ABYSSOBENTHIC 



abyssobenthic — See abyssal. 



abyssopelagic — Pertaining to tliat portion of the 

 ocean which lies below depths of 2,000 fathoms 

 (3,700 meters). {See figure for classification 

 of marine environments.) 



acceleration — The rate of change with time of 

 speed and/or velocity ; strictly, the rate of change 

 with time of the velocity of a particle. (5) 

 In the cgs system of physical measurements, it 

 is expressed in terms of centimeters per second 

 per second. See gal. (37) 



acceleration of gravity — The acceleration of a 

 freely falling body due to the gravitational at- 

 traction of the earth. Its true value varies with 

 latitude, altitude, and the nature of the under- 

 lying rocks. 



accelerometer — A device which measures the 

 forces of acceleration acting on a body within 

 the instrument. Among many uses it can be 

 used to measure wave effect on a ship at sea. 



accepted depth — (sometimes called observed 

 depth) . The best possible determination of the 

 true depth of each Nansen bottle at the time of 

 reversal. (67) 



accretion — 1. Natural accretion is the gradual 

 build-up of land over a long period of time 

 solely by the action of the forces of nature, on a 

 beach by deposition of water or airborne ma- 

 terial. Artificial accretion is a similar build-up 

 of land by reason of an act of man, such as the 

 accretion formed by a groin, breakwater, or 

 beach fill deposited by mechanical means. (61) 

 See aggradation. 



2. The process by which inorganic masses 

 grow larger, by the addition of fresh particles to 

 the outside. (2) 



accretionary limestone — A limestone which has 

 formed in situ by slow accumulation of organic 

 remains such as coral or shells. (2) See bio- 

 strome, bioherm, organic reef. 



accumulation — In glaciology, the quantity of 

 snow or other solid form of water added to a 

 glacier or snowfield by alimentation; the op- 

 posite of ablation. (5) 



accuracy — The degree of conformity with a stand- 

 ard. Accuracy relates to the quality of a result, 

 and is distinguished from precision which re- 

 lates to the quality of the operation by which the 

 result is obtained. (37) 



acicular ice — (also called fibrous ice, satin ice). 

 Fresh water ice consisting of numerous long 

 crystals and hollow tubes having variable form, 

 layered arrangement, and a content of air bub- 

 bles. This ice often forms at the bottom of an 

 ice layer near its contact with water. (59) 



acid rock — Igneous rock containing a high pro- 

 portion of silica, contrasted with basic rock in a 

 two-division classification of rocks. (2) 



aclinic line — (or dip equator, magnetic equator). 

 The line through those points on the earth's sur- 

 face at which the magnetic inclination is zero. 



The aclinic line is a particular case of an iso- 

 clinic line. 



In South America the aclinic line lies at about 

 15° S; while from central Africa to about Viet- 

 nam it coincides approximately with the parallel 

 oflO°N. (5) 



acorn barnacle — (or rock barnacle). A barna- 

 cle (Blanidae) whose shell is attached or 

 cemented directly to a firm surface. 



acoustic, acoustical — These two qualifying ad- 

 jectives can be confused and, in fact, are often 

 misused. The qualifying adjective acoustic is 

 used when the term which it modifies desig- 

 nates something which has the properties, 

 dimensions, or physical characteristics, asso- 

 ciated with sound waves. The adjective acousti- 

 cal, on the other hand, is used when the term 

 being qualified does not innately contain some 

 property, dimension, or physical characteristic 

 which is intimately associated with sound. 

 Thus, we speak of an acoustic impedance, but 

 we speak of the Acoustical Society of America. 

 (3) 



acoustic bearing — See sonic bearing. 



acoustic dispersion — 1. The scattering or 

 spreading of sound with frequency. 



2. The separation of a complex sound wave 

 into its various frequency components, usually 

 caused by a variation with frequency of the 

 wave velocity of the medium. The rate of 

 change of the velocity with frequency is used as 

 a measure of the dispersion. (6) 



acoustic impedance — For a given surface area of 

 an acoustic medium perpendicular, at every 

 point, to the direction of propagation of 

 sinusoidal acoustic waves of given frequency, 

 and having equal acoustic pressures and equal 

 volume velocities per unit area at every point 

 of the surface at any instant, the acoustic 

 impedance is the quotient obtained by dividing 

 (1) the phasor corresponding to the acoustic 

 pressure by (2) the phasor corresponding to 

 the volume velocity. (28) 



acoustic intensity — The limit approached by the 

 quotient obtained by dividing the power of the 

 acoustic energy being transmitted at a given 

 time through a given area by the magnitude of 

 this area as the magnitude of this area ap- 

 proaches zero. 



pC 



where intensity, /, in root mean square pressure, 

 P, of a plane wave, p is the density, and c the 

 sound velocity. Units are energy per square 

 centimeter per second. (28) 



acoustic pressure — The difference at a point be- 

 tween the instantaneous sound pressure and the 

 hydrostatic pressure. 



acoustics — The science of sound, including its 

 production, transmission, and effects. 



