The electric telegraph was of very great strategic and tactical benefit during a war, and where the lines of conflict between the two opposing sides were reasonably clearly defined as on the Western Front in the First World War the systems of electric telegraphy were usually reliable and efficient. However this was not always the position with highly mobile engagements, or where guerilla activity was strong, or where the terrain was difficult to secure. In 1888, A.P. Finley identified 12 problems inherent in military electric telegraphy. Furthermore, the laying of the land-lines was labour intensive and costly. Fortunately, there was one method of military signalling that could be used in the above mentioned situations which was not susceptible to these problems and conditions, namely optical telegraphy based on the use of mirrors and sunlight. The instrument that made this possible the heliograph was due to Mr. (later Sir) Henry C. Mance (1840-1926) of the Persian Gulf Telegraph Department of the Government of India, which he had joined in 1863. He had assisted in the laying of submarine cables in the Persian Gulf and had devised the method of detecting and localising defects in such cables. In 1869 he was stationed at the Jask telegraph station in Baluchistan, and adapted the principles of the heliostat and the heliotrope to the invention of the heliograph: it was adopted by the Indian Government in 1875. Among the instrument's advantages were portability, low cost, great range, secrecy of signalling except to observers directly in the signalling path, ease of setting-up and convenience in operation in difficult terrain or enemy infested territory.
Optical communications, Page 1 of 2
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