PER MILLE 



path function — The radiance per unit length in 

 tlie direction of the line of sight, generated by 

 the scattered light of a beam. 



peak — ( or sea peak ). See seamount. 



peak sound pressure — The peak sound pressure 

 for any specified time interval is the maximum 

 absolute value of the instanteous sound pres- 

 sure in that interval. (6) 



peak-to-peak value — For an oscillating quantity, 

 the algebraic difference between the extremes of 

 the quantity. (6) 



Pearly Nautilus — See nautilus. 



pebbles — Usually smooth and rounded stones 

 ranging in diameter between 4 and 64 millime- 

 ters. (2) See phi grade scale. 



pedicellaria — One of the minute pincerlike ap- 

 pendages of sea urchins (and some starfishes), 

 borne abundantly on the test between the spines, 

 and consisting of three movable jaws (rarely 

 two, four, or five) mounted on a stalk. In some 

 species these organs are provided with poison 

 glands. 



pelagic — See pelagic division. 



pelagic-abyssal sediments — Deep sea sediments 

 that are fi'ee of terrestrial material except for a 

 small proportion of very fine clay. 



pelagic division — A primary division of the sea 

 which includes the whole mass of water. The 

 division is made up of the neritic pro^'ince which 

 includes the water shallower than 100 fathoms 

 (200 meters), and the oceanic province which 

 includes that water deeper than 100 fathoms. 

 {See figure for classification of marine 

 environments.) 



pelagic limestone — A rock formed principally of 

 the calcareous tests of pelagic forminifera. It 

 usually is deposited in deep water. 



pelagite — See nodules. 



pelecypod — See bivalve. 



pelite — (or mudstone, pelyte) . Clastic sediments 

 composed of clay, minute particles of quartz, 

 rock flour, or volcanic ash (pelitic tuff) and 

 which may be calcareous. 



peltic tuff — See pelite. 



pelyte — See pelite. 



pendulum — 1. A body so suspended from a fixed 

 point as to swing freely to and fro under the 

 combined action of gravity and momentum. 



2. A vertical bar so supported from below 

 by a stiff spring as to vibrate to and fro under 

 the combined action of gravity and the restoring 

 force of the spring. 



peninsula — A body of land nearly surrounded by 

 water and connected with a large body by a neck 

 or isthmus; also any piece of land jutting out 

 into the water. (2) 



penknife ice — See candle ice. 



percolation — The process by whicli water is forced 

 by wave action thi-ough the interstices of the 

 bottom sediment and has a tendency to i-educe 

 wave heights. 



percolation factor — A quantity by wliich Jhe 

 wave heights are reduced through percolation 

 alone. 



perennial ice — Sea ice more than two years old. 



(59) 



performance figure — A numerical value (in deci- 

 bels) given to illustrate the operational effecti\e- 

 ness of a sonar device, that is, the source level 

 minus the background noise level. 



perigean range — (abbreviated Pn) . The average 

 of all monthly tide ranges occurring at the time 

 of perigee. It is larger than the mean I'ange, 

 wliere the type of tide is either semidiurnal or 

 mixed, and is of no practical significance where 

 the type of tide is diurnal. (73) 



perigean tidal currents — Tidal currents of in- 

 creased speed occurring at the time of perigean 

 tides. (68) 



perigean tide — Tides of increased range occurring 

 monthly near the time of the moon's perigee. 



perigee — The point in the orbit of the moon (or 

 any other earth satellite) nearest to the earth; 

 opposed to apogee. (50) 



perihelion — The point in the earth's orbit nearest 

 to the sun. (50) 



periodic current — See tidal current, reversing 

 current. 



periscope depth range — (abbreviated PDR). 

 The maximum range at which active sonar echo 

 ranging contact can be made with a submarine 

 operating at periscope depth. 



permafrost — 1. A layer of soil or bedrock at a 

 variable depth beneath the surface of the earth 

 in which the temperature has been below freez- 

 ing continuously from a few to several thou- 

 sands of years. Permafrost exists where the 

 summer heating fails to descend to the base of 

 the layer of frozen ground. A continuous 

 stratum of permafrost is found where the annual 

 mean temperature is below about 23°F. 



2. As limited in application by P. F. Svetsov : 

 soil -which is known to have been frozen for at 

 least a century. 



(5) 

 permanent aurora — See airglow. 



permanent current — A current such as the Kuro- 

 shio, which flows continuously and whose speed 

 and direction is little changed by the tide and 

 meteorological factors. A permanent current 

 also includes the continuous outflow of fresh 

 river water discharge. (50). See also ocean 

 current. 



permanent ice foot — An ice foot that does not 

 melt completely in summer. (68) 



permanent plankton — See holoplankton. 



per mille — (symliol °/oo)- Per thousand or lO"": 

 used in the same way as percent (7*, per hun- 

 dred or 10"-). Per mille (by weight) as com- 

 monly vised in oceanography for salinity and 

 chlorinity; for example, a salinity of 0.03452 

 (or 3.452 percent) is commonly stated as 34.52 

 per mille (parts per thousand). (5) 



121 



