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The sun, switched on and off.
Also known as: Sun telegraph, Mirror signalling
The signalling instrument flashing sunlight to a distant station by tilting a mirror.
Signals to a distant station by flashing reflected sunlight in coded pulses.
Contexts: History, Navigation
Reaches remarkable distances with no power source at all, which made it the only long-range signalling available in dry regions without infrastructure — the channel's requirement is weather rather than equipment.
Contexts: History, Navigation
Directional to a fine tolerance, so both stations must know where the other is before a message can begin — the system cannot broadcast and cannot find a correspondent.
Contexts: History, Navigation
A circular mirror on a tripod with a sighting rod and a hinged shutter, aimed at a distant station and flashed in long and short pulses.