signals from synchronized transmitters. The system 

 was developed by the British during World War II 

 and is now being used for commercial navigation 

 mostly in Europe. Decca has a range of 300 miles. 

 In the present system the Decca Company has in- 

 corporated a lane identification system which 

 operates automatically. Thus, the navigator can, 

 at a glance, see which lane and fractional part of 

 a lane he is in. (17) 



DECIBEL (db) . The decibel is one tenth of a bel. 

 Thus, the decibel is a unit of level when the base 

 of the logarithm is the tenth root of ten, and the 

 quantities concerned are proportional to power. 

 (1) 



DECLINATION . 1. Angular distance north or south 

 of the celestial equator, taken as positive (+) 

 when north and negative (-) when south of the 

 equator. The sun passes through its declinational 

 cycle once a year, reaching its maximum north 

 declination of 23 1/2° about June 21 and its maxi- 

 mum south declination of -23 1/2° about December 

 21. The moon has an average declinational cycle of 

 27 1/3 days which is called a tropical month. 

 Tides or tidal currents occurring near the times 

 of maximum north or south declination of the moon 

 are called tropic tides or tropic currents and 

 those occurring when the moon is over the equator 

 are called equatorial tides or equatorial currents. 

 The maximum declination which is reached by the 

 moon in successive months depends upon the longitude 

 of the moon's node and varies from 28 1/2° when 

 the longitude of the ascending node is zero to 

 18 1/2° when the longitude of the node is 180° . 

 The node cycle or time required for the node to 

 complete a circuit of 360 of longitude is ap- 

 proximately 18.6 years. (14) 



2. In terrestrial magnetism: at any given 

 location, the angle between the geographical 

 meridian and the MAGNETIC MERIDIAN; that is, the 

 angle between TRUE NORTH and MAGNETIC NORTH. 

 Declination is either "east" or "west" according as 

 the compass needle points to the east or west of 

 the geographical meridian. 



Lines of constant declination are called 

 ISOGONIC LINES and the one of zero declination is 

 called the AGONIC LINE. (24) 



DECLINATIONAL INEQUALITY . See DIURNAL INEQUALITY. 



DECOMPRESSION . The degree of body saturation by 

 various gases is dependent upon time of exposure, 

 depth of dive and circulatory efficiency. In order 

 to bring a diver safely to the surface, time for 

 this gas to escape without bubble formation in the 

 body tissues must be allowed. The escape of this 

 gas is called decompression and the time required 

 for the process is known as decompression time. 

 (37) 



DECOMPRESSION SICKNESS (OR BENDS) . Dissolved 

 nitrogen in the blood and tissues is only slowly 

 gained or lost by the body, and tends to remain in 

 solution except during rapid changes from greater 

 to lesser pressures. The dissolved nitrogen will 

 come out of solution and form bubbles in the blood 

 and other tissues in the same way that bubbles are 

 formed in carbonated beverages when they are decom- 

 pressed by removal of the cork. Nitrogen which 

 comes out of solution under these conditions may 

 form sufficiently large and numerous bubbles to 

 cause obstruction to blood flow in small vessels, 

 painful distention of tendons, joint tissues, 

 muscles, eld scars, fractures, etc. If the ascent 

 is sufficiently slow the excess nitrogen will be 

 harmlessly eliminated by the lungs and bends will 

 not occ ur. For each depth down to 130 feet there 

 is a diving duration which can be accomplished 

 without need for decompression other than that 

 accomplished by normal ascent rate. The danger of 

 bends during decompression is increased by ex- 

 posure to greater depths, by exposure over longer 

 periods of time, and by too rapid ascent. There 



is little tendency for bends on dives less than 

 40 feet, even while breathing air; if deeper div- 

 ing is to be performed using air, slow and regu- 

 lated ascent must be carried out. This requires 

 the swimmer to have a portable gas supply which 

 will provide not only for the period of work at 

 depth, but also for the period of slow decompres- 

 sion as he returns to the surface . Navy decom- 

 pression tables should be strictly observed in 

 regulating decompression. Pain evoked by too rapid 

 ascent can be eliminated by redescent or recompres- 

 sion in a chamber if available. This increase In 

 pressure will force the bubbles back into solution. 

 (37) 



DECTRA (TRUNK ROUTE DECCA) . Dectra is a navigation 

 position-fixing system intended for a specific 

 route such as a long ocean crossing. The system is 

 similar to the standard DECCA system. (35) 



DEEP . A relatively small area of exceptional depth 

 found in a depression. The term is generally 

 restricted to depths greater than 3,000 fathoms. 

 (27) 



DEEP EASTERLIES . See EQUATORIAL EASTERLIES. 



DEEPS . Those depths below 6000 m. 



DEEP SCATTERING LAYER (DSL) . Term applied to 

 widespread strata in the ocean which scatter or 

 return vertically directed sound such as in the 

 case of echo sounding. These layers, which are 

 evidently of biological origin, are located In 

 depths ranging from 150 to 200 fathoms during the 

 day with most of them migrating to or near the 

 surface during the night. 



DEEP-SEA CHANNEL . An elongate valley that cuts 

 slightly below the surface of many of the deep-sea 

 fans and may extend out to the basin floor. (27) 



DEEP-SEA FLOOR ACCRETIONS . Materials on the deep- 

 sea floor of possible economic interest, with rough 

 estimates of total reserves are: 



Tonnage Elements of 

 Material Estimates Interest 



Manganese nodules - 10 



12 



10 



10 



10^^ 

 10l3 



10 



15 



Phosphorite nodules- 

 Globigerina Ooze 

 Dlatomaceous Ooze 

 Red Clay 

 Barium sulfate 



concentrations 

 Magnetic Spherules - 



DEEP SEA LEAD . A heavy sounding lead (about 30 to 

 100 pounds), usually having a line 100 fathoms or 

 more in length. A light deep sea lead is sometimes 

 called a coasting lead. Sometimes called dipsey 

 lead. (17) 



DEEP-SEA SYSTEM . See BENTHIC DIVISION. 



DEEP-SEA TERRACE . A benchlike feature bordering 

 an elevation of the deep-sea floor at depths 

 generally greater than 300 fathoms. (27) 



DEEP TRADES . See EQUATORIAL EASTERLIES. 



DEEP WATER . Water of depth such that surface 

 waves are little affected by conditions on the 

 ocean bottom. It is customary to consider water 

 deeper than one-half the surface wave length as 

 deep water. (11) 



DEEP-WATER WAVE (SHORT WAVE. STOKES IAN WAVE) . A 

 surface wave the length of which is less than 



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