eleraf?nt) ; (e) the class of instruments which 

 depend upon the diffusion of water vapor through 

 a porous membrane (diffusion hygrometer); and (f) 

 the class of instruments which depend upon measure- 

 ments of the absorption spectra of water vapor 

 (spectral hygrometer). (Middleton, W. E. K., and 

 Spilhaus, A. F., Meteorological Instruments, 3rd 

 ed., rev., 1953, pp. 105-116) (12) 



HYPERPNEA . Abnormally rapid or deep breathing. 



HYPERVENTILATION . Hyperventilation is the term 

 applied to breathing more than is necessary to 

 keep the body's carbon dioxide tensions at the 

 proper level. If carried to an extreme, hyperven- 

 tilation can be as undesirable and dangerous as 

 conditions involving interference with breathing. 

 UNINTENTIONAL HYPERVENTILATION is most often 

 triggered by nervous tension and can be experienced 

 by otherwise normal individuals in stress situa- 

 tions anywhere. It is also brought on by ANOXIA 

 and is a common and serious problem in aviators. 

 Divers using self-contained equipment for the first 

 few times are likely to hyperventilate to some 

 extent largely because of anxiety. Hyperventila- 



tion has little effect on the body's oxygen levels, 

 but it can reduce carbon dioxide tensions to the 

 point of producing serious symptoms. 



Symptoms of abnormally low carbon dioxide ten- 

 sion (hypocapnia) can be produced by voluntary 

 hyperventilation - taking a number of deep breaths 

 over a short period of time as in preparation for 

 a BREATH-HOLD dive. (37) 



HYPOLIMNION . The layer of water below the THERMO- 

 CLINE in a fresh water lake or pool; the opposite 

 of EPILIMNION. (12) 



HYPO PLANKTON . See MARINE LIFE. 



HYPOTHECA . See EPITHECA. 



HY STEELS . High yield strength steels (e.g. HY80 = 

 yield strength of 80,000 psi, HYIOO = yield strength 

 100,000 psi, HY150 and HY185 = 150,000 and 185,000 

 psi yield strength respectively). These are 

 present and candidate steels for pressure vessels 

 including subs and research submersibles . Mild 

 steel in the way of contrast has a yield of about 

 30,000 psi. 



I 



lAC. International Advisory Committee on Research 

 in the Natural Sciences (Programme of UNESCO) . 



lACOMS . International Advisory Committee on Marine 

 Sciences . 



lAG . International Association of Geodesy. 



lAGA . International Association on Geomagnetism 

 and Aeronomy. 



lAHR . International Association of Hydraulic 

 Research. 



lAL . International Association of Limnology. 



lALA . International Union of Lighthouse Authorities. 



lAMAP . International Association of Meteorology 

 and Atmospheric Physics. 



lAPO . International Association of Physical 

 Oceanography. One of the associations constitut- 

 ing the International Union of Geodesy and Geo- 

 physics (lUGG) . 



lASH . International Association of Scientific 

 Hydrology. 



IAS PEL International Association of Seismology 

 and Physics of Earth's Interior. 



lATTC. Inter-American Tropical Tuna Commission. 



Established by treaty between the United 

 States and Costa Rica March 1950, providing for 

 the scientific investigation of the tuna fishery 

 of the east Pacific Ocean. 



Headquarters of the Commission is at the 

 Scripps Institution of Oceanography, La Jolla, 

 California, with branch laboratories at Panama 

 and San Jose, Costa Rica. 



Activities: The lATTC investigates the abun- 

 dance, biology and ecology of tunas and tuna-bait 

 fishes, and the effects of natural factors and 

 human activities on the abundance and catch; col- 



lects and interprets statistics and other informa- 

 tion regarding past and present operations and 

 results of the fishing; determines the past and 

 present conditions of the fish stocks, and publishes 

 reports on its findings. 



Members and Financing: United States, Costa 

 Rica and Panama share expenses in proportion to 

 their utilization of the tuna fisheries catch. 

 U. S. Share: 99.87.. 



lAV . International Association of Volcanology. 



IBP . International Biological Program. 



IBRD. 



International Bank of Reconstruction and 



Development . 



IBY . International Biological Year. 



ICA. 



1. International Co-operative Administration. 



2. International Cartographic Association. 



ICAS . Interdepartmental Committee for Atmospheric 

 Sciences (FCST) . 



ICE . The solid state of water formed either by 

 freezing or SUBLIMATION. 



Ice encountered at sea consists for the most 

 part either of ICEBERGS or other LAND ICE originat- 

 ing from CONTINENTAL ICE sheets and GLACIERS, or 

 of SEA ICE formed by the freezing of the top layers 

 of the sea itself. Sea ice proper accounts for 

 probably 95 per cent of the area of ice encountered 

 at sea, but ICEBERGS are important because of the 

 manner in which they drift far from their place of 

 origin, constituting grave menaces to navigation. 

 A certain amount of ice also may originate in 

 rivers or estuaries as fresh-water ice, but it is 

 in a state of decay by the time it reaches the 

 open sea and is of local importance only. (25) 



ICEBERG . A mass of land ice that has broken away 

 from land and floats in the sea, or becomes stranded 

 in shallow water; to be distinguished from FLOEBERG. 

 The unmodified term "iceberg" usually refers to the 

 irregular masses of ice formed by the calving of 

 GLACIERS along on orographically rough coast; 

 whereas TABULAR ICEBERGS and ICE ISLANDS are calved 

 from an ICE SHELF, and f Icebergs are formed from 

 sea ice. (12) 



ICEBLINK . A yellowish-white glare on the under- 



59 



