The Hydraulic Telegraph
In ancient times people ranging from the American Indian tribes to the Greeks and Persians used fire and smoke to communicate over long distances. The messages which could be communicated through those means were severely limited, and had to be agreed to in advance. For example, the display of two raised torches could refer to the sighting of ships, or troops, or some other agreed message. Messengers stationed themselves as far apart from each other as possible while remaining in sight, usually on a hilltop. In some instances further inland, signal towers were erected. By establishing a chain of such posts messages, though with little detail, could be transmitted over distances faster than if carried by messengers.
The Greek writer Aeneas was a student of military history and strategy, who applied his knowledge of both to the improvement of long distance communication. Aeneas recognized the value of using a system of signals and sought a means of using them to provide more detailed messages back and forth. His study led him to develop a code in which symbols referred to various words or phrases. Each signaling station was equipped with several different encoded rods, allowing the code to cover a large number of contingencies which could be communicated between the stations. Each station was also equipped with vessels filled to a marked point with water.
The agreed upon rod was placed in the vessel, which was equipped with a drain plug. When a station wished to send a signal a torch would be raised to alert the receiving station, which acknowledged by raising a torch in reply. The sender would drop his torch and simultaneously remove the drain plug, and the receiver, seeing the torch drop, did the same. The sender would then watch the rod in his vessel until the water level revealed the message he was trying to convey, at which point the plug was inserted and the torch raised, signaling the receiver to do likewise. Both vessels were then refilled for future messages. The system required quick responsiveness, but trained operators could send more detailed messages with impressive speed.
During the first Punic War, which was fought from 264 to 241 BC, the hydraulic telegraph invented by Aeneas was used to send messages from Sicily to Carthage. It allowed the movements of enemy troops to be observed and reported far in advance of their arrival at their destination, allowing time for the preparation of an adequate defense. It also allowed for civilians to prepare themselves for pending operations in a manner which had theretofore been unheard of. Variations of the hydraulic telegraph were used by other nationalities, but the Greeks had the most success applying them as a means of long distance communication.
The hydraulic telegraph was primarily used as a weapon of war, though its significance as a means of communicating across borders did not escape its inventor nor the Greeks. Modifications to the basic operation were studied by inventors and tinkerers as a means of developing it as a communications device for non-military purposes. The need to retain line of sight between sender and receiver was an obvious shortcoming of the device, as was its limitations imposed by the weather, which could impair visibility and thus the system’s usefulness. The British explored the use of hydraulic telegraphs as late as the nineteenth century.