CLEPSYDRA

The clepsydra is an instrument for measuring Time based on the regulated flow of a quantity of water (or even oil), by emptying or filling a container. Its name, from the Greek klepsydra — “that which steals water” —, originally referred to a simple bowl pierced with a hole at its base: the duration of a speech or a plea was measured by the time it took the bowl to empty, independent of any regular flow rate, which does not strictly make it a Clock. The oldest known clepsydra, a graduated conical bowl discovered at Karnak, dates back to the reign of Amenhotep III, around 1400 BCE.

The main flaw of the clepsydra — the drop in pressure, and thus in flow rate, as the vessel emptied — was resolved around 270 BCE by the Greek engineer Ctesibius of Alexandria, who maintained a constant water level through a system of communicating vessels and an overflow valve, transforming the instrument into a true water clock. The most sophisticated clepsydras were later produced by the Persians and the Chinese, including the engineer Su Song, who in 1088 built a time tower over ten meters tall in Kaifeng. Used at night or in bad weather, when the sundial fell silent, the clepsydra remained, until the rise of mechanical clockmaking, one of the few instruments capable of measuring time continuously.