a diurnal wave Is conspicuous by a large Inequality 

 in either the high or low water heights with two 

 high waters and two low waters usually occurring 

 each tidal day. In strictness all tides are mixed 

 but the name is usually applied without definite 

 limits to the tides intermediate to those predomi- 

 nantly semidiurnal and those predominantly diurnal. 

 (14) 



MIXING LENGTH . A mean length of travel, character- 

 istic of a particular motion, over which an eddy 

 maintains its identity; analogous to the mean free 

 path of a molecule. Physically, the idea implies 

 that mixing occurs by discontinuous steps, that 

 fluctuations which arise as eddies with different 

 characteristics wander about, and that the mixing 

 is done almost entirely by the small eddies. 

 Mathematically, the theory assumes that the fluc- 

 tuations can be adequately represented by the first 

 term in the Taylor expansion of the mean quantity 

 in question. Considering the velocity, for example, 

 the eddy velocity is represented by 



_ du 



where L is the mixing length and z the height. 

 The Reynolds stresses then have the form 



; - pu'w' 



pV 



2 du dw 



dz dz 



and the coefficient of eddy viscosity is 



2 du 



K 



M 



dz 



(24) 



MLR. Marine Life Resources (Program) . 



MMA . U. S. Navy ship designation for a Minelayer, 

 Auxiliary. 



MMF . U. S. Navy ship designation for a Minelayer, 

 Fleet. 



MOAT . An annular depression that may not be con- 

 tinuous, located at the base of many seamounts or 

 islands. (27) 



MODE OF VIBRATION . In a system undergoing vibra- 

 tion, a mode of vibration is a characteristic 

 pattern assumed by the system, in which the motion 

 of every particle is simple harmonic with the same 

 frequency. Two or more modes may exist concur- 

 rently in a multiple-degree-of-freedom system. (2) 



MODERATE GALE . See GALE. 



MODIFIED VAN PORN WATER SAMPLER . A large water 

 capacity* non toxic (usually non metallic) sampler 

 used especially by phytoplankton biologists to ob- 

 tain water samples for subsequent chlorophyll de- 

 terminations . 



The sampler contains 2 plumber 's -helper valves 

 (pure gum- rubber) and the tension between the valves 

 is applied by gum-rubber tubing. The valves are 

 kept in an open position by two connected chains. 



A number of these bottles may be attached to 

 the wire and tripped by messengers. Routinely, 4 

 to 5 samples are filtered at a time in the ship- 

 board laboratory. After the samples are filtered 

 they may be: (1) used for pigment analysis, (2) 

 measured -to determine their dry weight or some 

 other biochemical parameter, (3) examined by spe- 

 cial microtechniques, or (4) identified and indi- 

 vidually counted on the filter. (35) 



MODULATION . Modulation is the variation in the 

 value of some parameter characterizing a periodic 

 oscillation. The most well-known of these are 

 amplitude modulation and frequency modulation. AM 

 and FM radio receivers are common devices for de- 

 tecting such modulations. (9) 



MODULUS OF ELASTICITY . A measure of stiffness of 

 a material. Not to be confused with Modulus of 



rubber. A term applied to two different relation- 

 ships between stress and strain below the Elastic 

 Limit. Tangent Modulus of Elasticity la the slope 

 of a Stress-Strain Diagram at a specified stress. 

 Secant Modulus of Elasticity is the ratio of stress 

 to strain at a specified stress or strain. It is 

 also called the Stress-Strain Ratio. For a materi- 

 al exhibiting a true Proportional Limit of any sig- 

 nificant magnitude, both moduli are equal to the 

 slope of the straight-line portion of the Stress- 

 Strain Diagram and are therefore equal to each 



Tangent Modulus 

 of Elasticity = ^ 



Secant Modulus 

 of Elasticity =^ 



' Strain- 



other. For a material without a definite Propor- 

 tional Limit, the two moduli may differ signifi- 

 cantly. For many materials, the Proportional 

 Limit is nominal in that the stress-strain curve 

 is only approximately the straight line predicted 

 by Hooke's Law. For such materials the two moduli 

 are different but usually the difference is of no 

 practical significance. Depending on the type of 

 loading represented by the Stress-Strain Diagram, 

 Modulus of Elasticity may be known as Compressive 

 Modulus of Elasticity (or Modulus of Elasticity in 

 Compression). Flexural Modulus of Elasticity (or 

 Modulus of Elasticity in Flexure), Shear Modulus 

 of Elasticity (or Modulus of Elasticity in Shear), 

 Tensile Modulus of Elasticity (or Modulus of Elas- 

 ticity in Tension) , or Torsional Modulus of Elas- 

 ticity (or Modulus of Elasticity in Torsion). 

 Shear MDdulus of Elasticity is almost invariably 

 equivalent to Torsional Modulus of Elasticity and 

 they are more commonly known as Modulus of Rigid- 

 ity. The term [todulus of Elasticity alone general- 

 ly refers to Tensile Modulus of Elasticity. Moduli 

 of Elasticity in Tension, Compression or Flexure 

 are usually approximately equal for a given materi- 

 al and may be calculated from tfodulus of Rigidity 

 as follows: 



E = 2G (1 + r) 



where E = Modulus of Elasticity, psi 



G = Modulus of Rigidity, psi 



r - Poisson's Ratio 

 Below the Elastic Limit, Modulus of Elasticity may 

 be used to predict the stress corresponding to a 

 specified strain or vice versa. A higher Modulus 

 of Elasticity indicates greater stiffness. 



MODULUS OF RESILIENCE . The Resilience of a materi- 

 al subjected to a stress corresponding to its Pro- 

 portional Limit. 



Proportional 



Limit 



Stroin 



77 



