ISLAND SHELF. See INSULAR SHELF. 



ISLAND SLOPE. See INSULAR SLOPE. 



ISOPECTIC . A line connecting points at which ice 

 begins to form at the same time of the vjinter. A 

 line connecting points at which ice melts at the 

 same time of the spring is called an isotac. (17) 



ISOBAR ■ A line connecting points having the same 

 atmospheric pressure reduced to a common datum, 

 usually sea level. (17) 



ISOBATH . Depth contour. (17) 



ISOBATHIC . Having equal depth. (17) 



ISOBATHYTHERM . A line or surface showing the 

 depths in oceans or lakes at which points have 

 the same temperature. Isobathytherms are usually 

 drawn to show cross sections of the water-mass. 

 (24) 



ISOBRONT. A line connecting points at which some 

 specified phase of a thunderstorm occurs at the 

 same time. (17) 



ISOCERAUNIC (ISOKERAUNIC) . Indicating or having 

 equal frequency or intensity of thunderstorms. 

 (17) 



ISOCHASM . A line connecting points having the same 

 average frequency of auroras. (17) 



ISOPLETH . An isogram indicating the variation of 



ISOCHEIM. See ISOTHERE. 



ISOCLINIC LINE . A line drawn through all points 

 on the earth's surface having the same magnetic 

 INCLINATION. The particular isoclinic line drawn 

 through points of zero inclination is given the 

 special name of ACLINIC LINE. (24) 



ISODEE . A line connecting points of equal differ- 

 ence between pressure altitude and absolute alti- 

 tude above sea level. (17) 



ISODYNA>aC LINE . A line connecting points of equal 

 magnetic intensity, either the total or any com- 

 ponent. (17) 



ISOGONIC LINE . In the study of terrestrial mag- 

 netism, a line drawn through all points on the 

 earth's surface having the same magnetic DECLINA- 

 TION; not to be confused with MAGNETIC MERIDIAN. 

 The particular isogonic line drawn through all 

 points having zero declination is called the agonic 

 line. (24) 



ISOHALINE . Having no change in salt content along 

 a given reference plane. (15) 



ISOHEL . A line connecting points having the same 

 amount of sunshine during any specified period. 

 (17) 



an element with respect to two variables, one of 

 which is usually the time of year. The other may 

 be time of day, altitude, or some other variable. 

 (17) 



ISOPYCNIC (ISOPYCNIC LINE) . A line connecting 

 points of equal density, particularly of ocean 

 water. A line connecting points of equal atmospher- 

 ic density may be called an ISOSTERE. (17) 



ISOPYCNIC LINE. See ISOPYCNIC. 



ISOSTASY . A supposed equality existing in vertical 

 sections of the earth, whereby the weight of any 

 column from the surface of the earth to a constant 

 depth is approximately the same as that of any 

 other column of equal area, the equilibrium being 

 maintained by plastic flow of material from one 

 part of the earth to another. (17) 



ISOSTERE . See ISOPYCNIC. 



ISOTAC. See ISOPECTIC. 



ISOTHERE . A line connecting points having the same 

 mean summer temperature. A line connecting points 

 having the same mean winter temperature is called 

 an isocheim. (17) 



ISOTHERM . A line connecting points of equal 

 temperature. (17) 



ISOTHERMAL . Having no change in temperature along 

 a given reference plane,. (15) 



ISOTHERM FOLLOWER . This device directly measures 

 the temperature of internal waves and automatical- 

 ly traces isothermal vertical oscillations with 

 reference to time. This instrument is comprised 

 of (1) a sea sensing unit; (2) an electric winch 

 containing a cable to which the sea sensing unit is 

 attached; (3) electronic components (servo-mecha- 

 nism, amplifiers, etc.) and (4) two recorders 

 (depth and temperature). The sea sensing unit con- 

 tains a thermistor head balanced bridge circuit 

 with a resistance corresponding to the desired 

 isothermal temperature. If the bridge becomes un- 

 balanced, a thyratron tube is fired. This acti- 

 vates a winch and causes it either to wind in or 

 let out the sea unit, "locking it" onto the de- 

 sired isotherm. The isotherm depth is continuously 

 recorded on the ship by means of a pressure sensor 

 in the sea unit. The net result is a trace of the 

 given isotherm depth. (30) 



ISOHYET . A line connecting points having the same 

 amount of precipitation for any specified period. 

 (17) 



ISOKERAUNIC . See ISOCERAUNIC, 



ISONEPH . A line connecting points having the same 

 amount of cloudiness. (17) 



ISOPAG . -A line connecting points where ice is 

 present for the same number of days per year. (17) 



ISOTHERMOBATH . A line connecting points of equal 

 temperature in a verticle plane in the ocean. (17) 



ISOTOPE . A member of an element family which dif- 

 fers from the other members only by its weight . 

 Isotopes of the same element have no chemical dif- 

 ference - they have the same number of electrons 

 and protons but differ in their number of neutrons 

 which accounts for the difference in weight. Some- 

 times this excess or deficiency causes the atom to 

 be unstable or radioactive, the atom is then a 

 radioisotope. There are over 300 stable isotopes 

 in nature. A few elements exist in only one iso- 



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