Appletons' cyclopaedia of applied mechanics: a dictionary of mechanical engineering and the mechanical arts (1880)
D. Appleton and Company, Benjamin, Park, 1849-1922 · New York, D. Appleton · leaf 797
placed in view with its face toward the approaching train, it means danger ; and when concealed—that is, placed parallel with the track—it indicates safety. A lamp placed behind the disk showsa red light for danger at night. To make this signal positive in both its positions, it is now cus-tomary…
Mechanical engineering -- Dictionaries
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