the same bearings and at the same frequency when 

 receiving the same electric energy, if the trans- 

 mitting response of the transducer for that frequen- 

 cy were the same for any bearing as for the maximum 

 response reference bearing. 



2. The transmitting directivity factor may 

 also be defined as the ratio of (1) the power of 

 the acoustic energy radiated over all bearings 

 by the transducer when receiving sinusoidal elec- 

 tric waves of the specified frequency and having 

 a given power to (2) the power of the acoustic 

 energy which would be radiated over the same 

 bearings and when receiving the same electric 

 waves, if the transmitting response of the trans- 

 ducer for the specified frequency were the same 

 for any bearing as for the maximum response 

 reference bearing. (4) 



THUNDERSQUALL . See SQUALL. 



TRANS^aTTING SPECTRUM FACTOR . The transmitting 

 spectrum factor of a sonar transducer, for a 

 specified bearing, for a specified frequency band, 

 for a specified distribution of energy in that band, 

 and for a specified reference frequency, is defined 

 as the ratio of (1) the index intensity of the 

 acoustic energy radiated over the specified bearing 

 and in the specified frequency band by the trans- 

 ducer when receiving electric energy having a 

 power per unit band which varies with frequency in 

 the specified manner to (2) the index intensity of 

 the acoustic energy which would be radiated over 

 the same bearing and in the same frequency band 

 when receiving the same electric energy if the 

 transmitting response of the transducer for that 

 bearing were the same for any frequency as for the 

 specified reference frequency. (4) 



plify operation and provide better linearity. In 

 addition to the blue and red filters on the dual- 

 filter hydrophotometer, a green filter is used on 

 the tri-f liter hydrophotometer. This instrument 

 is designed for use In turbid estuarine and coastal 

 regions and is not considered suitable for deep 

 water transparency measurements . (35) 



TROCHOIDAL WAVE . A progressive oscillatory wave 

 whose form is that of a prolate cycloid or tro- 

 choid. It is approximated by waves of small ampli- 

 tude. (11) 



TROCHOPHORE . An invertebrate larva, coiranonly pear- 

 shaped and with an equatorial band of CELIA. (19) 



TROCHOID . As used in connection with wave motion, 

 a curve described by a point within a circle that 

 rolls along a straight line. The curve is also 

 known as a prolate cycloid. The cross section of 

 an ocean wave approximates in form a trochoid 

 described with the rolling circle below the straight 

 line. The resulting form has a comparatively sharp 

 crest and flattened trough. (14) 



TROPICAL MONTH . See DECLINATION. 



TROPICAL SUBMERGENCE . See BIPOLARITY. 



TROPIC CURRENTS . Tidal currents occurring semi- 

 monthly when the effect of the moon's maximum de- 

 clination is greatest. At these times the tendency 

 of the moon to produce a diurnal Inequality in the 

 current is at a maximum. (14) 



TROPIC HIGH WATER INEQUALITY . See DIURNAL INEQUAL- 

 ITY. 



TRANSPARENCY . The ability of water to transmit 

 light of different wave lengths, usually measured 

 in per cent of radiation which penetrates a dis- 

 tance of 1 meter. Transparency is the average of 

 the depths at which a SECCHI DISK (about 30 centi- 

 meters in diameter) disappears and reappears. (15) 



TRANSPONDER . An electronic device that receives 

 a challenging signal and automatically transmits 

 a response. The transponder consists of a receiver, 

 which receives the signal impulses, and a responder 

 (or transmitter) that returns signal impulses to 

 the interrogator-responser. (42) 



TRANS -TIDAL WAVES . Those waves having periods of 

 24 hours and greater. 



TRANSVERSE BAR . A BAR which extends at approximate- 

 ly right angles to the shoreline. (27) 



TRANSVERSE WAVE (DISTORTIONAL WAVE) . A wave in 

 which the direction of propagation of the wave is 

 normal to the displacements of the medium, e.g., 

 a vibrating string. The GRAVITY WAVE in which 

 fluid parcels move in circular orbits is an example 

 of a mixed transverse-longitudinal wave. The 

 Rossby wave is also mixed, except in the case of 

 zero current speed, when it is a transverse wave. 

 (12) 



TRAWL WINCH. DEEP SEA . A shipboard winch capable 

 of paying out 3000 fathoms of cable in 30 minutes, 

 it measures the cable tension and can be operated 

 so as to recover 3000 fathoms of cable in 90 

 minutes with a ton on each end of the line, (35) 



TRENCH . A long and narrow depression with relative- 

 ly steep sides. 



TRIAXON. See SPICULE. 



TRI-FILTER HYDROPHOTOMETER . This device is basic- 

 ally identical to the DUAL-FILTER HYDROPHOTOMETER. 

 It is, however, lighter and easier to maintain. 

 The electrical circuit has been redesigned to sim- 



TROPIC INEQUALITIES . Tropic high water inequality 

 is the average difference between the two high 

 waters of the day at the times of TROPIC TIDES. 

 Tropic low water inequality is the average differ- 

 ence between the two low waters of the day at the 

 times of tropic tides. Those terms are applicable 

 only when the type of tide is semidiurnal or mixed. 

 See TROPIC TIDES. (14) 



TROPIC LOW WATER INEQUALITY . 

 EQUALITY . 



See DIURNAL IN- 



TROPIC RANGE. See GREAT TROPIC RANGE. 



TROPIC TIDES . Tides occurring semimonthly when 

 the effect of the moon's maximum declination is 

 greatest. At these times there is a tendency for 

 an increase in the diurnal range. The tidal planes 

 pertaining to the tropic tides are designated as 

 tropic higher high water, tropic lower high water, 

 tropic higher low water, and tropic lower low water. 

 (14) 



TROPIC VELOCITY . The greater flood or greater ebb 

 velocity at the time of TROPIC CURRENTS. (14) 



TROPO PAUSE . The boundary or transition zone 

 between the troposphere and the stratosphere. (42) 



TROPOSPHERE . The lower layer of the earth's at- 

 mosphere, extending from the surface of the earth 

 to an altitude of ten miles. Although the com- 

 position of the air remains more or less constant, 

 its density decreases rather rapidly with altitude; 

 757=, of the atmosphere's weight is found in the 

 troposphere. (42) 



TROUGH . 1. In meteorology, an elongated area of 

 relatively low atmospheric pressure; the opposite 

 of a RIDGE. The axis of a trough is the trough 

 line. 



This term is commonly used to distinguish the 

 above from the closed circulation of a low (or 

 cyclone); but a large-scale trough may include one 

 or more lows, an upper-air trough may be associated 



122 



