hourly height readings. In rivers subject to oc- 

 casional freshets, the river level may undergo wide 

 variations and for practical purposes certain 

 months of the year may be excluded in the determi- 

 nation of tidal datum planes. For charting pur- 

 poses, tidal datum planes for rivers are usually 

 based on observations during selected periods when 

 the river is at or near low water stage. (14) 



MEAN SEA LEVEL . (Abbreviated NBL; popularly called 

 sea level.) The average height of the sea surface, 

 based upon hourly observation of tide height on the 

 open coast or in adjacent waters which have free 

 access to the sea. These observations are to have 

 been made over a "considerable" period of time. In 

 the United States, mean sea level is defined as 

 the average height of the surface of the sea for 

 all stages of the tide over a nineteen-year period. 



Selected values of mean sea level serve as the 

 sea level datum for all elevation surveys in the 

 United States. In meteorology, mean sea level is 

 used as the reference surface for all altitudes in 

 upper-atmospheric work; and in aviation it is the 

 level above which altitude is measured by a pres- 

 sure altimeter. 



Along with MEAN HIGH WATER, MEAN LOW WATER, 

 and MEAN LOWER LOW WATER, mean sea level is a 

 type of tidal datum. (24) 



MEAN SPHERE DEPTH . The uniform depth to which the 

 water would cover the earth if the solid surface 

 were smoothed off and were parallel to the surface 

 of the geoid. The accepted figure for this depth 

 is 2440 meters. The MEAN DEPTH of the sea is 3800 

 meters. (27) 



MEAN STEEPNESS . The ratio of the MEAN DEPTH to 

 the horizontal distance over which the mean depth 

 was determined. (27) 



MEAN TIDE LEVEL (KTL) . Same as HALF -TIDE LEVEL. 



MEAN WATER LEVEL . The mean surface level as deter- 

 mined by averaging the height of the water at equal 

 intervals of time, usually at hourly intervals. 

 (14) 



MECHANICAL ANALYSIS OF SEDIMENTS . The operation 

 of determining the distribution of grains of a 

 granular material in accordance with size. The 

 separations of the relatively large grains are 

 made by the use of sieves with varying sized mesh, 

 the material passing through mesh of one size and 

 being retained on the sieve of the next smaller 

 size being considered as coming within one size 

 range or bracket. Generally expressed in percen- 

 tage of weight of materials within specific limits 

 of size. Other methods are used for silt and clay 

 separation. (27) 



MECHANICAL IMPEDANCE . The mechanical impedance of 

 a given surface area of an acoustic medium perpen- 

 dicular, at every point, to the direction of propa- 

 gation of sinusoidal acoustic waves of given fre- 

 quency and having equal acoustic pressures and equal 

 volume velocities per unit area at every point of 

 the surface at any instant, is the quotient obtained 

 by dividing (1) the phasor corresponding to the 

 force, due to the acoustic pressure on the given 

 area, by (2) the phasor corresponding to the volume 

 velocity per unit area. (4) 



MECHANICAL OHMS . Mechanical impedance is measured 

 in mechanical ohms . One mechanical ohm is equal to 

 one gm/sec, or to one dyne sec/cm. (4) 



MEDIAN DIAMETER . The diameter which marks the 

 division of a given sample into two equal parts by 

 weight, one part containing all grains larger than 

 that diameter and the other part containing all 

 grains smaller. (27) 



MEDIASTRINAL EMPHYSEMA. See EMPHYSEMA. 



MEDIUM FLOE . See FLOE. 



MEDUSA . An umbrella-shaped coelenterate with ten- 

 tacles around the periphery and having no hard 

 parts which represents the sexual generation of a 

 hydrozoan or scyphozoan. Commonly known as a jelly- 

 fish, and ranges in size from about 12mm to over 

 2m in diameter, with the largest having tentacles 

 up to 10m long. (19) 



MEG . A prefix indicating one million. 

 MEGALOPLANKTON. See MARINE LIFE. 



MESOGLEA . The gelatinous filling between the outer 

 and inner cell layers of a two-layered animal such 

 as a jellyfish. 



MESON . Sub-atomic particles, varying in weight 

 but heavier than electrons and lighter than protons, 

 highly unstable, very short-lived and having either 

 a positive or negative charge, which are supposed 

 to form the binding energy in atomic nuclei or at 

 least are very closely related to nuclear forces. 

 Their complete nature is highly disputed. (39) 



MESOPLANKTON. See MARINE LIFE. 



MESSENGER . A brass weight, usually hinged and with 

 a latch so that it can be fastened around a wire, 

 used to actuate NANSEN BOTTLES and other oceano- 

 graphic instruments after they have been lowered 

 to the desired depth. (12) 



METABOLISM . The utilization of oxygen by all cells 

 of the body for the production of energy and heat. 

 In this process carbon dioxide is produced. (37) 



METACENTER . Metacenter is the point of intersec- 

 tion of a vertical line through the center of 

 buoyancy of a floating body and a vertical line 

 through the new center of buoyancy. (33) 



METAGENESIS . See ALTERNATION OF GENERATIONS . 



METAL ION CONCENTRATION CELL . A cell established 

 on a metallic surface caused by different concen- 

 trations of its ions in the electrolyte where it is 

 in contact with the metallic surface. 



METAMORPHIC ROCK . Rock formed by the alteration of 

 pre-existing rocks which have developed new physi- 

 cal and chemical characteristics as the result of 

 pressure, heat, or other geologic agents within the 

 earth's crust. (27) 



METER (OR HALF-METER) PLANKTON SAMPLER . This samp- 

 ler consists essentially of three parts: a metal 

 ring, one or one-half meter in diameter, provided 

 with rope bridles for attaching to a tow line; a 

 conical bag or net made of silk bolting which acts 

 as a sieve; and a metal collecting bucket at the 

 cod end of the net. 



This sampler is easy for one man to rig and 

 operate and will capture a greater number of macro- 

 plankton than nets of smaller aperture. For quali- 

 tative work of a reconnaissance type, this net is 

 rapid and effective and a typical net is a half- 

 meter (diameter) conical net with a No. mesh. A 

 special bottle or bucket fits the small end of the 

 cone. The bottle is secured to the net by several 

 turns of strong thread or twine. The bucket is 

 secured by fastening to a metal ring on the net. 

 Three lines, each about 18 inches long, are at- 

 tached at equidistant points to the brass ring at 

 the large end of the net and are joined at 1 point 

 to the towline, forming a bridle. The towline 

 should be approximately 150 feet long. 



The mesh size of a plankton net varies. The 

 selection of mesh size depends on the size of the 

 plankton organisms sought. An identification number 

 is on every net; the larger the number, the finer 



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