cam raises and then lets a hammer fall that strikes the bell. The first tower clocks, mounted in towers in cathedrals, abbeys, and monasteries to call the community to prayer, which originated in Medieval Europe before the invention of the mechanical clock in the 13th century, were water clocks which used the passing strike mechanism; they rang once for each canonical hour. Before European clocks, China developed a water-driven astronomical clockwork technology, starting with the first century AD scientist Zhang Heng (78–139). The Tang dynasty Chinese Buddhist monk and inventor Yi Xing (683–727) created a rotating celestial globe that was turned by a water clock mechanism driven by a waterwheel This featured two wooden gear jacks on its horizon surface with a drum and a bell, the bell being struck automatically every hour and the drum being struck automatically every quarter-hour. It is recorded that Confucian students in the year 730 were required to write an essay on this device in order to pass the Imperial examinations. The use of clock jacks to sound the hours were used in later clock towers of Song dynasty China, such as those designed by Zhang Sixun and Su Song in the 10th and 11th centuries, respectively. A striking clock outside of China was the clock tower near the Umayyad Mosque in Damascus, Syria, which struck once every hour. It is the subject of a book, On the Construction of Clocks and their Use (1203), by Riḍwān ibn al-Sāʿātī, the son of clockmaker. The Florentine writer Dante Alighieri made a reference to the gear works of striking clocks in 1319. One of the older clock towers still standing is St Mark's Clocktower in St Mark's Square, Venice. The St Mark's Clock was assembled in 1493, by the famous clockmaker Gian Carlo Rainieri from Reggio Emilia, where his father Gian Paolo Rainieri had already constructed another famous device in 1481. In 1497, Simone Campanato moulded the great bell, which was put on the top of the tower where it is alternately beaten by the Due Mori (Two Moors), two bronze statues handling a hammer. Counting the hours During the great wave of tower clock building in 14th-century Europe, around the time of the invention of the mechanical clock itself, striking clocks were built that struck the bell multiple times, to count out the hours. The clock of the Beata Vergine (later San Gottardo) in Milan,