DICTIONARY OF

Hydrographic Department 176 Hysteresis
Hydrographic Department. Branch of Admiralty, founded in 1795y to 'select and compile
such information as might appear to be requisite for the purpose of improving navigation'. Is
responsible for all hydrographic work and for the publication of requisite information
regarding surveys, tidal data, chart preparation, etc.
Hydrographic Office. Branch of U.S. Navy corresponding to British Hydrographic
Department.
Hydrography. The science and practice of surveying oceans and seas, the charting of
coasts, the collection of information relating to navigation, the production of charts and
sailing directions, the investigation of tides, and the placing of the ascertained facts at the
disposal of navigators and others concerned.
Hydrometer. Instrument for measuring the density of liquids. Usually consists of a ballasted
float carrying a scale that indicates density as related to fresh water. This zero is 1000; the
density of sea water is about 1026, but varies, in different ports and areas, between 1000 and
about 1040.
Hydrophone. Instrument for collecting sound transmitted by water.
Hydrostatic. Pertaining to the principles of equilibrium of fluids.
Hygrometer. Dry and wet bulb thermometers mounted alongside each other so that
depression of wet bulb can be ascertained and humidity of atmosphere deduced. Special
hygrometers are used by engineers for determing the dryness fraction of steam.
Hygroscopic. Capable of abstracting moisture from the atmosphere.
Hyperacme Block. Heavy lift purchase consisting of upper sheave having a toothed disc,
into which a worm engages, and a sprocket wheel of smaller diameter. Length of chain is
secured to framework, passes through a single shave hook block and then into sprockets on
upper shave. Worm is actuated by endless chain going over sprocket wheel secured at end of
worm shaft.
Hyperbola. A curve, any point upon which has the same difference of distances between
two fixed points, called loci: e.g. point X, 10 units from one locus and 8 from the other:
difference 2; point Y, 13 from one locus and 11 from the other: difference 2.
Hyperbolic Navigation. Finding a Position Line or Position Lines corresponding to a
hyperbola or hyperbolae associated with a chain of two or more radio stations.
Hysteresis. The lagging behind of a certain amount of magnetism when an object has been
temporarily magnetised and the magnetising agent has been withdrawn.