Heligraphy

Heligraphy, a method of communicating swiftly between distant points by means of the sun's rays reflected from mirrors. Either successive flashes or obscurations of a continuous reflection of the sun's light may be combined so as to read like Morse's telegraphic system (see TELEGRAPH). Heligraphy may be used for geodetic measurement, or for military and other signalling. The instruments which contain the mirrors are variously called heliograph and heliostat. The instruments have been so perfectly contrived as to be available at a distance of over 190 miles (in California); French engineers in Algeria have found the signals serviceable at a distance of 170 miles. As early as the 11th century A.D. Algeria possessed a system of heliographs: 'At the summit of this tower was an apparatus of mirrors, corresponding to similar ones established in different directions, by aid of which one could communicate rapidly with all the towns from one end of the empire to the other' (Athenæum, 28th January 1882). Recently there has been a great development in heligraphy, or sun-telegraphy, for signalling messages between the sections of an army in the field, as during the British campaign in Afghanistan in 1880. Drummond's and Begbie's heliostats, and the heliographs (differing in details) of Mance and Anderson, are favourably known. The name heliostat was originally used of an Equatorial (q.v.) revolving on its polar axis.—Heliotrope was the name given to a mirror placed at the distant station, and adjusted by clockwork, so that at a particular hour of the day (arranged beforehand) the light of the sun shall be reflected from the mirror directly to the surveyor's station. See SIGNALLING.