The smoke signal is one of the oldest forms of communication in recorded history. It is a form of visual communication used over long distance.
In Ancient China, soldiers stationed along the Great Wall would alert each other of impending enemy attack by signaling from tower to tower. In this way, they were able to transmit a message as far away as 300 miles in just a few hours.
Polybius, a Greek historian, came up with a more complex system of alphabetical smoke signals around 150 BC. He invented a system of converting Greek alphabetic characters into numeric characters. It was devised to enable messages to be easily signaled by holding sets of torches in pairs. This idea, known as the "Polybius square", also lends itself to Cryptography and Steganography. This cryptographic concept has been used with Japanese Hiragana and the Germans in the later years of the First World War.
The North American Indians also communicated via smoke signal. Each tribe had their own signaling system and understanding. A signaler started a fire on an elevation typically using damp grass, which would cause a column of smoke to rise. The grass would be taken off as it dried and another bundle would be placed on the fire. Reputedly the location of the smoke along the incline conveyed a meaning. If it came from half way up the hill, this would signify all was well, but from the top of the hill it would signify danger.
Smoke signals are still in use today. In Rome, the Vatican uses smoke signals to indicate the selection of a new Pope. Eligible cardinals conduct a secret ballot until someone receives a vote of two-thirds plus one. The ballots are burned after each vote. Black smoke indicates a failed ballot; white smoke means a new Pope has been elected.
In general smoke signals are used to transmit news, signal danger, or gather people to a common area.