Hydraulic telegraph
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The hydraulic telegraph was a semaphore system invented in the 4th century BC by Aeneas Tacticus; according to Polybius, it was used during the First Punic War to send messages between Sicily and Carthage.
The system involved identical containers on separate hills; each container would be filled with water, and a vertical rod floated within. The rods were inscribed with various predetermined codes.
To send a message, the sending operator would use a torch to signal the receiving operator; once the two were synchronized, they would simultaneously open the spigots at the bottom of their containers. Water would drain out until the water level reached the desired code, at which point the sender would lower his torch, and the operators would simultaneously close their spigots.
