OTE Museum of Telecommunications Systems, Technology Museum of Thessaloniki
Aeneas Tacticus included a description of this system in his book on Siege Warfare, which has unfortunately not survived. There is, however, a description of what Evanghelos Stamatis has called the "hydraulic telegraph" in Polybius, although there are no illustrations of the device in any ancient sources. In this system, the transmission of messages required two identical sets of equipment. A large jar or cylindrical tank was fitted with a cork float that supported a rod engraved with a series of messages. As water was let out of the tank, by means of a stopcock at the bottom, the float descended and the rod with it. The stopcock was closed when the desired message on the rod was level with the top of the jar. The sender signalled by torch when he was ready to open the stopcock, whereupon the receiver opened the one on his apparatus, thus synchronising the two sets of apparatus. When the desired message was in the proper position the sender extinguished his torch, and the receiver closed his stopcock and read off the corresponding message.
- Diels H., Antike Technik, Leipzig 1920.
- Πολύβιος, Ιστοριών Θ΄-Ι΄-ΙΑ΄, Μετάφραση-σχόλια Θ.Γ. Μαυρόπουλος, Αθήνα 1996.
- Σταμάτης Ε., Αι μυστικαί τηλεπικοινωνίαι των αρχαίων Ελλήνων, Αθήναι 1969.