twice the depth of the water. When this relation- 

 ship exists the following approximation is valid: 



where c is the wave velocity, g is the acceleration 

 of gravity, and L is the wave length. Thus, the 

 velocity of deep-water waves is independent of the 

 depth of the water. (24) 



DEER . Directional Explosive Echo Ranging. 



PEG . U. S. Navy ship designation for a 

 Missile Escort. 



Guided 



DELRAC . Delrac, a navigation system, resembles the 

 standard DECCA Navigator. In Delrac, a network of 

 hyperbolic lines are laid down in space from fixed 

 transmitting stations. Instead of the star forma- 

 tion, however, the stations work in pairs, one 

 master to each slave, on very long baselines (750 

 to 1000 miles). Present Decca Navigator trans- 

 missions are on different, but related frequencies, 

 so that the receiver can discriminate between them, 

 and the signals can be brought in the receiver to 

 a common comparison frequency. 



In Delrac, the necessary common frequency is 

 produced at the source, by sending a short burst 

 of the same frequency, first from the master and 

 then from the slave. A memory circuit in the 

 receiver stores these signals and enables their 

 phase to be compared. This time sharing technique 

 embraces two or three pairs of a group of stations. 



Each pair of stations has an estimated range 

 of 3000 miles even in regions of high atmospheric 

 noise, and the overall accuracy of fixing is es- 

 timated at 10 miles or better over the entire 

 coverage at the 957,, level of probability. The use 

 of the time-sharing process makes it possible to 

 accomodate 21 pairs of Delrac stations (more than 

 enough to cover the whole world) in the 10 to 14 

 kilocycles band. (29) 



DELTA . An area of alluvial deposit, usually trian- 

 gular in outline, near the mouth of a river. (27) 



DELTAIC DEPOSITS . Sedimentary deposits laid down 

 in a river delta. (27) 



DENARY BAND . A band having a band ratio (See) of 

 f^j/fg" 10 is properly described as a denary band. 

 This band is often spoken of, incorrectly, as a 

 "frequency decade." (4) 



DENSIGRAPH . An instrument that continuously 

 measures density of sea water. 



DENSITOMETER . An instrument for measuring fluid 

 density. 



DENSITY CURRENT . See TURBIDITY CURRENT. 



DENSITY OF ICE . The density of fresh-water ice at 

 its freezing point is 0.917. Newly-formed sea ice, 

 due to its salt content, is more dense, 0.925 being 

 a representative value. The density decreases as 

 the ice freshens. By the time it has shed most of 

 its salt, sea ice is less dense than fresh-water 

 ice, because ice formed in the sea contains more 

 air bubbles. Ice having no salt but containing 

 air to the extent of eight per cent by volume (an 

 approximately maximum value for sea ice) has a den- 

 sity of 0.845. (12) 



DENSITY OF SEA WATER . The weight of a given volume 

 of sea water at a specified temperature as compared 

 with the weight of the same volume of fresh water 

 at a temperature of 4°C. As the weight of a given 

 volume of sea watTer will vary with its own tempera- 

 ture It is now the practice of the Coast and 

 Geodetic Survey to adopt 15°C. as the standard 

 temperature for sea water for the purpose of com- 

 parison and the hydrometers used for the observa- 

 tions are graduated accordingly. The average den- 



sity of sea water is approximately 1.026 at a tem- 

 perature of 15°C. (14) 



DEPTH ANGLE . The angle between the horizontal and 

 the bearing of the submerged target as seen from 

 own ship. (7) 



DEPTH, CONTROLLING . The least depth of water in 

 the navigable parts of a waterway, which limits 

 the allowable draft of vessels. (11) 



DEPTH FACTOR . See SHOALING COEFFICIENT. 



DEPTH RECORDER . This instrument is used to obtain 

 a pictorial record of the ocean bottom. The device 

 consists primarily of a projector, a hydrophone and 

 a recorder. The recorder controls the keying of 

 the transmitter and the gating of the receiver for 

 precision drive. The recorder's primary function 

 is to display sonic travel time intervals in a 

 precise integrated fashion. A continuous record 

 of the reflecting surface, which is the ocean bot- 

 tom is thus obtained. (30) 



DER . U. S. Navy designation for a Radar Picket 

 Escort Vessel. 



DERELICT . Any property abandoned at sea, often of 

 sufficient size as to constitute a menace to navi- 

 gation; especially an abandoned vessel. See 

 JETTISON. (17) 



DERRICK STONE . Stone of sufficient size as to 

 require handling in individual pieces by mechanical 

 means, generally 1 ton up. (11) 



DESLANT . Destroyer Forces, Atlantic (USN) . 



PES MAS . See SPICULE. 



DETRITUS . Particles worn from rocks by mechani- 

 cal means; also broken organic material. (27) 



DEVIATION LOSS . When relative response is con- 

 sidered as a function of bearing the transmission 

 loss thus measured is known as the deviation loss 

 of the transducer in question. (4) 



DEW POINT . The temperature to which a given parcel 

 of air must be cooled at constant pressure and 

 constant water-vapor content in order for saturation 

 to occur. When this temperature is below 0°C, it 

 is sometimes called the frost point. The dew point 

 may alternatively be defined as: the temperature 

 at which the saturation vapor pressure of the par- 

 cel is equal to the actual vapor pressure of the 

 contained water vapor. 



Isobaric heating or cooling of an air parcel 

 does not alter the value of that parcel's dew point, 

 as long as no vapor is added oi; removed. Therefore, 

 the dew point is a conservative property of air 

 with respect to such processes. However, the dew 

 point is non-conservative with respect to vertical 

 APIABATIC motions of air in the atmosphere. The 

 dew point of ascending moist air decreases at a 

 rate only about one-fifth as great as the dry- 

 adiabatic lapse-rate. 



The dew point of the atmosphere can be deter- 

 mined directly by any of several types of PEW- 

 POINT HYGROMETERS, but it is more commonly deter- 

 mined with the aid of a psychrometric calculator 

 or tables after the direct reading of a PSYCHROM- 

 ETER. (14) 



DEW-POINT HYGROMETER . An instrument for determin- 

 ing the PEW POINT; a type of hygrbmeter. It 

 operates in the following manner. A parcel of air 

 is cooled at constant pressure, usually by contact 

 with a refrigerated polished metal surface. Con- 

 densation appears upon the metal surface at a tem- 

 perature slightly below that of the thermodynamic 

 dew point of the air. The observed dew point will 

 differ from the thermodynamic dew point depending 



33 



