Water clocks are reported as early as 4000 B.C. and were used in the ancient world, but these were domestic clocks. Beginning in the Middle Ages around 1000 A.D. striking water clocks were invented, which rang bells on the canonical hours for the purpose of calling the community to prayer. Installed in clock towers in cathedrals, monasteries and town squares so they could be heard at long distances, these were the first turret clocks. By the 13th century towns in Europe competed with each other to build the most elaborate, beautiful clocks. Water clocks kept time by the rate of water flowing through an orifice. Since the rate of flow varies with pressure which is proportional to the height of water in the source container, and viscosity which varies with temperature during the day, water clocks had limited accuracy. Other disadvantages were that they required water to be manually hauled in a bucket from a well or river to fill the clock reservoir every day, and froze solid in winter.
