Since the earliest times people have wanted to send messages to each other over distances. Once you get too far away to shout or see hand signals you have to use something else. All kinds of things have been tried – banging on hollow trees or drums and even tapping on walls. You know the kind of thing: one bang for 'yes' and two bangs for 'no'. Smoke signals by day and fires by night have used light to transmit information. In places like Egypt where sunshine was reliable, people developed a system of signalling using mirrors to reflect the sun and send 'flashes'. The most advanced version of this device (called a heliograph) had two moving mirrors so that one could pick up sunlight and the other could flash signals in any direction.
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