Draining water to match a messageSourced
Greek signalling synchronised two separate water vessels
An ancient Greek system placed matching water vessels on separate hills, with coded rods floating inside. Torch signals coordinated the operators as they drained and stopped the water at the desired message. The vessels were not connected: timing and matching codes conveyed the information.
The message travelled through coordinated observation, not a pipe.
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Hydraulic telegraph · Wikimedia contributors
Adapted, summarized and translated from the source. Adapted text is also available under CC BY-SA 4.0.
Reviewed: 2026-10-10