Coal miners with a caged canary. Canaries were used in coal mining as an early warning system for toxic gases such as carbon monoxide, methane or carbon dioxide. These gases would affect the bird before the miners, and signs of distress in the bird indicated to the miners that conditions were unsafe. The idea of placing a canary or other warm-blooded animal in a mine to detect carbon monoxide was first proposed by John Scott Haldane. The use of miners' canaries in British mines continued from 1911 until 1986 when they were replaced by electronic gas detectors. This photograph is from an archive of images dating from the 1860s to the 1920s, collected for 'The Pageant of America: A Pictorial History of the United States' (1925-1929).