IFS . U. S. Navy designation for an Inshore Fire 

 Support Ship. 



IGC . International Geographical Congress. 



IGIPAS . Interagency Group on International Pro- 

 grams in Atmospheric Science. 



IGN . Institut Geographique Nationale. 



IGNEOUS ROCK . Rock formed by solidification from 

 a molten state. (27) 



IGPP . Institute of Geophysics and Planetary 

 Physics . 



IGU. 1. International Geophysical Union. 

 2. International Geographical Union. 



IGY . International Geophysical Year. 



IHB (BHI) . International Hydrographic Bureau 

 (Monaco) . 



IHP . International Hydrographic Program. 



IIOE . International Indian Ocean Expedition. 



IIP . International Ice Patrol. 



A treaty organization of North Atlantic mari- 

 time nations organized in 1914 to improve safety 

 of navigation in the vicinity of the Grand Banks 

 and along the transatlantic steamship lanes where 

 icebergs are a menace to shipping. Makes oceano- 

 graphic observations during iceberg season and com- 

 putes dynamic currents to predict location of ice- 

 bergs . 



Financing: A share of the cost of the Patrol 

 is charged to member nations at the end of each 

 year, based on their proportionate share of the 

 gross tonnage passing through the iceberg area. 



IMCO. 



International Maritime Consultative Organi- 



IMPEDANCE . If we apply a sinusoidal force at a par- 

 ticular frequency to a system, we can expect some 

 sort of motion to ensue. If the velocity which is 

 produced as a result of the excitation is small, we 

 say that the impedance of the system at the point 

 of application of the force is high; conversely, if 

 the velocity is high, we say that the impedance at 

 the point is low. In order to make this idea quan- 

 titative, we include two notions in the specifica- 

 tion of impedance. The first is the ratio of the 

 magnitude of the exciting force to the magnitude of 

 the resulting velocity. The second is the phase 

 difference between the excitation and resulting 

 motion. These two aspects of impedance are usually 

 represented in the form of a complex number. In 

 general, the impedance will change with a change in 

 frequency of the excitation. (9) 



IMPINGEMENT ATTACK. This is a form of erosion- 



corrosion in which aerated water strikes the sur- 

 face of a metal causing removal of protective films 

 making small anodic areas with respect to adjacent 

 areas. The adjacent areas are depolarized cathodes 

 causing ideal electrochemical circuits. (35) 



IMPULS E ■ Impulse is the product of a force and the 

 time during which the force is applied; more spe- 

 cifically,, the impulse if J ^ Fdt where the force 

 F is time dependent and equal to zero before time 

 t]^ and after t2. (2) 



INAFC . 



International North American Fisheries 



Commission, 



INCLINATION . In terrestrial magnetism, the angle 

 through which a freely suspended magnet would dip 

 below the horizon in the magnetic north-south 

 meridional plane; one of the MAGNETIC ELEMENTS. 



At the ACLINIC LINE the inclination is zero; 

 at either MAGNETIC POLE the inclination is 90°. 

 (24) 



INDEX LEVEL . The index level of a sound is defined 

 as the level which that sound would have at a point 

 one yard from the point of its apparent origin, as- 

 suming such a point to exist, if it were generated 

 at this apparent source point but produced the same 

 effects at distant points as the effects it actual- 

 ly does produce. (4) 



INDIAN SPRING LOW WATER (INDIAN TIDE PLANE, 

 HARMONIC TIDE PLANE) . A datum originated by Prof. 

 G. H. Darwin when investigating the tides of India. 

 It is a plane depressed below mean sea level by an 

 amount equal to the sum of the amplitudes of the 

 harmonic CONSTITUENTS 'M2, S2, Ki, and Ol. (14) 



INDIAN TIDE PLANE. See INDIAN SPRING LOW WATER. 



INDUCTION-CONDUCTIVITY-TEMPERATURE INDICATOR . This 

 device operates through the inductive measurement 

 of an electric current which has been induced in 

 a sea water path. The sensing head consists of 

 two iron-core toroidal windings potted in an in- 

 sulating resin. One winding is excited by a 115- 

 volt, 60-c.p.s. electrical signal; the sea water 

 path through the hole in the center of this toroid 

 acts as a one-turn secondary of a transformer and, 

 consequently, has about 0.2 volt induced in it. 

 The amount of current flowing depends primarily 

 upon the length and diameter of the hole and upon 

 the conductivity of the water in the hole. The 

 current flowing in the sea water is measured by 

 means of the second toroid which is mounted adjacent 

 to and coaxially with the existing toroid. No sea- 

 water-to metal contact exists in this system, so 

 the problems inherent to electrodes are eliminated. 

 This system is essentially independent of line 

 frequency variations over a moderate range of 

 several cycles per second. Stability of the in- 

 strument depends wholly upon the dimensional stabil- 

 ity of the resin "doughnut" and the stability of 

 the electrical components. 



Laboratory tests have shown this device to be 

 capable of measuring conductivity to + 0.02 millim- 

 ho (+ 0.02% of salinity) over the entire ranges of 

 temperature and salinity encountered in estuarine 

 and marine environments. (35) 



INDURATED . Hardened; rocks hardened by heat, pres- 

 sure, or the addition of some ingredient not com- 

 monly contained in the rock referred to; for 

 example, sand indurated by limonite. (27) 



INERTIAL GUIDANCE . Guidance by means of accelera- 

 tions measured and integrated within the craft. 

 (17) 



INERTIAL NAVIGATION . Dead reckoning performed 

 automatically by a device which gives a continuous 

 indication of position by double integration of 

 accelerations since leaving a starting point. (17) 



INFRA-GRAVITY WAVES . Those waves with periods 

 from 30 seconds to 5 minutes. 



INFRARED DETECTORS . Sea surface temperatures have 

 been taken from aircraft by measuring black-body 

 radiation from the surface. Infrared detectors 

 (thermopiles or bolometers) are used in chopped 

 optical systems so that they "look" alternately at 

 the sea surface and a reference black-body in the 

 instrument. (35) 



INFRASONIC FREQUENCY . An infrasonic frequency is a 

 frequency lying below the audio frequency range. 

 Vibrations in this frequency range can be felt but 

 not heard. (9) 



INJECTION SIGNAL . The sawtooth frequency-modulated 



61 



