ISOSTATIC EQUILIBRIUM 



isostatic equilibrium — See isostasy. 



isostere — A line of equal or constant specific 

 volume. It is equivalent to an isopycnic. (5 ) 



isotac — A line connecting points at which ice 

 melts at the same time of the spring. A line con- 

 necting points at which ice begins to form at the 

 same time of the winter is called an isopectic. 

 (68) 



isotach — A line connecting points of equal rates 

 of speed ; for example, the flow of currents. 



isotherm — ^A line on a chart connecting all points 

 of equal or constant temperature; an isopleth 

 of temperature. ( 5 ) 



isothermal — Of equal or constant temperature, 

 with respect to either space or time. (5) 



isothermal layer — See mixed layer. 



isothermal layer depth — See layer depth. 



isotherm follower — 1. A device which plots the 

 time change in depth of an isotherm on a chart. 

 2. An instrument used to study the movement 

 of subsurface layers of oceanic water. 



isotope — 1. One of several nuclides having the 

 same number of protons in their nuclei, and 

 hence belonging to the same element, but differ- 

 ing in the number of neutrons and therefore in 

 mass number J., or in energy content (isomers) . 

 Small quantitative differences in chemical prop- 

 erties exist between isotopes. 



2. A synonym for isotopic tracer. 



3. A radionuclide or a preparation of an ele- 

 ment with special isotopic composition (allobar) 

 as an article of commerce, so called because of 

 the principal use of such materials as isotopic 

 tracers. 



4. In common usage, synonym for nuclide 

 (not recommended). 



(41) 



isotopic tracer — The isotope or non-natural mix- 

 ture of isotopes of ail element which may be in- 

 corporated into a sample to make possible obser- 

 vation of the course of that element, alone or in 

 combination, through a chemical, biological, or 

 physical process. The observations may be 

 made by measurement of radioactivity or of 

 isotopic abundance. (70) 



isotropic — Having the same physical properties in 

 all directions. (68) 



isovelocity — Having equal values of sound 

 velocity in all parts of a given water column; 

 no change in sound velocity with depth. 



isovelocity conditions — Those conditions which 

 occur when the sound velocity within a layer is 

 constant, that is, the temperature gradient is 

 <0.2°C per 30 meters. 



isthmus — A narrow strip of land, bordered on 

 both sides by water, that connects two larger 

 bodies of land. (2) 



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