D 



daily — See diurnal. 



daily inequality — See diurnal inequality. 



Daily Memorandum — A publication issued by the 

 U.S. Naval Oceanographic Office and several of 

 its Branch Offices to disseminate ephemeral nau- 

 tical information for a siDecific area, namely East 

 Coast Edition; West Coast Edition; Pacific 

 Edition ; and Far East Edition. 



daily retardation — The amount of time by Avhich 

 corresponding tidal phases grow later day by 

 day (averages approximately 50 minutes). 

 (61) 



daily variation — See magnetic diurnal varia- 

 tion. 



damping — The dissipation of energy with time or 

 distance. (6) 



dark bottle — See black bottle. 



date-time group — (abbreviated DTG) . The date 

 and time, expressed in digits and zone suffix, at 

 which the message was prepared for transmis- 

 sion. (Expressed as six digits followed by zone 

 suffix; [generally Z for Greenwich time zone] 

 first pair of digits denoting the date, second pair 

 the liour, tliird pair the minutes.) (63) 



datum — 1. Any numerical or geometrical quantity 

 or set of such quantities which may serve as a 

 reference or base for other quantities. 



For a group of statistical references, the plural 

 form is data ; as geographic data for a list of lati- 

 tudes and longitudes. Where the concept is 

 geometrical and particular, rather tlian statisti- 

 cal and inclusive, tlae plural form is datums, as, 

 for example, two geodetic datums have been used 

 in the U. S. in recent years. (37) 



2. /S'ee chart datum. 



3. See geodetic datum, 

 datum level — See chart datum, 

 datum plane — See chart datum. 



datum point — Any reference point of known or 

 assumed coordinates from which calculations 

 or measurements may be taken. (63) 



daughter — A synonyn for a decay product. (70) 



Davidson Current — (also called Davichon. In- 

 shore Current). A coastal countercurrent set- 

 ting north inshore of the California Current 

 along the west coast of the United States (from 

 northern California to Washington to at least 

 48°N) during the winter months. 



Davidson Inshore Current — See Davidson 

 Current. 



dead reckoning — A method of navigation utiliz- 

 ing only the speed and heading of the craft. 



without reference to external aids. See fix. 

 (5) 



dead water — The phenomenon which occurs when 

 a ship of low propulsive power negotiates water 

 which has a tliin layer of fresher water over a 

 deeper layer of more saline water. As the ship 

 moves, part of its energy goes into generation 

 of an internal wave wliich causes a noticeable 

 drop in efficiency of propulsion. 



debacle — The rush of water or ice in a stream 

 immediately following the breakup. (59) 



debouchure — See mouth. 



debris — See detritus. 



debris ice — 1. Sea ice which contains soil, stones, 

 shells, and other materials. 



2. Another name for brash ice. 

 (59). 



debris line — A line near the limit of storm wave 

 uprush marking the landward limit of debris 

 deposits. (61) 



decade scaler — A scaler whose scaling factor is 

 a power of ten. (70) 



decapod — 1. One of an order (Decapoda) of 

 crustaceans which includes the shrimps, 

 lobsters, and crabs. 



2. One of an order (Decapoda) of cepha- 

 lopods which contains the squids. 



decapod mollusk — See squid. 



decay — 1. As apj^lied to ocean surface waves : The 

 gradual decrease of surface wave heights owing 

 principally to angular spreading, dispersion, 

 and opposing winds. 

 2. See disintegration. 



decay area — Tlie area of lesser winds through 

 which ocean waves travel after emerging from 

 the generating area. (73) 



decay constant — See attenuation constant. 



decay distance — The distance through which 

 ocean waves travel as swell after leaving the 

 generating area. (61) 



decay of waves — Tlie change that waves undergo 

 after they leave a generating area (fetch) and 

 pass through a calm or region of lighter winds. 

 In the process of decay, the significant wave 

 height decreases and the significant wavelength 

 increases. (61) 



Decca* — A continuous-wave, hyperbolic radio 

 aid to navigation in which a receiver measures 

 and indicates the relative phase differences be- 

 tween signals received from two or more 

 synchronized ground stations. 



45 



