pp . Pulse Doppler. 



PPM . Pulse Puration Modulation. 



PDR . Precision Depth Recorder. 



PEAK . A SEAMOUNT rising more than 500 fathoms from 

 the sea floor and having a pointed or round top. 

 (26) 



PEAK LEVEL . The peak level is the maximum instan- 

 taneous level that occurs during a specified time 

 Interval. In acoustics, peak sound pressure level 

 is to be understood, unless some other kind of 

 level is specified. (2) 



PEAK SOUND PRESSURE . The peak sound pressure for 

 any specified time interval is the maximum absolute 

 value of the instantaneous sound pressure in that 

 Interval. (2) 



PEAK-TO-PEAK VALUE . The peak-to-peak value of an 

 oscillating quantity is the algebraic difference 

 between the extremes of the quantity. (2) 



PEBBLES . Small, usually rounded rock fragments 

 ranging in size from 0.16 to 2.59 (4 to 64 milli- 

 meters) in diameter. Pebbles are classed as gravel 

 on bottom sediment charts. 



PELAGIC . Pertaining to the open sea, when used in 

 connection with sediment type; it also refers to 

 fish which do not spend their whole life on the 

 bottom, although they may remain fairly near the 

 shore, such as herrings or sardines. (27) 



PELAGIC DIVISION . A primary division of the sea 

 which includes the whole mass of water. The 

 division is made up of the NERITIC PROVINCE which 

 includes that water shallower than 200 meters, 

 and the OCEANIC PROVINCE which includes that water 

 deeper than 200 meters . 



PELAGIC ORGANISMS . Pertaining to all organisms in- 

 habiting the open sea, except bottom dwellers. 

 (15) 



PELORUS . An instrument used on a boat in connec- 

 tion with a log line to obtain the direction of 

 current. In its simplest form, it is a disk about 

 8 inches in diameter and graduated clockwise for 

 every 5° or 10°. It is mounted rigidly on the boat, 

 usually with the 0° mark forward and the diameter 

 through this mark parallel with the keel of the 

 boat. (14) 



PENINSULA. An elongated portion of land nearly 

 surrounded by water, and connected to a larger body 

 of land. (11) 



PERIGEAN TIDES OR TIDAL CURRENTS . Tides of in- 

 creased range or tidal currents of increased velo- 

 city occurring monthly as the result of the moon 

 being in perigee or nearest the earth. The peri- 

 gean range of tide is the average semidiurnal range 

 occurring at the time of perigean tides and i£ most 

 conveniently computed from the harmonic constants. 

 It is larger than the mean range where the type of 

 tide is either semidiurnal or mixed, and is of no 

 practical significance where the type of tide is 

 diurnal. (14) 



PERIOD. Interval required for the completion of a 

 recurring event, such as the revolution of a celes- 

 tial body, or the time between two consecutive like 

 phases of the tide or current. A period may be ex- 

 pressed in angular measure and is then taken as 

 360°. The word is also used to express any speci- 

 fied duration of time. (14) 



PERIODIC CURRENT . An oscillating current the values 

 of which recur at somewhat regular intervals. (17) 



PERIODIC CURRENT . See OCEAN CURRENT. 



PERMANENT CURRENT . A current that runs continuous- 

 ly independent of the tides and temporary cause. 

 Permanent currents include the fresh water dis- 

 charge of a river and the currents that form the 

 general circulatory systems of the oceans. (14) 



PERMANENT CURRENT . See OCEAN CURRENT. 



PERMANENT ICE FOOT . An ICE FOOT that does not melt 

 completely during the summer months. (25) 



PERMANENT PLANKTON . See MARINE LIFE. 



PERMANENT THERMOCLINE . Frequently used in acoustics 

 to describe the decrease in temperature which always 

 occurs at great depths. (3) 



PERMEABILITY . The property of a material which 

 permits appreciable movement of water through it 

 when saturated and actuated by hydrostatic pressure. 

 (27) 



PERTROGRAPHY . The description and systematic 

 classification of rocks. (11) 



PERU CURRENT (HUMBOLDT CURRENT) . The cold ocean 

 current flowing north along the coasts of Chile 

 and Peru. It is one of the swiftest of ocean cur- 

 rents. The Peru current originates where part of 

 the water that flows toward the east across the 

 subantarctic Pacific Ocean is deflected toward the 

 north as it approaches South America. The north- 

 ern limit of the current can be placed a little 

 south of the equator, where the flow turns toward 

 the west, joining the SOUTH EQUATORIAL CURRENT. 



The southern portion of the Peru current is 

 sometimes called the CHILE CURRENT. (24) 



PETERSEN GRAB . In the Petersen (or van Veen) type 

 of grab two semicircular buckets of varying sizes 

 are hinged along a central axis. The buckets are 

 held apart for lowering to the bottom by some form 

 of catch. On striking the bottom this is released 

 so that on hoisting the buckets move round on their 

 axis, take a bite out of the sediment, and come 

 together to form a closed container. With this con- 

 figuration the rate at which the grab hits the bot- 

 tom affects the bite and when the ship is drifting 

 a poor sample may be obtained if the grab does not 

 hit the bottom vertically. (35) 



PETERSEN (GRAB) 



PERIOD. See PRIMITIVE PERIOD . 



JAW OF CRAB 



89 



