see SPRING, NEAP, PERIGEAN, APOGEAN, and TROPIC 

 TIDES. (14) 



RANGE RESOLUTION . The minimum range separation of 

 two targets, on the same bearing, for which the two 

 are Individually detectable. (5) 



RAPHE . The median line or slit of a valve of a 

 DIATOM. (20) 



"RAPTURES OF THE DEEP" . Inert gas narcosis. See 

 NITROGEN NARCOSIS. 



RASTER . The rectangular pattern developed on a CRO 

 screen by the combined effects of the horizontal 

 and vertical sawtooth sweeps. (5) 



RATE OF DECAY . The rate of decay is the time rate 

 at which the sound pressure level, or any other 

 stated characteristic, decreases at a given point 

 in a given time. A commonly used unit to express 

 the rate of decay is the decibel per second. (9) 



RATE RANGE . Rate of change of range between own 

 ship and target. (5) 



RAWIN . Radar wind sounding or the determination 

 of winds by the radar observation of a balloon. 



RAWINSONDE . RADIOSONDE and RAWIN methods combined. 

 Rawinsonde is an electronic means of observing tem- 

 perature, humidity, pressure and winds. 



RAYDIST (TYPES E. R. ER. AND N) . Raydist is a radio 

 system for medium range precision surveying in 

 which the phases of two continuous -wave signals ar6 

 compared. It is based on the heterodyne principle 

 and uses low or medium frequencies. It requires a 

 minimum number of frequencies and these frequencies 

 usually need bear no fixed relationship with each 

 other. 



A number of designs of the Raydist system 

 enable position lines of various configurations, 

 such as circular, hyperbolic, and elliptic. The 

 mobile portion of the apparatus can be made very 

 light and the ground equipment both highly trans- 

 portable and free from complex or bulky antenna 

 structures . It operates automatically and requires 

 no specially trained personnel. Its range, due to 

 the waves used, is not limited to line-of sight 

 operation. The range varies for each type, depend- 

 ing upon the power of the transmitters. (29) 



RAYLEIGH WAVE . A Rayleigh wave is a surface wave 

 associated with the free boundary of a solid. The 

 wave is of maximum Intensity at the surface and 

 diminishes quite rapidly as one proceeds into the 

 solid. Hence, it has a tendency to hug the surface 

 of the solid. Such waves have been used quite 

 effectively in detecting surface cracks and flaws 

 in castings . (9) 



RAY PATH . The energy associated with a point on a 

 wave front moves along an Imaginary line known as a 

 ray path. The ray paths encountered in acoustics, 

 which are commonly called sound rays , are analogous 

 to the light rays of optics . Ray paths and wave 

 fronts are mutually perpendicular. (4) 



RDX. A type of high explosive. 



REAR RESPONSE . The maximum pressure with +60° from 

 the rear of the transducer in db relative to the 

 pressure on the acoustic axis. (6) 



RECEIVING BAND RESPONSE . The receiving band re- 

 sponse of a hydrophone, for a given bearing and for 

 a given frequency band, may be defined as the re- 

 sponse measured by the quotient obtained by divid- 

 ing (1) the available power of the electric energy 

 generated by the hydrophone in the given frequency 

 band when receiving over the given bearing plane 

 acoustic waves occupying the same frequency band by 



(2) the free-field intensity of these received 

 acoustic waves. (4) 



RECEIVING DIRECTIVITY FACTOR . The receiving direc- 

 tivity factor of a sonar transducer, for a speci- 

 fied frequency, is the ratio of (1) the available 

 power per unit band of the electric energy gener- 

 ated at the specified frequency by the transducer 

 when receiving over all bearings and at the speci- 

 fied frequency acoustic energy having a given free- 

 field intensity per unit band which is the same for 

 all bearings to (2) the available power per unit 

 band of the electric energy which would be gener- 

 ated at the same frequency and when receiving the 

 same acoustic energy over the same bearings, if 

 the receiving response of the transducer for that 

 frequency were the same for any bearing as for the 

 maximum response reference bearing. (4) 



RECEIVING RESPONSE . The receiving response of a 

 hydrophone, for a given bearing and for a given 

 frequency, may be defined as the response measured 

 by the quotient obtained by dividing (1) the avail- 

 able power of the sinusoidal electric waves of the 

 given frequency generated in the hydrophone when 

 receiving over the given bearing plane sinusoidal 

 acoustic waves of the same frequency by (2) the 

 free-field Intensity of these received acoustic 

 waves . (4) 



RECESSION (OF A BEACH) (RETROGRESSION) . 



1. A continuing landward movement of the shore 



line. 



RECIPROCAL TRANSDUCER . A reciprocal transducer is 

 a transducer that satisfies the principle of 

 reciprocity. (1) 



RECIPROCITY CALIBRATIONS ■ By means of the recipro- 

 city relation it is possible to measure the pro- 

 jector loss and the hydrophone loss of a reversible 

 transducer by comparisons with the known transmis- 

 sion losses of an electric network without knowing 

 the actual value of either the electric power or 

 the acoustic power. Measurements made in this 

 manner are known as reciprocity calibrations. (4) 



RECIPROCITY RANGE . The reciprocity range is the 

 frequency range over which a reversible transducer 

 obeys the reciprocity principle; i. e., the fre- 

 quency range over which It may be used as the 

 reversible transducer in reciprocity calibration 

 measurements. (1) 



RECIPROCITY THEOREM . The reciprocity theorem 

 states that if, in any electric network composed 

 of linear elements, a given electromotive force 

 applied between two given terminals produces a 

 current at a point in some branch of the network 

 then the same voltage acting at this second point 

 in the network will produce the same current be- 

 tween the two original terminals if they are short- 

 circuited. (4) 



RECOGNITION DIFFERENTIAL . The recognition differ- 

 ential for a specified listening system is that 

 amount by which the signal level exceeds the noise 

 level presented to the ear when there is a fifty 

 per cent probability of detection of the signal. 

 This quantity is most valuable in evaluating the 

 effectiveness of military communication channels, 

 since it is most Important that no order be mis- 

 understood. (9) 



RECORDING FYRHEL IOMETER . An instrument used to 

 provide information on the heat budget between 

 atmosphere and ocean. 



The pyrheliometer is a thermopile enclosed in 

 a glass bulb and measures radiation incident on a 

 horizontal surface. The receiving surface consists 

 of two flat concentric disks, a black disk forming 

 an absorbing surface and a white disk forming a 

 reflecting surface. The resulting temperature dif- 



97 



