...Tacitus, a Hellenistic writer on the art of war, invented the hydraulic telegraph.It used a set of predetermined messages attached to a vertical rod floating in a container of water. A twin of the system sat with at the second location, and its controller could mimic the messages sent by the first via a signal sent by torchlight.The system was adopted in situations that required urgent communication, such as war, but it had its drawbacks... the optical telegraph, also known as a semaphore telegraph, already used long-distance, line-of-sight signaling for rapid communication, typically with torches. With a predetermined set of...