M 



machinery noise — Noise produced by the main 

 propulsion plant, reduction gears, propeller 

 shafts, auxiliary machinery, and the under- 

 water discharges from the ship. 



macroplankton — Plankton organisms within the 

 size range 1 millimeter to 1 centimeter. Some- 

 times referred to as mesoplankton. Formerly 

 the term included megaloplanktonic fonns. See 

 megaloplankton, mesoplankton. 



madrepore — A branching or stag-horn coral, also 

 any perforated stone coral. (2) 



maelstrom — A confused and often destructive 

 current usually caused by the combined effects of 

 high, wind-generated waves and a strong oppos- 

 ing tidal current : the rapid flows may follow 

 eddying patterns or circular paths with whirl- 

 pool characteristics. Named after the fre- 

 quently cited phenomenon along the south shore 

 of Moskenesoy Island in the Lofoten Islands off 

 the Norway coast; hei'e, the maelstrom reaches 

 its strength when the tidal current ebbs west- 

 ward with speeds up to 9 knots at springs during 

 a strong opposing westerly wind. Similar 

 phenomena occur in Pentland Firth, Scotland 

 and off Cape de la Hague, Normandy. 



magma — Mobile rock material generated within 

 the earth from which igneous rock is derived by 

 solidification. When extruded it is called lava. 

 {See figure for compound volcano.) 



magnetic anomaly — A distortion of the regular 

 pattern of the earth's magnetic field due to local 

 concentrations of ferromagnetic minerals. 



magnetic declination — At any point, the angle 

 between the direction of tlie horizontal com- 

 ponent of the earth's magnetic field and true 

 north. 



magnetic deviation — The angle between the mag- 

 netic meridian and the axis of a compass card, 

 expressed in degrees east or west to indicate the 

 deviation in which the northern end of the com- 

 pass card is offset from magnetic north. Devia- 

 tion is caused by disturbing magnetic influences 

 in the immediate vicinity of the compass, as 

 within the craft. (68) 



magnetic dip — ( also called dij). incUnation) . The 

 angle which the magnetic lines of force make 

 with the plane of the horizon. (42) 



magnetic disturbance — (also called magnetic 

 storm) . Irregular, large amjjlitude, rapid time 

 changes of the earth's magnetic field which oc- 

 cur at approximately the same time all over the 

 earth. (24) 



magnetic diurnal variation — (also called dmrnal 

 rariation, daily variation) . Oscillations of the 

 earth's magnetic field which have a periodicity 

 of about a day and that depend to a close ap- 

 proximation only on local time and geographic 

 latitude. (15) 



magnetic elements — The declination {D), the 

 horizontal intensity (//), the vertical inten- 

 sity (Z), the total magnetic intensity (F), the 

 inclination or dip (/) , the strength of the force 

 toward geographic north (X), and the strength 

 of the force toward geographic east (T). 



magnetic equator — (also called aclinic line, dip 

 equator). The imaginary line on the earth's 

 surface where the magiietic inclination is zero 

 degrees, that is, the magnetic field is horizontal. 



magnetic field — A region in which a magnetic 

 influence exists. See magnetic field intensity. 



magnetic field intensity — (also called magnetic 

 inteihslty, niagnetic field, magnetic field 

 strength). The magnetic force exerted on an 

 imaginary unit magnetic pole placed at any 

 specified point of space. It is a vector quantity. 

 Its direction is taken as the direction toward 

 which a north niagnetic pole would tend to move 

 under the influence of the field. If the force is 

 measured in dynes and the unit jiole is a cgs 

 unit pole, the field intensity is given in oersteds. 

 Prior to 1932 the oersted was called the 

 gauss: but the latter term is now used to meas- 

 ure magnetic induction (within magnetic ma- 

 terials), while oersted is reserved for magnetic 

 force. By definition, one magnetic line of force 

 per square centimeter (in air) represents the 

 field intensity of one oersted. (5) 



magnetic field strength — See magnetic field 

 intensity. 



magnetic inclination — (or dip). See magnetic 

 dip. 



magnetic intensity — See magnetic field inten- 

 sity. 



magnetic latitude — At any point on the earth's 

 surface the angle whose tangent is one-half the 

 tangent of the magnetic dip at that point. (36) 



magnetic meridian — At any point, the direction 

 of the horizontal component of the earth's 

 magnetic field. 



magnetic pole — 1. (also called dip pole) . A place 

 wiiere the actual earth's magnetic pole is vertical. 

 2. In magnetic theory, a fictitious entity anal- 

 ogous to a unit electric charge of electrostatic 



100 



