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The smoke signal is one of the oldest forms of long-distance communication. It is a form of visual communication used over long distance. In general smoke signals are used to transmit news, signal danger, or gather people to a common area. ==History and usage== 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 in just a few hours. Polybius, a Greek historian, devised a more complex system of alphabetical smoke signals around 150 BC, which converted Greek alphabetic characters into numeric characters. It enabled 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 indigenous peoples also communicated via smoke signal. Each tribe had its 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 halfway up the hill, this would signify all was well, but from the top of the hill it would signify danger. Smoke signals remain in use today. In Rome, the College of Cardinals uses smoke signals to indicate the selection of a new Pope during a papal conclave. 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, while white smoke means a new Pope has been elected. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Smoke signal」の詳細全文を読む スポンサード リンク
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