This water clock or clepsydra is based on an Egyptian clock made in the third century BC. Water is supplied to the funnel (1) and passes to the cylinder in which the float (2) rises. This is connected to a rack-and-pinion that actuates the hour hand. The rate of water flow is regulated by the graduated stopper (3) and the water is kept at a constant level by means of an overflow tube.

The clepsydra was an ancient forerunner of the clock, used by the Greeks, Egyptians, and Romans to measure time. The clepsydra measured the controlled flow of water, mercury, or other liquid through a narrow opening. "Clepsydra" is Greek for "water thief."