Clepsidra o reloj de agua (del griego κλέπτειν kléptein 'robar'; ὕδωρ hydōr 'agua') es cualquier mecanismo para medir el tiempo mediante el flujo regulado de un líquido hacia o desde un recipiente graduado, dando así dos tipos diferentes de relojes según la dirección del flujo. Las clepsidras datan de la antigüedad egipcia (mrḫyt) y se usaban especialmente durante la noche, cuando los relojes de sol perdían su utilidad. Los primeros relojes de agua consistían en una vasija de cerámica que contenía agua hasta cierto nivel, con un orificio en la base de un tamaño adecuado para asegurar la salida del líquido a una velocidad determinada y, por lo tanto, en un tiempo prefijado. El recipiente disponía en su interior de varias marcas, de tal manera que el nivel de agua indicaba los diferentes períodos, tanto diurnos como nocturnos. Los relojes de agua también se usaron por los atenienses para señalar el tiempo asignado a los oradores. Más tarde fueron introducidos con el mismo fin en los tribunales de Roma y además se usaban en las campañas militares para señalar las guardias nocturnas. El reloj de agua egipcio, más o menos modificado, siguió siendo el instrumento más eficiente para medir el tiempo durante muchos siglos.

Leer el artículo completo en Wikipedia · CC-BY-SA

A water clock or clepsydra (Greek κλεψύδρα from κλέπτειν kleptein, 'to steal'; ὕδωρ hydor, 'water') is any timepiece in which time is measured by the regulated flow of liquid into (inflow type) or out from (outflow type) a vessel where the amount is then measured. Water clocks, along with sundials and hourglasses, are likely to be the oldest time-measuring instruments, with the only exceptions being the vertical gnomon and the day-counting tally stick. Where and when they were first invented is not known, and given their great antiquity it may never be. The bowl-shaped outflow is the simplest form of a water clock and is known to have existed in Babylon and in Egypt around the 16th century BC. Other regions of the world, including India and China, also have early evidence of water clocks, but the earliest dates are less certain. Some authors, however, claim that water clocks appeared in China as early as 4000 BC. Some modern timepieces are called "water clocks" but work differently from the ancient ones. Their timekeeping is governed by a pendulum, but they use water for other purposes, such as providing the power needed to drive the clock by using a water wheel or something similar, or by having water in their displays. The Greeks and Romans advanced water clock design to include the inflow clepsydra with an early feedback system, gearing, and escapement mechanism, which were connected to fanciful automata and resulted in improved accuracy. Further advances were made in Byzantium, Syria and Mesopotamia, where increasingly accurate water clocks incorporated complex segmental and epicyclic gearing, water wheels, and programmability, advances which eventually made their way to Europe. Independently, the Chinese developed their own advanced water clocks, incorporating gears, escapement mechanisms, and water wheels, passing their ideas on to Korea and Japan. Some water clock designs were developed independently and some knowledge was transferred through the spread of trade. These early water clocks were calibrated with a sundial. While never reaching a level of accuracy comparable to today's standards of timekeeping, the water clock was the most accurate and commonly used timekeeping device for millennia, until it was replaced by more accurate pendulum clocks in 17th-century Europe. A water clock, also known as a clepsydra, uses a flow of water to measure time. If viscosity is neglected, the physical principle required to study such clocks is the Torricelli's law. There are two types of water clocks: inflow and outflow. In an outflow water clock, a container is filled with water, and the water is drained slowly and evenly out of the container. This container has markings that are used to show the passage of time. As the water leaves the container, an observer can see where the water is level with the lines and tell how much time has passed. An inflow water clock works in basically the same way, except instead of flowing out of the container, the water is filling up the marked container. As the container fills, the observer can see where the water meets the lines and tell how much time has passed.

Leer el artículo completo en Wikipedia · CC-BY-SA