upon the nature of the condensing surface, the 

 condensation nuclei, and the sensitivity of the 

 condensate-detecting apparatus. (14) 



DEZ INC IF ICAT ION . Corrosion of an alloy containing 

 zinc in which the principal product of corrosion 

 is metallic copper. The mechanism may involve 

 overall corrosion of the alloy followed by re- 

 deposition of the copper from the corrosion 

 products or selective corrosion of zinc (or a high 

 zinc phase) to leave a copper residue. Dezincifi- 

 cation is commonly encountered in brasses that 

 contain more than 157. zinc. (35) 



DG . Directional Gyro. A gyroscopic device used 

 to indicate a selected horizontal direction for a 

 limited time, as during a turn. When used for an 

 extended period, it is checked and reset at fre- 

 quent intervals. (17) 



DHL 



Deutsches Hydrographisches Institute. 



DIKE . An igneous intrusion that cuts across the 

 bedding or other layered structure of the surround- 

 ing rock. (27) 



DIMORPHISM . Existing under two distinct forms. 

 (19) 



DIP . In geology, the angle at which the rock 

 structure is inclined with a horizontal plane. In 

 terrestrial magnetism, the angle formed by the 

 lines of total magnetic force with the horizontal 

 plane at the earth's surface; reckoned positive if 

 downward. In mine warfare, the increase in depth 

 of a moored mine case, due to current force against 

 the case and cable. (27) 



DIP EQUATOR . See ACLINIC LINE. 



DIPLOID . Having two sets of chromosomes; the 2n 

 number characteristic of the sporophyte generation. 

 (18) 



DIAPHONE . A device for producing a distinctive fog 

 signal by means of a slotted reciprocating piston 

 actuated by compressed air. Blasts may consist of 

 two tones of different pitch, in which case the 

 second tone is of lower pitch. Alternate pitch 

 signals are called two tone. A diaphone is some- 

 what similar to a siren, but the diaphone sound is 

 lower and, if of a single tone, ends with an abrupt 

 "grunt" at the end of the blast. (17) 



DIAGENETIC DEPOSITS . Deposits consisting dominant- 

 ly of minerals crystallized out of sea-water, such 

 as manganese nodules. (27) 



DIASTROPHISM . Refers to all processes of earth 

 movement and rock deformation, many of which 

 result in changes of relative position, both ver- 

 tical and horizontal. (27) 



DIATOM . A microscopic alga with an external skele- 

 ton of silica, found in both fresh and salt water. 

 Part of the ocean bed is composed of a sedimentary 

 ooze consisting principally of large collections of 

 the skeletal remains of diatoms. (17) 



DIATOMIN (FHYCOXANTHIN) . A yellow or yellowish 

 brown pigment found in certain algae and diatoms. 

 (20) 



DIELECTRIC . An insulator. A term applied to the 

 insulating material between the plates of a 

 capacitor. (36) 



DIFFERENTIAL SENSITIVITY . The 50 per cent 

 detectable ratio between the sum of echo strength 

 and background noise and the background noise. (5) 



DIFFRACTED WAVE . A diffracted wave is one whose 

 front has been changed in direction by an obstacle 

 or other non-homogeneity in the medium other than 

 by reflection or refraction. (9) 



DIFFRACTION . Diffraction is the name given to 

 that process which allows sound waves to bend 

 around obstacles that are in their path. (9) 



DIFFRACTION OF WATER WAVES . The phenomenon by 

 which energy Is transmitted laterally along a wave 

 crest. When a portion of a train of waves is 

 interrupted by a barrier such as a BREAKWATER, the 

 effect of diffraction is manifested by propagation 

 of waves into the sheltered region within the bar- 

 rier's geometric shadow. (11) 



DIGITAL MODULATION (DM) . Digital modulation, a 

 third method (AM, FM, DM) of radio transmission 

 consisting of on-off pulses of radio energy coded 

 into specific patterns. Among the claimed advan- 

 tages of DM is the ability to collect, transmit 

 and receive data In digitally-coded form without 

 having to modify It for transmission over voice 

 channels as with FM. 



DIP POLE . See MAGNETIC POLE. 



DIRECTIONAL GAIN (DIRECTIVITY INDEX) . The direc- 

 tional gain of a transducer, in decibels, is 10 

 times the logarithm to the base 10 of the direc- 

 tivity factor. See DIRECTIVITY FACTOR. (1) 



DIRECTIONAL HYDROPHONE . A directional hydrophone 

 is a hydrophone the response of which varies sig- 

 nificantly with the direction of sound incident. 

 (1) 



DIRECTIONAL RESPONSE PATTERN (BEAM PATTERN) ■ The 

 directional response pattern of a transducer used 

 for sound emission or reception Is a description, 

 usually presented graphically of the response of 

 the transducer as a function of the direction of 

 the transmitted or Incident sound waves in a 

 specified plane and at a specified frequency. In 

 general, the beam pattern will change with a change 

 in the operating frequency. (9) 



DIRECTIVITY FACTOR . (1) The directivity factor of 

 a transducer used for sound emission is the ratio 

 of the sound pressure squared, at some fixed dis- 

 tance and specified direction, to the mean-square 

 sound pressure at the same distance averaged over 

 all directions from the transducer. The distance 

 must be great enough so that the sound appears to 

 diverge spherically from the effective acoustic 

 center of the sources. Unless otherwise specified, 

 the reference direction is understood to be that 

 of maximum response. (2) The directivity factor 

 of a transducer used for sound reception is the 

 ratio of the square of the open-circuit voltage 

 produced in response to sound waves arriving in a 

 specified direction to the mean-square voltage 

 that would be produced in a perfectly diffused 

 sound field of the same frequency and mean-square 

 sound pressure. (1) 



DIRECTIVITY INDEX . A measure of the directional 

 properties of a transducer. It is the ratio, in 

 decibels, of the average Intensity or response 

 over the whole sphere surrounding the projector or 

 hydrophone to the intensity or response on the 

 acoustic axis. (5) 



DIRECTIVITY PATTERN . A chart in which relative 

 response is plotted as a function of bearing in 

 some specified plane, either against rectangular 

 coordinates or against polar coordinates, is known 

 as a directivity pattern, or as a beam pattern. 

 (4) 



DIRECTIVITY RATIO . A measure of the directional 

 properties of a transducer. It is the numerical 

 ratio of the intensity, or response, on the acous- 

 tic axis to the average intensity, or response, 

 over the whole sphere surrounding the projector, 

 or hydrophone. (7) 



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