mirage. A strong inversion over a relatively warm 

 sea may satisfy this requirement. The instability 

 of the air layer lying immediately over the sea 

 may lead to sporadic breakdown of the stratifica- 

 tion there, accompanied by rapid changes in the 

 mirage characteristics, a circumstance that sug- 

 gests the fairy-like features of the mirage forms. 

 (Humphreys, W. J., in Physics of the Air, 3rd ed., 

 1940, pp. 474-475) (12) 



FATHOM . The common unit of depth in the ocean, 

 equal to six feet (or 1.83 meters). It is also 

 sometimes used in expressing horizontal distances, 

 in which case 100 fathoms make one CABLE or very 

 nearly one-tenth nautical mile. (24) 



FATHOMETER . An instrument used in measuring the 

 depth of water by the time required for a sound 

 wave to travel from surface to bottom and for its 

 echo to be returned. It may be used also for 

 measuring the rise and fall of the tides in off- 

 shore localities . (14) 



FATIGUE . A weakening or deterioration of metal or 

 other material, or of a member, occurring under 

 load, especially under repeated, cyclic, or con- 

 tinued loading. (31) 



FATIGUE LIFE . A clue as to the length of useful 

 life of a material subjected to repeated loading. 



FAUDOE . Florida Atlantic University-Department of 

 Ocean Engineering, Boca Raton, Florida. 



FAULT. A break or shear in the earth's crust, with 

 an observable displacement between the two sides of 

 the break, and parallel to the plane of the break. 

 (27) 



FAX CHART . See FACSIMILE CHART. 



FAX MAP . See FACSIMILE CHART. 



FCC . Federal Communications Commission. 



FCST . Federal Council of Science and Technology. 



FEEDER BEACH . An artificially widened beach serv- 

 ing to nourish downdrift beaches by natural littoral 

 currents or forces. (11) 



FEEDER CURRENT . The current which flows parallel 

 to shore before converging and forming the neck of 

 a RIP CURRENT. (11) 



FEEL THE BOTTOM . The action of a vessel proceeding 



in shoal water, when its speed is reduced and it 



sometimes becomes hard to steer. Also called smell 

 the bottom. (17) 



FERREL'S LAW . See CORIOLIS . 



FETCH . 1. In wave forecasting, the continuous 

 area of water over which the wind blows in essen- 

 tially a constant direction. Sometimes used 

 synonymously with FETCH LENGTH. Also, GENERATING 

 AREA. 



2. In wind set-up phenomena, for enclosed 

 bodies of water, the distance between the points 

 of maximum water surface elevations. This would 

 usually coincide with the longest axis in the 

 general wind direction. (11) 



FETCH LENGTH . In wave forecasting, the horizontal 

 distance (in the direction of the wind) over which 

 the wind blows . (H) 



FIGURE OF MERIT . Ratio, in decibels, of pressure 

 in transmitted ping at a distance of 1 yard to pres- 

 sure of the minimum detectable echo under prevail- 

 ing conditions. (7) 



FILAMENT WINDING . Basically filament winding is 

 the technique of coating small filaments of mater- 

 ials, usually glass, with a resin, usually an epoxy. 



Filament windings are used to especially im- 

 part higher compressive strengths and better cor- 

 rosion resistance to sea structures. 



FILTER. 1. In ocean-wave forecasting, a set of 

 formulas that define the particular wave frequen- 

 cies and directions in the fetch area which are of 

 significance at the point of forecast. (24) 



2. A combination of resistances, induc- 

 tances, and capacitances, or any one or two of 

 these, which allows the comparatively free flow of 

 certain frequencies or of direct current while 

 blocking the passage of other frequencies . An 

 example is the filter used in a power supply, which 

 allows the direct current to pass, but filters out 

 the ripple. (20) 



FILTERING EFFECT . The differential damping of pres- 

 sures or of vertical oscillation of water particles 

 with increasing depth, depending upon the wave 

 period . Longer waves are damped less than shorter 

 waves at a given depth. (15) 



FILTER PHOTOMETER FOR USE AT SEA . This instrument 

 determines the optical density of coloured solu- 

 tions. It is suitable for the colorimetric analy- 

 sis of nutrient salts. The device uses two similar 

 photocells enclosed in cases which are light tight 

 except for a circular aperture. Between the cells 

 and equidistant is a lamp, the cells being so 

 mounted that all light passing through the aperture 

 falls on the active surface of the cathode. The 

 two cells are arranged in a bridge circuit followed 

 by an amplifier, the difference in the output of 

 the cells being measured on a centre-zero meter. 

 With the lamp at a fixed reference point, the aper- 

 ture before one cell is adjusted until the meter 

 reads zero. A suitable length of the solution, the 

 optical density of which is to be measured is in- 

 serted between the lamp and one cell causing the 

 meter to deflect. The lamp is moved toward the 

 solution until the meter again reads zero. The 

 distance the lamp has been moved is related to the 

 optical density of the solution. (30) 



FINAL ICE CLEARANCE. EARLIEST . Earliest reported 

 date after breakup that open water (less than 1/10 

 coverage) was first observed over a specific area. 



FINAL ICE CLEARANCE, LATEST . Latest reported date 

 after breakup that open water (less than 1/10 

 coverage) was first observed over a specific area. 



FIORD. See FJORD. 



FIRN (Also called FIRN SNOW) . Old snow that has 

 become granular and compacted (dense) as the result 

 of various surface metamorphosis, mainly melting 

 and refreezing but also including sublimation. The 

 resulting particles are generally spherical and 

 rather uniform. Firnification, the process of firn 

 formation, is the first step in the transformation 

 of snow into land ice (usually GLACIER ICE) . Some 

 authorities restrict the use of firn to snow that 

 has lasted through one summer, thereby distinguish- 

 ing it from spring snow. 



Originally the French term, "neve'," was 

 exactly equivalent to the German term, "firn", but 

 there is a growing tendency, especially among 

 British glaciologists, to use "nev^" for an area 

 of firn, i.e., generally for the accumulation area 

 above or at the head of a GLACIER. (12) 



FIRST APPEARANCE OF ICE. EARLIEST . The earliest 

 re-reported date on which sea ice in any form was 

 observed at a specific location. 



46 



