The first mechanical timekeeping device was a water clock called a clepsydra, which was used from about 1500 b.c. through the Middle Ages (500–1350). One very elaborate clepsydra was constructed for Holy Roman Emperor Charlemagne (742–814) in a.d. 800: Upon the hour, it dropped a metal ball into a bowl. Because of problems with water (it evaporated, froze, and eroded the surfaces of its container), a more accurate device was needed. It is believed that the first completely mechanical clock was developed by a monk around 1275: The clock was driven by the slow pull of a falling weight that had to be reset to its starting position after several hours. The clocks in monasteries were among the first to be fitted as alarm clocks: striking mechanisms were added to the timekeeping devices so the monks would know when to ring the monastery bell.