MULTIPLE SHOT UNDERWATER CAMERA . 

 CAMERAS. 



See UNDERWATER 



MULTIPLE TIDE STAFF . A succession of tide staffs 

 on a sloping shore so placed that the vertical 

 graduations on the several staffs vTill form a con- 

 tinuous scale referred to the same datum. (14) 



MULT IS POT ARRAY . The factors which determine the 

 directive properties of any transducer may be con- 

 sidered most logically by first examining the be- 

 havior of a number of discrete receiving points 

 uniformly spaced along a straight line. Such an 

 arrangement is known as a linear multispot array. (4) 



MUSSELS . Marine, brackish, or fresh-water molluscs 

 sometimes called clams. (15) 



MUTUAL ADMITTANCE . The mutual admittance of a 

 transducer may be defined as the quotient obtained 

 by dividing the phasor representing the short- 

 circuited current at one pair of terminals by the 

 phasor representing the voltage across the second 

 pair of terminals when the first pair is short- 

 circuited. (4) 



MUTUAL IMPEDANCE . The mutual impedance of a trans- 

 ducer is the quotient obtained by dividing the 

 phasor representing the open-circuit voltage 

 between one pair of its terminals by the phasor 

 representing the current through the other pair. 

 (4) 



N 



NADC. Naval Air Development Center, Johnsville, 

 Pennsylvania. 



NANNOPLANKTON . Those plankton that will pass 

 through an ordinary net, but can be removed from 

 the water by centrifuging or passage through filter 

 paper. (See MARINE LIFE) 



NANSEN BOTTLE . The Nansen bottle is a metal revers- 

 ing water sampler with a 1.25-liter capacity. Its 

 purpose Is to bring an uncontaminated water sample 

 from a desired depth to the surface. It is fitted 

 with a tapered plug valve at either end and is 

 lowered on the oceanographic wire in the open posi- 

 tion, thus flushing itself during the lowering. 

 When the bottle reaches any predetermined depth, a 



BOTTI n EN RACKS 



brass messenger is dropped from the deck. The 

 messenger serves to disconnect the top of the bot- 

 tle from the wire; the bottle then reverses, making 

 a 180° arc with the wire. The plug valves close 

 when reversal occurs , entrapping a water sample 

 from the desired depth, and a second messenger is 

 released which in turn effects the reversal of a 

 lower bottle, and so on. 



To keep contamination of water samples to a 

 minimum, Nansen bottles are constructed of brass. 

 To provide resistance to action of salt water the 

 exterior parts are chromium plated and the interior 

 is silver or tin plated. The exterior is painted 

 yellow to Increase visibility in the water. Should 

 a Nansen bottle be inadvertently lowered with the 

 plug valves closed, the pressures encountered will 

 crush it. 



Each Nansen bottle is fitted with a frame to 

 hold deep-sea reversing thermometers. Most frames 

 hold 2 thermometers , although frames for 3 ther- 

 mometers are sometimes used. Brass tubes in the 

 frame, into which the thermometers are inserted, 

 are slotted to permit easy reading of the scale. 

 One end of each tube is perforated to permit water 

 circulation so that the thermometers may come to 

 temperature more rapidly. The ends of the tubes 

 contain coil springs and rubber pads to hold the 

 thermometers securely yet provide a certain amount 

 of protection against shock. The thermometers are 

 always inserted into the tubes in such a manner 

 that the large mercury reservoir is in the end of 

 the tube having the small perforations. (35) 



NANSEN CAST . A series of NANSEN-BOTTLE water 

 samples and associated temperature observations 

 resulting from the release of a MESSENGER. (12) 



NARTU. Naval Air Reserve Training Unit. 



NAS . National Academy of Sciences. 



NASA. National Aeronautics and Space Administration. 



NAS CAS . Committee on Atmospheric Sciences of the 

 National Academy of Sciences - National Research 

 Council. 



NAS CO . National Academy of Sciences Committee on 

 Oceanography. 



NAS/NRC. National Academy of Sciences-National 

 Research Council. 



NATO. North Atlantic Treaty Organization. 



NATURAL FREQUENCY . The natural frequency of a sys- 

 tem is the frequency of free oscillation of the 

 system. If a system has many degrees of freedom, 

 there will be as many natural frequencies as there 

 are degrees of freedom. (9) 



80 



