clepsydra

clepsydra, ancient device for measuring time by the gradual flow of water. The word clepsydra (plural clepsydrae or clepsydras) is derived from the Greek words kleptein, meaning “to steal,” and hydōr, meaning “water,” and is commonly translated as “the thief of water.” Developed as a complement to timekeeping methods based on astronomical bodies such as the Sun and stars, clepsydras offered several practical advantages. Unlike sundials they could measure time indoors and functioned even under overcast skies. Because their operation relied on the controlled flow of water rather than the movement of celestial bodies, they were not constrained by the limitations of solar-based timekeeping. This allowed clepsydras to provide a more consistent and often more accurate measure of time than sundials, whose use depended on sunlight and the Earth’s rotation.

Types

Water-based time-telling devices may have been an invention of the Chaldeans of ancient Babylonia; such devices were probably also used in ancient Egypt. One form, used by the North American Indigenous peoples and some African peoples, consisted of a small boat or floating vessel that shipped water through a hole until it sank. These forms resembled timers more than they did clocks. Similar timekeeping methods have been documented among several earlier cultures, including the Romans and the Druids. In another form the vessel was filled with water that was allowed to escape through a hole, and the time was read from graduated lines on the interior measuring the level of the remaining water. A drawback of these early designs involved the nonlinear rate of flow from the vessels: The flow rate of water decreases as its depth in a vessel decreases, making the remaining water level an unreliable indicator of time. Some Egyptian specimens dating from the 14th century ʙᴄᴇ attempted to solve this problem by using a vessel with a smaller cross-sectional area at the bottom, which somewhat compensated for the decreased water flow.

The Romans invented a clepsydra consisting of a cylinder into which water dripped from a reservoir; a float provided readings against a scale on the cylinder wall. This and similar designs, augmented with siphons, made clepsydras more accurate by producing a more uniform flow rate. The stairstep water clocks developed in China around 200 ʙᴄᴇ consisted of identical bowls arranged in a series of steps, with all but the lowermost bowl initially filled with water. They were also designed to improve the linearity of water clocks. Because the water flowing out of each bowl was replenished by water flowing in from the bowl above, the arrangement maintained a nearly constant flow rate and, consequently, a more uniform measure of time.

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Uses

Clepsydras served many purposes throughout history. In ancient Rome they were often used to time the speeches of orators. The Roman general and statesman Julius Caesar is said to have relied on one to time the length of British summer nights. Medieval doctors used clepsydras to determine their patients’ pulse rates. They remained in service as late as the 16th century when astronomers such as Tycho Brahe and Galileo employed them in their scientific investigations. Galileo, for instance, used a mercury clepsydra to time his experimental falling bodies. As late as the 20th century, clepsydras were used to time irrigational flow in Algeria.