THERMOMETER 



terrain correction — (or topographic correction) . 

 A correction apijlied to observecl values obtained 

 in geophysical surveys in order to remove the 

 effect of variations to the observations due to 

 the topography in the vicinity of the sites of 

 observation. 



terrestrial magnetism — See geomagnetism. 



terrestrial radiation — (also called eradhition. 

 eartli radiation) . The total infrared radiation 

 emitted from the eaith's surface including the 

 seas and oceans; to be carefully distinguished 

 from effective terrestrial radiation, atmo- 

 spheric radiation, and insolation. (5) 



terrigenous sediments — Deposits consisting of 

 debris derived from the erosion of land areas 

 and usually deposited in the shallow parts of the 

 sea. 



test — The hard covering or supporting structure 

 of many invertebrates, it may be enclosed 

 within an outer layer of living tissue; a shell. 



(2) 



test board — See fouling panel. 



test panel — s.ee fouling panel. 



test plate — See fouling panel. 



Texas Tower — A fixed tower mounted offshore on 

 the continental shelf or a shoal to provide 

 oceanographic and meteorological observations. 



thalassic rocks — Strata formed in deep, still 

 water far from land, generally composed of very 

 fine grains. (48) 



thallatogenic — The vertical movement of the sea 

 floor. (2) 



thallophyte — One of an artificial grouping (Thal- 

 lophyta) of simple plants lacking true stems, 

 leaves, and roots and generally having one-celled 

 sex organs. The algae, fungi, and bacteria are 

 included in this group. 



thalweg — The line connecting the deepest jDoints 

 of the channel of a sea valley or submarine 

 canyon. 



thawing holes in the ice — Ice pocked with open 

 holes, usually circular. These holes represent 

 a further stage of development of snow waters 

 by ice melting. (74) 



theoretical gravity — The value of gravity calcu- 

 lated for a particular latitude according to an 

 accepted formula such as the 1924 Intematiorial 

 Formula of Gravity. 



thermal — Pertaining to temperature or heat. (5) 



thermal capacity — See heat capacity. 



thermal conductivity — (also called lieat conduc- 

 tivity., coefficient of thermal conduction, coeffi- 

 cient of Ixeat conduction) . The time rate of 

 transfer of heat by conduction, through unit 

 thickness, across unit area for unit difference 

 of temperature. It is measured as calories per 

 second per square centimeter for a thickness of 

 one centimeter and a difference of temperature 

 of 1°C. (27) 



thermal energy — See heat. 



thermal equator — See oceanographic equator. 



thermal expansion — That property of a substance 

 which causes it to change its volume with 

 changes in temperature. 



thermal layer — See thermocline. 



thermal noise — A very low level noise produced 

 by molecular movements in the sea. 



thermal structure — The temperature variation 

 with depth in sea water. 



thermal wake — A temperature change produced 

 on the sea surface by ])assage of a submerged 

 submarine; although small, it can sometimes be 

 detected with special sensors. 



thermistor — A thermally sensitive resistor em- 

 ploying a semiconductor material (usually me- 

 tallic oxides) with a large negative resistance- 

 temperature coefficient. The response charac- 

 teristics of thermistors are generally dependent 

 upon ambient conditions, especially temperature, 

 and the nature and purity of the semiconductor 

 material. (34) 



thermistor bolometer — A device which is very 

 sensitive to temperature changes resulting from 

 absorption of long wave infrared radiation. 



thermistor chain — An instrument-carrying chain 

 (up to 1,200 feet long) generally towed astern 

 to get continuous temperature recordings from 

 upper water layere at sea. ( 35 ) 



thermocline — A vertical negative temperature 

 gradient in some layer of a body of water, w'liich 

 IS appreciable greater than the gradients above 

 and below it ; also a layer in which such a gradi- 

 ent occurs. The principal thermoclines in the 

 ocean are either seasonal, due to heating of the 

 surface water in summer, or permanent. (5) 



thermodynamics — The science that treats of the 

 mechanical action or relations of heat. 



thermogram — The record of a thermograph. 

 (5) 



thermograph — A self-recording thermometer. 

 The thermometric element is most commonly 

 either a bimetal strip or a Bourdon tube. (5) 

 In oceanography, the two most commonly 

 used thermographs are the Geodyne* (Bour- 

 don) and Braincon* (mercury thermometer, 

 radioactive source, and film). 



thermohaline — Pertaining to both temperature 

 and salinity acting together; for example, 

 thermohaline circulation. 



thermohaline circulation — Vertical circulation 

 induced by surface cooling, which causes convec- 

 tive overturning and consequent mixing. 



thermohaline convection — Vertical movement of 

 water observed when sea water, because of its 

 decreasing temperature or increasing salinity, 

 becomes heavier than the water underneath it 

 and a disturbed vertical equilibrium results. 

 (25) 



thermometer — An instrument for measuring tem- 

 perature by utilizing the variation of the physi- 

 cal properties of substances according to their 

 thermal states. (5) 



163 



