N 



nadir — The point on the celestial sphere vertically 

 below the observer, or 180 degrees from zenith. 

 (66) 



nannoplankton — (or centrifuge plankton). 

 Plankton within the size range 5 to 60 microns. 

 Includes many dinoflagellates and smaller 

 diatoms. Individuals will pass through most 

 nets and usually are collected by centrifuging 

 water samples. This spelling is as originally 

 coined ; the spelling nanoplankton used by some 

 authorities is etymologically correct. 



nanocurie — Equals 10"^ curie. 



Nansen bottle — A device used by oceanographers 

 to obtain subsurface samples of sea water. 



The "bottle" is lowered by wire ; its valves are 

 open at both ends. It is then closed in situ by 

 allowing a weight (called a messenger) to slide 

 down the wire and strike the reversing mech- 

 anism. This causes the bottle to turn upside 

 down, closing the valves and reversing the re- 

 versing thermometers which are mounted in a 

 special thermometer case on it. If, as is usually 

 done, a series of bottles is lowered, then the re- 

 versal of each bottle releases another messenger 

 to actuate the bottle beneath it. (5) 



Nansen cast — See oceanographic cast. 



narrows — A narrow passage or strait. (2) 



natural arch — See sea arch. 



natural frequency — The characteristic frequen- 

 cy, that is, the number of vibrations or oscilla- 

 tions per unit time of a body controlled by its 

 physical characteristics (dimensions, density, 

 etc.). In a harbor, the natural frequency gives 

 rise to waves, called seiches, which have periods 

 and amplitudes dependent on the physical char- 

 acteristics of the harbor. 



natural radioactivity — 1. The property of radio- 

 activity exhibited by more than fifty naturally 

 occurring radionuclides. 



2. Tlie natural radioactive constituents of sea 

 water include Potassium^", Rubidium*', Ura- 

 nium^^% Uranium-^*, Thorium-^-, Radium--", 

 Carbon^'', and Hydrogen^ (Tritium). Of these 

 Potassium^" is by far the largest contributor to 

 the oceans natural radioactivity. 

 (70) 



natural scale— The ratio between the linear di- 

 mensions of a cliart, drawing, etc., and the actual 

 dimensions represented, expressed as a propor- 

 tion. (68) 



nature of the bottom — See character of the 

 bottom. 



nauplius — A limb-bearing early larval stage of 

 many crustaceans. 



nautical almanac — A periodical publication (us- 

 ually annual) of astronomical statistics useful 

 to and designed primarily for marine naviga- 

 tion. (68) 



nautical mile — ( abbreviated n. mile) . In general 

 a unit used in marine navigation equal to a 

 minute of arc of a great circle on a sphere. 

 Depending upon the radius of the sphere, vari- 

 ous lengths of nautical miles have been defined. 

 The adopted value in the United States since 

 July 1, 1959 is one international nautical mile 

 equals 6,076.11549 U.S. feet (approximately). 



nautilus — 1. Any of a genus {Nautilus) of 

 cephalopods of the South Pacific and Indian 

 Oceans having a spiral chambered shell, pearly 

 on the inside, whence the name Pearly Nautilus. 

 2. An eight-armed cephalopod (Argonauta) 

 related to the octopus, the female having a 

 fragile, papery, unchambered shell, called also 

 Paper Nautilus. 



navigation season — The average statistical dates 

 for commencement and ending of shipping with- 

 in or through a port or given area and controlled 

 generally by ice and weather conditions. 



Naval Oceanographic and Meteorological 

 Automatic Device — (abbreviated NOMAD). 

 A deep sea moored buoy which provides auto- 

 matic radio transmission of surface weather 

 and subsurface temperature. 



naze — See headland. 



neap high water — See mean high water neaps, 



neap low water — See mean low water neaps. 



neap range — See mean neap range. 



neap rise — See mean neap rise. 



neaps — See neap tide. 



neap tidal currents — Tidal currents of decreased 

 speed occurring at the time of neap tides. (68) 



neap tide — (or neajjs). Tide of decreased range 

 which occurs about every two weeks when the 

 moon is in quadrature. (50) {See figure for 

 tide cycle.) 



nearshore circulation — The ocean circulation 

 composed of the nearshore currents and coastal 

 curents. (61) 



nearshore current system — The current system 

 caused by wave action in and near the surf 

 zone. The nearshore current system consists of 

 four parts: the shoreward mass transport of 

 water, longshore currents, rip currents, and 

 longshore movement of expanding heads of rip 



108 



