the sonar set and performs the time measuring 

 function. (35) 



PRECISION GRAPHIC RECORDER (PGR) . This Instrument 

 is similar to the Precision Depth Recorder, but it 

 is considered to be more versatile in that it has 

 many scale and depth combinations readily available. 

 However, the PGR is also rather more complex than 

 the PDR and It uses a helix instead of the stylus 

 of the PDR for recording. 



PRESSURE COMPONENT . Any change in velocity with 

 depth not accounted for by a change in temperature 

 or in salinity may best be described as the pressure 

 component of the velocity gradient. (4) 



PRESSURE GAGE . A tide gage that is operated by 

 the change in pressure at the bottom of a body of 

 water due to the rise and fall of the tide. It 

 has sometimes been used for tide observations on 

 shoals at some distance from land. (14) 



PRESSURE-GRADIENT HYDROPHONES . A pressure-gradient 

 hydrophone is a hydrophone in which the electric 

 output substantially corresponds to a component of 

 the gradient (space derivative) of the sound 

 pressure. (1) 



PRESSURE-GRADIENT TRANSDUCER . Transducer, such as 

 a moving-ribbon hydrophone, in which the moving 

 element responds to pressure difference rather than 

 to pressure. (6) 



PRESSURE HYDROPHONE . A pressure hydrophone in which 

 the electric output substantially corresponds to the 

 instantaneous sound pressure of the impressed sound 

 wave. (1) 



PRESSURE ICE . Ice having any readily observed 

 roughness of the surface. FLAT ICE is the result 

 of undisturbed ice growth and development. Such 

 disturbed development is the result of wind, cur- 

 rent, tide, and/or temperature change. Types of 

 pressure ice are: RAFTED, RIDGED, HUMMOCKED, 

 TENTED, ROPAK, and WEATHERED. (25) 



PRESSURE IN THE OCEAN (General) . In oceanographic 

 work pressure is measured in atmospheres or in units 

 of the centimeter-gram-second system. An atmosphere 

 is defined as the pressure exerted per square cen- 

 timeter by a column of mercury 760 mm high at a 

 temperature of degrees C, where the acceleration 

 of gravity is 980. 665 cm/sec^. In chemical ocean- 

 ography a related unit, the Torr, is used which 

 equals the pressure exerted per square centimeter 

 by a column of mercury 1 mm high at a temperature 

 of degrees C and at the above-mentioned acceler- 

 ation of gravity. The c.g.s. unit of pressure is 

 dyne/cm^, and 1 atmosphere is equal to 1.0133 X 10° 

 dynes/cm^. One million dynes/cm^ was designated as 

 1 bar by V. Bjerknes. The corresponding practical 

 unit used in physical oceanography is 1 decibar, 

 which equals 0.1 bar. The pressure exerted per 

 square centimeter by 1 m of sea water very nearly 

 equals 1 decibar; that is, the hydrostatic pres- 

 sure in the sea increases by 1 decibar for ap- 

 proximately every meter of depth. Therefore, the 

 depth in meters and the pressure in decibars are 

 expressed by nearly the same numerical value. The 

 distribution of temperature and salinity in the 

 ocean, and the slight compressibility of water 

 are among the factors responsible for the small 

 difference in these values. 



In dealing with the pressure in the oceans, 

 the atmospheric pressure is always neglected and 

 the pressure at the sea surface is entered as zero. 

 Since the pressure is essentially a function of 

 depth and the numerical value in decibars nearly 

 equals the depth in meters, the range in pressure 

 will be from zero at the sea surface to over 10,000 

 decibars in the deepest part of the ocean. (12,13) 



PRESSURE-OPERATED PLANKTON NET . This device has 

 a pressure mechanism for opening and closing a 

 plankton sampler using a spring-loaded damper ac- 

 tuated by the pressure element from a conventional 

 900-foot bathythermograph. The sampler fishes 

 between depth Intervals preselected by inserting 

 pins of different lengths into the arresting gear. 

 The first pin governs the point at which the 

 sampler will open as the BT element is compressed 



MRESriNG GEAR 



by the water pressure. The sampler remains open 

 until further pressure releases the second and 

 longer pin. Larger plankton nets are not as easily 

 opened and shut, one of the means that we have 

 considered is to use so-called strangle lines 

 which are successive attachments to the net for 

 pursing it off. Pressure pistons can be used to 

 sever the strangle attachments. 



PRESSURE RIDGE . PRESSURE ICE in the form of a 

 ridge. Pressure ridges may be several miles long 

 and up to 100 feet high. (25) 



PRICE CURRENT METER. The Price current meter was 



originally designed by Assistant Engineer VJ. G. 

 Price, Corps of Engineers, U. S. Army. It is 

 designed to measure current speeds from 0.1 to 11 

 feet per second (0.06 to 6.6 knots). It does not 

 measure current direction. Current direction must 

 be determined by other means when using this in- 

 strument. 



The meter consists of a number of cone-shaped 

 cups mounted on a vertical rotating shaft called 

 the cup shaft. For every 1, 5, or 10 revolutions 

 of the cup shaft, depending on the model being 

 used, an electrical circuit in a contact chamber is 

 closed and transmits a signal to the observer 

 through a 2-conductor electrical cable and a set 

 of earphones. In place of earphones, an automatic 

 recorder with a time-marking system may be con- 

 nected to the circuit. Power for the circuit can 

 be supplied by dry-cell batteries. The number of 

 revolutions of the cup shaft, when applied to the 

 calibration or rating table, indicates the speed of 

 the current. (35) 



PRIMARY LOBES . See LOBES. 



PRIMARY PRESSURE INSTRUMENTS . Primary pressure 

 instruments are manometers and gauges which can 

 be calibrated without reference to another pres- 

 sure measuring instrument. The mercury barometer 

 and dead weight scale are examples of such primary 

 pressure instruments. (35) 



PRIMING OF TIDE . The periodic acceleration in the 

 time of occurrence of high and low waters due to 

 changes in the relative positions of the moon and 

 the sun. (14) 



PRIMITIVE PERIOD . The primitive period of a 

 periodic quantity is the smallest increment of 

 the independent variable for which the function 

 repeats itself. (2) 



PRINCIPAL AXIS ■ The principal axis of a transducer 

 used for sound emission or reception is a reference 

 direction for angular coordinates used in describ- 



S8 



