MEAN SOUNDING VELOCITY 



measured from a fixed predetermined reference 

 level (chart datum). 



mean sounding velocity — Mean values for veloc- 

 ity of sound through the vertical water column 

 of specific depths based on different velocities of 

 sound in different sections of the column. These 

 values yield more nearly correct depths when 

 sonic depth finding machines are used. The 

 velocity of sound at any specific depth may dif- 

 fer considerably from the mean. 



mean sphere depth — The uniform depth to which 

 the water would cover the earth if the solid sur- 

 face were smoothed off and were parallel to the 

 surface of the geoid. This depth would be 

 about 8,000 feet ( 2,440 meters) . ( 5 ) 



mean spring range — (or spring range). The 

 average semidiurnal range of tide at time of 

 syzygy. It is greater than the mean range 

 where the type of tide is either semidiurnal or 

 mixed and is of no practical significance where 

 the type of tide is diurnal. {See figure for tide 

 range.) 



mean spring rise — (or spring rise) . The surface 

 water level height of mean high water springs 

 measured above the basic reference plane (chart 

 datum). (See figure for tide range.) 



mean tide level — (abbreviated MTL ; or half -tide 

 level, ordvnary tide level) . The reference plane 

 midway between mean high water and mean 

 low water. (/See figure for tide range.) 



mean tropic range — The mean of the great tropic 

 range and the small tropic range. (68) 8ee 

 lunar declination. 



mean water level — (abbreviated MWL). The 

 mean surface level determined by averaging the 

 height of the water at equal intervals of time, 

 usually at hourly intervals, over a considerable 

 period of time. 



mechanical analysis — Measurement of the par- 

 ticle size distribution of a sediment. (2) 



mechanical mixing — Action of waves in mixing 

 the surface layer of the ocean. 



median — Pertaining to a series of numbers, the 

 median is the middle term when the numbers are 

 arranged in algebraic order. If the number of 

 terms is even, the median is taken halfway be- 

 tween the two middle terms. ( 5 ) 



medimarimeter — An instrument for measuring 

 mean sea level. The term is not generally used 

 in the United States. ( 68 ) 



mediterranean — A large body of salt water or 

 inland sea surrounded by land, which^may have 

 one or more narrow openings to the ocean or an- 

 other sea. 



medium field of ice — See medium ice field. 



medium floe — See medium ice floe. 



medium ice field — (or medium -field of ice). An 

 ice field 15 to 20 kilometers (8.1 to 10.8 nautical 

 miles) across. (74). 



medium ice floe — 1. An ice floe of sea ice 600 to 

 3,000 feet across. (59) 

 2. See ice floe. 



medium scale — See scale. 



medium winter ice — 1. Winter ice thicker than 

 15 to 30 centimeters (6 to 12 inches). (74) 

 2. See winter ice. 



medusa — See jellyfish. 



megaloplankton — Plankton larger than 1 centi- 

 meter; includes the larger forms of the plank- 

 ton, such as salps and large jellyfishes. 



megaripple — (or sand wave). A large wavelike 

 sediment feature in very shallow water composed 

 of sand. The wavelength may reach 100 meters, 

 and amplitude is about 0.5 meter. 



meizoseismal — Refers to the maximum destruc- 

 tive effects of an earthquake. A line or curve 

 connecting points of maximum destruction 

 around an earthquake epicenter is a meizoseis- 

 mal curve. (48). 



meizoseismal curve — See meizoseismal. 



melting point — The temperature at which a solid 

 substance changes from the solid to the liquid 

 form. Although sea ice melts at a specific tem- 

 perature, it actually begins to disintegrate and 

 weaken when its temperature approaches the 

 melting point. 



membrane filter — A strainer used for the quan- 

 titative extraction of phytoplankton and bac- 

 teria from water samples by vacuum filtration 

 and usually consisting of a collodion membrane 

 with fine pores of uniform size. 



Mercator projection — (also called equatorial 

 cylindrical orthomorpMc projection) . A con- 

 formal projection derived by mathematical 

 analysis in which the meridians and parallels 

 are portrayed as parallel straight lines at right 

 angles to one another. The scale is chosen to be 

 true along the Equator. This projection can be 

 equivalently described as the development of a 

 rhumb line on the earth, being portrayed as a 

 straight line on the projection. 



meridian — A north-south reference line, through 

 the geographical poles of the earth from which 

 longitudes and azimuths are measured. (68) 



meridional flow — The current moving along a 

 meridian. (32) 



mermaid's hair — See blue-green alga. 



mermaid's purse — See sailor's purse. 



meroplankton — Chiefly the floating develop- 

 mental stages (eggs and larvae) of the benthos 

 and nekton. These forms are especially abun- 

 dant in neritic waters. 



mesopelagic — Tliat portion of the oceanic prov- 

 ince extending from about 100 fathoms (200 

 meters) down to a depth of about 500 fathoms 

 (1,000 meters). {See figure for classification 

 of marine environments.) 



mesoplankton — Plankton within the size range 

 0.5 to 1.0 millimeter. Rarely used in this mean- 

 ing since it is also used to designate all plankton 

 living in middepths. 



messenger — A cylindrical metal weight approxi- 

 mately 3 inches long and 1 inch in diameter ; it 

 is usually hinged and with a latch and is at- 



104 



