gabbro — See igneous rock. 



gal — A unit of acceleration equal to one centi- 

 meter per second per second ( 1 cm/sec') or 1,000 

 milligals. A milligal is 0.001 gal. The term 

 gal is not an abbreviation. It was invented to 

 honor the memory of Galileo. (37) 



Galofaro — A whirlpool in the Strait of Messina. 

 Formerly called Charybdis. ( 68 ) 



galvanometer — An instrument for measuring a 

 small electric current, or for detecting its pres- 

 ence or direction by means of the movement of a 

 magnetic needle or of a wire, or coil, in a mag- 

 netic field. String (wire) or mirror galvanom- 

 eters are used in oscillographs and other instru- 

 ments of applied geophysics. ( 35 ) 



gam — (or school). A large school of whales 

 (sperm whales travel about in small parties 

 which sometimes join up to form schools, or 

 gams, of considerable size) . 



gametophyte — In the sexual generation of an 

 alga, the male or female plant which produces 

 motile or nonmotile gametes. 



gamma — A small unit of magnetic field intensity 

 generally used in describing the earth's mag- 

 netic field. It is defined as being equal to lO"'^ 

 oersted. 



gamma ray — A quantum of electromagnetic 

 radiation emitted by a nucleus, each such pho- 

 ton being emitted as the result of a quantum 

 transition between two energy levels of the 

 nucleus. Gamma rays have energies usually be- 

 tween 10 thousand-electron-volts and 10 million- 

 electron-volts, with correspondingly short wave- 

 lengths and high frequencies. They are often as- 

 sociated with alpha and beta radioactivity, fol- 

 lowing transitions that leave the product nuclei 

 in excited states ; in general, they are more pen- 

 etrating than alpha or beta particles. They also 

 occur in isomeric transitions and in many in- 

 duced nuclear reactions. A gamma ray spectrum 

 consists of one or more sharp lines, each corre- 

 sponding to an energy and intensity that are 

 characteristic of the source. (41 ) 



gamma ray spectrometer — An electronic device 

 which counts and measures the energy of in- 

 cident gamma rays. These data are presented 

 as the number of gamma ray counts in small 

 energy increments over a selected energy range ; 

 usually to 3 million-electron-volts. Thus the 

 spectra of emitted gamma ray energy from a 

 radioactive substance (such as sea water) can 

 be determined. Since each gamma ray emitting 



radioisotope gives off one or more gamma rays 

 of characteristic energy such measurements per- 

 mit radioisotope identification and quantitative 

 assessment. Techniques such as spectrum 

 stripping are used to separate the spectrum con- 

 tribution of individual radioisotopes in mixtures 

 such as sea water. Some types of spectrometers 

 are the shipboard gamma ray spectrometer and 

 deep underwater nuclear counter (DUNG). 



gamma scintillation spectrometer — See gamma 

 ray spectrometer. 



gap — A steep-sided depression cutting trans- 

 versely across a ridge or rise. ( 62 ) 



gas — A state of matter in which the molecules are 

 practically unrestricted by cohesive forces. A 

 gas has neither shape nor volume. ( 27 ) 



gas bladder — See air bladder. 



gas chromatography — A separation technique 

 involving passage of a gaseous moving phase 

 through a column containing a fixed phase. It 

 includes (1) gas-liquid chromatography (GLC), 

 or more precisely, gas-liquid partition chroma- 

 tography, in which the fixed phase (column 

 packing) is a liquid solvent distributed on an 

 inert solid support; and (2) gas-solid chroma- 

 tography (GSC), in which the fixed phase is a 

 surface-active sorbent (charcoal, silica gel, acti- 

 vated alumina). Gas chromatography is used 

 principally as an analytical technique for the 

 determination of volatile compounds (gases and 

 liquids) with boiling points up to 400°C or even 

 higher. However, it is useful also as a research 

 method for determining certain physical quanti- 

 ties such as distribution or partition coefficients 

 and adsorption isotherms, and as a preparative 

 technique for isolating pure components or cer- 

 tain fractions from complex mixtures. 



gastroliths — Stones which have been transported 

 in the stomach of marine animals such as 

 walruses. 



gastropod — (or snail). One of a class (Gastro- 

 poda) of mollusks in which the animals possess 

 a distinct head, generally with eyes and tentacles, 

 and a broad flat foot and usually are enclosed in 

 a spiral shell. 



gat — A natural or artificial passage or channel 

 extending inland through shoals or steep banks. 



(68) , . , ... 



gauss — ^A unit of magnetic induction (or magnetic 



flux density) equal to one dyne per unit cgs 



magnetic pole. Prior to 1932, the gauss was 



used both as a unit of magnetic induction and as 



68 



