HYDRODYNAMIC NOISE 



simplified versions of the hydrodynamic equa- 

 tions. 



hydrodynamic noise — Noise produced by the mo- 

 tion of the ship or sonar housing through tlie 

 water. 



hydrodynamics — The study of fluid motion. 

 "Fluid" here refers ambiguously to liquids and 

 gases. (5) 



hydrofoil — Any surface, such as a wing or rudder, 

 designed to obtain reaction upon it from the 

 water through which it moves. In recent usage, 

 it connotes a ship equipped with planes which 

 provide lift when the ship is propelled forward. 



hydrogen sulfide mud — See black mud. 



hydrograph — A graphical representation of stage 

 or discharge at a point on a stream as a function 

 of time. The most common type, the observed 

 hydrograph, represents river gage readings 

 plotted at time of observation. Other types of 

 hydrograph which are statistically derived from 

 observed data include the distribution graph 

 and the unit hydrograph. The possible hydro- 

 graph varieties are numerous, representing aver- 

 ages, storm units, seasonal characteristics, etc., 

 in cumulative or differential form, and utilizing 

 different graphing or tabulating methods. (5) 



hydrographic datum — See chart datum. 



hydrographic station — See serial station. 



hydrographic survey — A survey of a water area, 

 with particular reference to submarine relief, 

 and any adjacent land. See oceanographic 

 survey. (68) 



hydrography — That science which deals with the 

 measurement and description of the physical 

 features of the oceans, seas, lakes, rivers, and 

 their adjoining coastal areas, with particular 

 reference to their use for navigational purposes. 

 (66) 



hydroid — The polyp form of a hydrozoan, as dis- 

 tinguished from the medusa or jellyfish form. 



hydrologic cycle — The composite picture, includ- 

 ing change of state and vertical and horizontal 

 transport, of the interchange of water substance 

 between the earth, the atmosphere, and the sea. 



(5) 

 hydrology — The scientific study of the waters of 



the earth, especially with relation to the effects 

 of precipitation and evaporation upon the oc- 

 currence and character of water in streams, lakes, 

 and on or below the land surface. ( 5 ) 



hydromedusa — See hydrozoan. 



hydrometeor — See meteor. 



hydrometer — An instrument used for measuring 

 the specific gravity of a liquid. ( 5 ) 



hydromica — See illite. 



hydrophone — An electroacoustic transducer 

 that responds to water-borne sound waves and 

 delivers essentially equivalent electric waves. 



hydrophone effect — See machinery noise, pro- 

 peller noise. 



hydrophone loss — Applied to a sonar transducer 

 used for the reception of acoustic energy at a 

 specified frequency; it may be defined as the 

 transmission loss measured by the ratio of 

 (1) the source power of the free-field acoustic 

 energy available as plane sinusoidal waves from 

 a water surface having an area of one square 

 centimeter and lying perpendicular to the direc- 

 tion of the maximum response reference axis of 

 the transducer at the point to be occupied by its 

 effective center to (2) the resulting output power 

 of the electric energy available from the trans- 

 ducer. (28) 



hydrophotometer — An instrument iised to meas- 

 ure the extinction coefficient or transmission of 

 light in water. It consists of a constant light 

 source placed at a specific distance from a photo- 

 cell. When placed in the water, the electrical 

 output of the photocell is proportional to the 

 amount of light striking the cell which, in turn, 

 depends upon the transparency of the water. 

 The instrument is calibrated to read 100 per- 

 cent light transmission in air. 



Hydroplastic corer — (or polyvinyl (PVC) 

 cover) . A deep sea coring device, developed at 

 the U.S. Naval Oceanographic Office, to obtain 

 31/^-inch diameter cores up to 12 feet in length. 

 The barrel is constructed of high-impact grade 

 polyvinyl chloride plastic without an inner liner. 



hydropsis — That part of oceanography concerned 

 with continuous observations and data collecting 

 and the reporting of oceanographic phenomena 

 on a regular and prompt basis with the aim of 

 supplying those that use the seas, particularly in 

 commercial fishing, current information. 

 Oceanographic counterpart to synopsis, or 

 synoptic meteorology. (25 ) 



hydrosphere — The water portion of the earth as 

 distinguished from the solid part, called the 

 lithosphere, and from the gaseous outer enve- 

 lope, called the atmosphere. (5) See bio- 

 sphere, geosphere. 



hydrostatic equation — The form assumed by the 

 vertical component of the vector equation of 

 motion when all coriolis force, earth curvature, 

 frictional, and vertical acceleration terms are 

 considered negligible compared with those in- 

 volving the vertical pressure force and the 

 force of gravity. Thus 



dp 



ds 



=—pff, 



where p is the pressure, p the density, g the accel- 

 eration of gravity, and 3 the geometric height. 



(5) 

 hydrostatic equilibrium — The state of a fluid 

 whose surfaces of constant pressure and constant 

 mass (or density) coincide and are horizontal 

 throughout. Complete balance exists between 

 the force of gravity and the pressure force. 

 The relation between the pressure and the geo- 



80 



