AUTOCHTHONOUS 



chemical compounds are formed of atoms, the 

 difference between compounds being attributable 

 to the nature, number, and arrangement of their 

 constituent atoms. (27) 



atomic energy — All forms of energy released in 

 the course of nuclear fission or nuclear transfor- 

 mation ( P.L. 703, 83d Congress) . (63) 



atomic weight — The relative weight of the atom 

 on the basis of oxygen as 16. For a pure isotope, 

 the atomic weight rounded off to the nearest 

 integer gives the total nimiber of nucleons 

 (neutrons and protons) making up the atomic 

 nucleus. If these weights are expressed in 

 grams they are called gram atomic weights. 

 (27) 



Note: According to the International Unified 

 Scale of 1961, the relative weight of the atom is 

 based on Carbon as 12. 



attenuance — The sum of absorptance and scat- 

 terance, (8) 



attenuation — 1. In physics, any process in which 

 the flux density, power, amplitude, intensity, 

 illuminance, etc. of a "parallel beam" of energy 

 decreases with increasing distance from the en- 

 ergy source. Attenuation is always due to the 

 action of the transmitting medium itself. It 

 should not be applied to the divergence of flux 

 due to distance alone, as described by the in- 

 verse-square law. (5) 



2. The reduction in sound or light intensity 

 caused by the absorption and scattering of 

 sound or light energy in air or water. 



3. A lessening of the amplitude of a wave with 

 distance from the origin. (66) 



4. The decrease of submarine motion with in- 

 creasing depth. Submarine motion resulting 

 from surface waves attenuates rapidly with 

 depth and practically disappears at a depth 

 equal to a surface wavelength. 



attenuation coefficient — A measure of the space 

 rate of diminution, or attenuation, of any 

 transmitted electromagnetic radiation. This 

 quantity may be identified in a form of Bou- 

 guer's law (or Beer's law) : 



or 



dl=—al dx; 



where / is the flux density at the selected point 

 in space, /» is the flux density at the source, x is 

 the distance from the source, and a is the atten- 

 uation coefficient. 



In general, the attenuation coefficient is speci- 

 fied only when the attenuation is known to be 

 due to both absorption and scattering or when 

 it is impossible to determine which is the cause. 

 More common is the use of either the absorption 

 coefficient or the scattering coefficient. 



Extinction coefficient is a synonymous term, 

 but it is used mostly in reference to visible 

 radiation. (5) 



attenuation constant — (also called attenv/ition 

 factor, decay constant) . A measure of the space 

 rate of attenuation, that is, the rate of flux- 

 density (or power) reduction as energy (visual, 

 electromagnetic, acoustic) propagates from its 

 source. (5) 



attenuation factor — See attenuation constant. 



Atterberg grade scale — (or Atterherg scale). A 

 decimal grade scale for particle size, with 2 

 millimeters as the reference point, and involv- 

 ing the fixed ratio 10. Subdivisions are the 

 geometric means of the grade limits : 0.2, 0.6, 2.0, 

 6.3, 20.0. (2) 



Atterberg limits — The indices (LL, PL) of the 

 water content of a sediment at the boundary be- 

 tween the semiliquid and plastic state (liquid 

 limit) and the plastic and the semisolid state 

 (plastic limit). (31) 



Atterberg scale — See Atterberg grade scale. 



attitude — The position of a body as determined 

 by the inclination of the axes to some frame of 

 reference. If not otherwise specified this frame 

 of reference is fixed to the earth. (63) 



audio frequency — (also called sonic freqency). 

 A frequency within the audible range, about 20 

 to 20,000 cycles per second. (68) 



augmenting factor — A factor used in the har- 

 monic analysis of tides or tidal currents. It 

 is a correction factor for tabulating the hourly 

 heights or speeds used in the summation for 

 constituents other than S which do not occur 

 on the exact constituent hours to which they are 

 assigned, but at times may differ by as much as 

 a half hour. (50) 



aural null — A null detected by listening for a 

 minimum signal or the complete absence of an 

 audible signal. (68) 



auricularia larva — The planktonic young of a 

 sea cucumber. 



aurora — The sporadic radiant emission from the 

 upper atmosphere over middle and high lati- 

 tudes. 



In northern latitudes these displays are called 

 aurora borealis, aurora polaris, or northern 

 lights; and in southern latitudes they are called 

 aurora australis. ( 5 ) 



aurora australis — The aurora of southern lati- 

 tudes. (5) 



aurora borealis — (also called aurora polaris, 

 northern lights). The aurora of northern lati- 

 tudes. (5) 



aurora polaris — See aurora, aurora borealis. 



auroral zone — A zone of maximum auroral activ- 

 ity at about 23 degrees from the geomagnetic 

 pole. 



authalic projection — See equal area projection. 



authigenic — A term applied to products of chemi- 

 cal and biochemical action which originated in 

 sediments at the time of or after deposition, and 

 before burial and consolidation, such as calcium 

 carbonate or manganese oxide deposition. (2) 



autochthonous — Of local origin ; indigenous. 



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