Clepsydras (water clocks) and hourglasses. These devices use the flow of a substance (water or sand) to measure time. The Egyptians and Chinese used clepsydras in the 15th century BC. Improvements were made by adding a float fitted with a hand which moved around a dial marking the hours. Famous clepsydras include the one by CTESIBIVS, which has since vanished but can be seen in replica at the Munich Science Museum. Another is the bronze clepsydra received by Emperor Charlemagne as a gift from Sultan Haroun al-Raschid. It consisted of a striking mechanism which, when activated, opened doors to release balls that tumbled onto a bell.