Hydraulic telegraph, Ancient Greece. Visual system of hydraulic fire signals devised by Aeneas Tacticus, an Ancient Greek writer on the art of war in around 336 BC. This system was described by 2nd century BC Greek military historian Polybius. An improvement on the beacon system, identical water vessels containing a graduated column floated in a vessel. Adjusted by draining the water, and synchronised by visual signals from hand-held torches, the columns could indicate a limited range of predetermined messages. Artwork from 'Electricite' (1911) by Max de Nansouty, part of the 'Les merveilles de la science' series of 1867-1891 by Louis Figuier.