For millennia, humankind has kept track of the progress of time by observing natural bodies, most notably the sun and the stars. In cloudy periods, however, these cannot be seen. The water clock, or clepsydra in Greek, is a timekeeper that works by measuring a regulated, uniform flow of water out of, or into, a vessel. With sufficient water, and a large enough vessel, this timekeeper can "run" for a day or two without needing to be refilled, or emptied.
Imagine a cylindrical water container with a hole in the bottom. The rate at which water drips out of the container is a function of the pressure exerted by the water that it contains; so the more water in the vessel (that is, the greater the "head" of water) the faster is the flow rate. When the container is full, the water level goes down quickly, but the flow is slower when it is nearly empty. Around 1500 B.C.E. the Egyptians realized that if the sides of the bucket tapered parabolically, the water level would go down at a uniform rate, and this would make a reasonable basis for a clock. Others introduced multiple cistern systems that ensured that the head of water remained constant. The dripping of the clepsydra in ancient times has found an echo in the ticking of a modern clock.
Later on, the Romans and Chinese constructed complicated float systems that followed the changing water level and moved an hour hand on a circular dial, or rang bells at specific times.
Simple forms of the clepsydra were also used to measure specific time intervals, for the regulation of religious services or political debates, in the way that sand glasses are now used as egg timers.
