2. Wolf smoke (bonfire). It was recorded in China during the Spring and Autumn Period that military warnings were transmitted by fire and smoke. After Qin Shihuang, a beacon tower was built every ten miles on the Great Wall of Wan Li, using dried wolf dung as fuel. After ignition, the flame and smoke soar, which may spread the emergency alarm to a long distance. This medium was also used in the business activities of businessmen in ancient society. For example, there are records of night teams and caravans "lighting fires for the number" in different periods.
In addition, there are running messages; Drift bottle transmits information; Flying pigeons pass books and transmit information; Signal flares transmit information; Letters convey information and so on.