Water clock, also called clepsydra, ancient instrument that measured time by the amount of water flowing from it. Used as early as the 14th century B.C. by the Egyptians, it consisted of a hollow container, such as a cylinder or a glass jar, with measuring lines. Water was poured into the container and then allowed to flow out. The lines indicated how much water had escaped, and from this it could be determined how much time had passed. Water clocks were used by Greeks and Romans as well. Among its various purposes was the timing of orators' speeches.