A relief of the Greek hydraulic telegraph of Aeneas, depicting one half of a telegraph system. The soldier holding the raised torch is watching the descent of a floating message indicator stick (which is marked with message codes) as water is released from the reservoir chamber the stick is floating in. When the float indicator reaches the appropriate level (corresponding to a certain predetermined message), the soldier's flame will be hidden or extinguished. On the distant hill shown near the top left of the relief, an observer with an identical hydraulic chamber is observing the flame, and waiting for it to be extinguished, at which time that reservoir spout will be closed and the message read from the identical floating message indicator stick.