sitivity to the reference sensitivity, where the 

 amplitude is a quantity proportional to the square 

 root of power. The kind of sensitivity and the 

 referense sensitivity must be indicated. (1) 



SENSOR. The component of an instrument that con- 

 verts an input signal into a quantity which is 

 measured by another part of the Instrument. Also 

 called 'sensing element*. (31) 



SEQUENCE OF CURRENT . The order of occurrence of 

 the four tidal current strengths of a day with 

 special reference as to whether the greater flood 

 immediately precedes or follows the greater ebb. 

 (14) 



SEQUENCE OF TIDE . The order in which the four tides 

 of a day occur with special reference as to whether 

 the higher high water immediately precedes or fol- 

 lows the lower low water. (14) 



SERPAC . Service Forces, Pacific (USN) . 



SERPLANT . Service Forces, Atlantic (USN). 



SESSILE . Permanently fixed, sedentary, not free- 

 moving. (19) 



SET . The direction towards which an ocean current 

 flows. (24) 



SETA . A bristle or slender, stiff bristle-like 

 structure (evident in many marine worms). (19) 



SET-UP WIND . (1) The vertical rise in the still 

 water level on the leeward side of a body of water 

 caused by wind stresses on the surface of the water; 



(2) The difference in still water 

 level between the windward and the leeward sides of 

 a body of water caused by wind stresses on the sur- 

 face of the water; 



(3) Synonymous with wind tide. Wind 

 tide is usually reserved for use on the ocean and 

 large bodies of water. Wind set-up is usually 

 reserved for use on reservoirs and small bodies of 

 water. (11) 



SEVFLT . Seventh Fleet (Pacific) (USN). 



SHADING . Shading is a method of controlling the 

 directional response pattern of a transducer 

 through control of the distribution of phase and 

 amplitude of the transducer action over the active 

 face. (1) 



SHADOW ZONE . Region In which refraction effects 

 cause exclusion of echo-ranging signals (sound). 



SHALLOW WATER . Commonly; water of such a depth 

 that surface waves are noticeably affected by bot- 

 tom topography. It Is customary to consider water 

 of depths less than half the surface wave length 

 as shallow water. (11) 



SHALLOW-WATER BLACKOUT . A carbon dioxide accumula- 

 tion or excess in a breathing system which causes 

 the diver to lose consciousness without the usual 

 warning of DYSPNEA or other symptoms such as head- 

 ache, nausea, dizziness or weakness. (37) 



SHARKI . See KAUS. 



SHEAR STRENGTH . The Ultimate Strength of a materi- 

 al subjected to shear loading. The maximum Shear 

 Stress that can be sustained by a material without 

 rupture. It may be obtained from the Torsion Test, 

 a modified Flexure Test or the Shear Test. In the 

 Torsion Test it is equal to Torsional Strength. In 

 the Shear Test, it is calculated by substituting 

 maximum sustained load for P in the punch test 

 formula for Shear Stress . It may be calculated in 

 the Flexure Test by substituting maximum sustained 

 load for P In the flexure formula for Shear Stress. 

 For Shear Stress to exceed Flexural Stress in a 



Flexural Test, however, the span of a rectangular 

 specimen must be less than half the square of Its 

 thickness, and the span of a cylindrical specimen 

 must be less than one-third its diameter. 



SHEAR STRESS . The maximum nominal biaxial stress 

 developed by a material subjected to a specified 

 load. In the Torsion Test, it is calculated as 

 Torsional Stress although the assvmiptlons under- 

 lying the formula for Torsional Stress are not 

 valid for stresses above the Elastic Limit. In 

 the Shear Test, Shear Stress (psi) is calculated 

 as follows: 



^^ -d? 



Shear Stress, psi 



punch load, lb 



punch diameter, in. 



specimen thickness, in. 

 In the Flexure Test, Shear Stress (psi) may be 

 calculated as follows: 



where Ss 

 P 

 d 



t 



0.75P 



bh 

 0.85P 



(for rectangular specimen) 

 (for round specimen) 



where Sg - maximum nominal Shear Stress, psi 



P - bending load, lb. 



b ~ specimen width, psi 



h = specimen thickness, psi 



d •■ specimen diameter, psi. 



SHEAR WAVE (ROTATIONAL WAVE) . A shear wave is a 

 wave, usually In a solid, which causes an element 

 of the solid to change its shape without at the 

 same time undergoing a change in volume. (9) 



SHEET FLOW . See LAMINAR FLOW. 



SHEET ICE . Ice formed In a thin, smooth layer over 

 a water surface. This should not be confused with 

 ICE SHEET, a continuous layer of ice covering a 

 large land area. (17) 



SHEET PILE . A PILE with a generally flat cross- 

 section to be driven into the ground or sea bed 

 and meshed or interlocked with like members to 

 form a diaphragm, wall, or BULKHEAD. (11) 



SHELF EDGE . A line along which there is a marked 

 increase of slope at the outer margin of a CON- 

 TINENTAL SHELF or INSULAR SHELF. Conventionally 

 the shelf edge has been taken at 100 fathoms. (27) 



SHELF ICE . A thick ice formation with level sur- 

 face extending seaward from the land but attached 

 thereto. Shelf ice may be formed in three ways: 

 (1) By an extension of LAND ICE onto water, (2) by 

 the accumulation of snow upon SEA ICE which has 

 persisted for several seasons, and (3) by a com- 

 bination of (1) and (2), resulting in areas of 

 land ice extending onto the water Interspersed 

 with areas of persistent sea ice covered with 

 accumulations of snow. 



The chief characteristics of shelf ice are: 

 (1) A shape conforming to the boundaries of the 

 coast, (2) a seaward edge usually floating freely 

 in deep water, (3) vertical cliffs up to 150 feet 

 high on the seaward edge, and (4) prominent hori- 

 zontal banding and clean-cut joint faces from 

 which TABULAR ICEBERGS are CALVED periodically. (25) 



SHELL-NATRON . A commercial compound used as a 

 carbon dioxide absorbent in diving. (37) 



SHINGLE . (1) Strictly and properly, beach gravel 

 composed of smooth, well-rounded pebbles of rough- 

 ly the same size. The space between pebbles are 

 not filled with finer materials as they are in or- 

 dinary gravel. Shingle gives out a musical note 

 when stepped on. 



(2) Loosely and commonly, any beach 



106 



