In the 9th century, during the Arab–Byzantine wars, the Byzantine Empire used a semaphore system of beacons to transmit messages from the border with the Abbasid Caliphate across Asia Minor to the Byzantine capital, Constantinople. The main line of beacons stretched over some 720 km (450 mi). In the open spaces of central Asia Minor, the stations were placed over 97 km (60 mi) apart, while in Bithynia, with its more broken terrain, the intervals were reduced to ca. 56 km (35 mi). Based on modern experiments, a message could be transmitted the entire length of the line within an hour. The system was reportedly devised in the reign of Emperor Theophilos (ruled 829–842) by Leo the Mathematician, and functioned through two identical water clocks placed at the two terminal stations, Loulon and the Lighthouse. Different messages were assigned to each of twelve hours, so that the lighting of a bonfire on the first beacon on a particular hour signalled a specific event and was transmitted down the line to Constantinople.