flux (e.g., of 

 momentum, heat, water vapor, etc.) in turbulent 

 flow, defined in analogy to those of the kinetic 



theory of gases. The exchange-coefficient hypo- 

 thesis states that the mean eddy flux per unit 

 area of a conservative quantity (suitably expressed) 

 is proportional to the gradient of the mean value 

 of this quantity, that is, 



4 dl 



mean flux per unit area 



dn' 



where A is the exchange coefficient, E the mean 

 value of the quantity, and n the direction normal 

 to the surface. In strict analogy to molecular 

 properties A would be constant, for turbulent flow 

 A turns out to depend on time and location. (24) 



EXOSKELETON . An external supporting structure or 

 covering. (19) 



EXPENDABLE BT . The Expendable Bathythermograph 

 (BT) has been successfully launched from destroyers 

 at speeds of 30 knots. It requires no winch, it 

 can be used in rough weather, and it is accurate, 

 yet expendable. The BT system consists of four 

 units: an expendable transducer unit, a surface 

 float, a shipboard telemetry bale, and a recorder. 

 Operating sequence is as follows; the float and 

 transducer are launched from a moving vessel; 

 the float unit remains essentially stationary 

 in the water because the wire pays out readily 

 from the shipboard telemetry bale; as the float 

 hits the wafer it releases the BT transducer which 

 falls freely and vertically, paying out wire from 

 its bale at the rear; while falling, the transducer 

 transmits an analog temperature signal through the 

 telemetry wire to the shipboard recorder. The 

 rate of descent is constant, so depth is a direct 

 function of time. (35) 



EXPENDABLE INSTRUMENTS . Expendable or "throw away" 

 instruments are generally interpreted as a class of 

 relatively inexpensive oceanographic research in- 

 struments which may be simply tossed overboard 

 from a ship at sea to telemeter certain desired 

 oceanographic information back to the ship. (35) 



EXPIRATORY RESERVE VOLUME . The amount of air that 

 can be expelled by forcible expiration at the end 

 of a normal expiration. It normally amounts to 

 about 1 liter during rest and becomes smaller as 

 the tidal volume increases. Note that the sum of 

 the tidal volume and inspiratory and expiratory 

 reserve volumes equals the vital capacity. (37) 



EXTERNAL WAVE . A wave in fluid motion having its 

 maximum amplitude at an external boundary such as 

 a free surface. 



Any surface wave on the free surface of a 

 homogeneous incompressible fluid is an external 

 wave, (12) 



EXTINCTION COEFFICIENT . A measure of the space 

 rate of diminution, or extinction, of any trans- 

 mitted light; thus, it is the attenuation coeffi- 

 cient applied to visible radiation. The extinction 

 coefficient a is identified in a form of Bouguer's 

 law (or Beer's law) : dI=-CTldx orI= le"™- 

 where I is the illuminance (luminous flux density) 

 at the selected point in space, I is the illumi- 

 nance at the light source, and x is the distance 

 from the source. 



When so used, the extinction coefficient equals 

 the sum of the medium's absorption coefficient and 

 scattering coefficient, each computed as a weighted 

 average over all wavelengths in the visible spec- 

 trum. As long as scattering effects are primary, 

 as in the lower atmosphere, the value of the ex- 

 tinction coefficient is a function of the particle 

 size of atmospheric suspensoids. It varies in 

 order of magnitude from 10 km"^ with very low visi- 

 bility to 0.01 km"' in very clear air. 



The extinction coefficient is related to the 

 transmission coefficient t as follows: t= e"". 



In oceanography, it is a measure of the atten- 



>tk 



