Anti-aircraft device, First World War, 1914-1918, (c1920). 'The War against German Aircraft: a "Listening Post" in the French anti-aircraft Section. These listening posts in the French army consist of four huge horns, which gather up the slightest sound and magnify it by means of a microphone, so that it is impossible for an enemy or other aviator to approach unheard. The horns may be described as reverse megaphones.' From "The Great World War - A History" Volume IV, edited by Frank A Mumby. [The Gresham Publishing Company Ltd, London, c1920]

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