Smoke signal - Wikipedia
The smoke signal is one of the oldest forms of long-distance communication. It is a form of visual communication used over a long distance. In general smoke signals are used to transmit news, signal danger, or to gather people to a common area.
In ancient China, soldiers along the Great Wall sent smoke signals on its beacon towers to warn one another of enemy invasion.[1][2] The colour of the smoke communicated the size of the invading party.[1] By placing the beacon towers at regular intervals, and situating a soldier in each tower, messages could be transmitted over the entire 7,300 kilometres of the Wall.[1] Smoke signals also warned the inner castles of the invasion, allowing them to coordinate a defense and garrison supporting troops.[2]
In ancient Sri Lanka, soldiers stationed on the mountain peaks alerted each other of impending enemy attack (from English, Dutch or Portuguese people) by signaling from peak to peak. In this way, they could transmit a message to the King in just a few hours.[3]
