English

In PHOTOGRAM­METRY, an imaginary line, in the plane of a PHOTOGRAPH, which represents the IMAGE of the TRUE HORIZON; it corresponds to the intersection of the plane of a PHOTOGRAPH and the horizontal plane containing the internal PERSPECTIVE CENTRE or rear NODAL POINT of the LENS. Also called horizon line.

FOG SIGNAL using compressed air or electricity to vibrate a diaphragm. Horns exist in a variety of types which differ greatly in their SOUND and power. Further to the recent action taken by the HYDROGRA­PHER of the U.K. such terms as NAUTOPHONE, TYPHON and others are gradually being embraced by the general term 'Horn'.

ANGULAR DISTANCE west of a CELESTIAL MERIDIAN, the arc of the CELESTIAL EQUATOR, or the angle at the CELESTIAL POLE, between the UPPER BRANCH of a CELESTIAL MERIDIAN and the HOUR CIRCLE of a CELESTIAL BODY or the VERNAL EQUINOX, measured westward through 24 HOURS (360°). It is usually further designated as local, Greenwich,or sidereal, as the origin of measurement is the LOCAL or GREENWICH MERIDIAN, or the HOUR CIRCLE of the VERNAL EQUINOX.

A GREAT CIRCLE on the CELESTIAL SPHERE passing through the CELESTIAL POLES. Also called declina­tion circle, circle of declination, or circle of right ascension.

The hull of a wrecked or condemned ship, from which the fittings and superstructure have usually been removed, which is MOORED in a permanent position or grounded.

A natural ELEVATION of the EARTH's surface resembling a HILLOCK, but smaller and lower. In ice terminology, a mound of ICE raised by pressu­re. May be fresh or weathered.

A BASE APPARATUS designed for measuring short BASES by the subtended method. It consists of a jointed STEEL TAPE made up of four sections suspended in CATENARY between regularly spaced supports. The Hunter short base apparatus may be used over rough GROUND which is unsuitable for ground TAPING.

Fluctuation about a midpoint due to INSTABI­LITY, as OSCILLA­TIONS of the needle of an instrument about the zero point, or alternate LEAD and LAG of a synchronous motor with respect to the alternating current.

Term, derived from a Caribbean word, first applied to TROPICAL CYCLONES of the Caribbean Sea. WIND with a speed equal to or greater than 58 KNOTS. (Beaufort scale wind force 12.)

Used in oceanography. The tidal current vector hydrograph is the figure traced out by the tip of a vector representing the current over the tidal cycle.

A person who studies and practices the science of HYDROGRAPHY; the term is often applied to the person in charge of a hydrographic department or office of a country.

A system for measuring the depth of water by sending a LASER pulse from an aircraft and recording the time difference between the sea surface and the bottom reflection.