ESTUARY 



summer, when tlie southwest monsoon forms a 

 continuation of the southeast trade winds, the 

 countercurrent flows with the monsoon drift. 



Equatorial Current — See North Equatorial 

 Current, South Equatorial Current. 



equatorial cylindrical orthormorphic projec- 

 tion — See Mercator projection. 



equatorial tidal currents — Tidal currents occur- 

 ring approximately every two weeks when the 

 moon is over the Equator. At these times, the 

 diurnal inequality between successive periods 

 of flood and ebb is at a minimum and the cur- 

 rents are most nearly semidiurnal. 



equatorial tides — Tides that occur approximately 

 every two weeks when the moon is over the 

 Equator. At these times, the moon jjroduces 

 minimum inequality between two successive 

 high waters and two successive low waters. 



equilibrium — 1. In thermodynamics, any state of 

 a system which would not undergo change if the 

 system were to be isolated. Processes in an 

 isolated system not in equilibrium are irrever- 

 sible and always in the direction of equilibrium. 

 2. In mechanics, a state in which the vector is 

 zero. In hydrodj-namics, it is usually further 

 required that a steady state exists throughout 

 the atmosphere or fluid model. The equilibrium 

 may be stable or unstable with respect to dis- 

 placements therefrom. (5) 



equilibrium argument — The theoretical phase of 

 a constituent of the equilibrium tide. See 

 Greenwich argument. 



equilibrium range — The high frequency band of 

 wind waves whose frequency spectrum satisfies 

 a minus five power law. 



equilibrium spheroid — The shape that the earth 

 would attain if it were entirely covered by a tide- 

 less ocean of constant depth. tSee geoid. (5) 



equilibrium theory — A hypothesis which assumes 

 an ideal earth which has no continental barriers 

 and is uniformly coA^ered with water of consider- 

 able depth. It also assumes that the water re- 

 sponds instantly to the tide-producing forces 

 of the moon and sun to form a surface in equilib- 

 rium and moves around the earth without vis- 

 cosity or friction. 



equilibrium tide — (or astronomic tide, astro- 

 nomical tide, gravitational tide) . The hypothet- 

 ical tide due to the tide-producing forces 

 under the equilibrium theory ; tide relating to 

 the attractions of celestial bodies, particularly 

 the sun and moon. (50) 



equilibrium vapor pressure — The vapor pres- 

 sure of a system in which two or more phases 

 of a substance coexist in equilibrium. In 

 oceanography the reference is to water substance 

 unless otherwise specified. If the system con- 

 sists of moist air in equilibrium with a plane 

 surface of pure water or ice, the more special- 

 ized term saturation vapor pressure is usually 



employed, in which case the vapor pressure is 

 a function of temperature only. (5) 



equinoctial— Tlie celestial equator. (50) 



equinoctial spring tides — Tiiose tides occurring 

 near tlie equinoxes when the full or new moon 

 and the sun have little or low declination and 

 spring tides of greater range than tne average 

 occur, particularly if the moon is also nearly 

 in perigee. (30) 



equinoctial tide — Tide occurring when the .sun 

 is near equinox. During this period spring tide 

 ranges are greater than average. ( 5 ) 



equinoxes — The two points in the celestial sphere 

 where the celestial equator intersects the eclip- 

 tic; also the times when the sun crosses the 

 celestial equator at these points. (50) 



eradiation — See terrestrial radiation. 



erect bryozoan — (or stolonate hryosoan). A 

 bryozoan which forms branching upright 

 growths attached basally to underwater surfaces. 



erg — The unit of energy in the centimeter-gram- 

 second system of physical units; that is, one 

 dyne centimeter. One erg is equal to 10"^ 

 Joule or to 2.389 X lO"" calories. (5) 



ergodic — A hypothesis asserting statistical equiv- 

 alence of ensemble averages and time averages 

 when steady state conditions occur. 



erosion — Any or all jDrocesses by which soil or 

 rock is broken up and transported from one 

 place to another. It is regarded as including 

 weathering, corrasion, and transportation. (2) 



erosion ramp — A sloping belt of reef rock im- 

 mediately above the reef flat on an atoll islet 

 where marine erosion is active. (2) 



erratic — A transported rock fragment different 

 from the bedrock on which it lies, either free 

 or as part of a sediment. Such fragments are 

 ice rafted or transported by glacier ice or by 

 floating ice and are widely distributed on the 

 sea floor in high latitudes. 



eruptive rocks — Eocks which have formed from 

 molten magma and which have either been in- 

 truded into older rocks or extruded at the earth's 

 surface through volcanic cones or fissures. 



escarpment — (or fault scarp, scarp). Aii elon- 

 gated and comparatively steep slope of the sea 

 floor, separating flat or gently sloping areas. 

 (62) 



establishment — See lunitidal interval. 



establishment of the port — (or high toater full 

 and change, conunon establishtivent, vulgar 

 establishment). The average interval between 

 upper and lower lunar transit near time of new 

 and full moon and the next high water. See 

 lunitidal internal. 



estuarine muds — Silts, often containing sufficient 

 clay to impart some plasticity, and containing 

 a considerable proportion of decomposed organic 

 matter. 



estuary — (or drovmed nver mouth, hranching 

 iay, firth, frith) . A tidal bay formed by sub- 



207-109 O— 66- 



59 



