Clepsydre

The clepsydre is a Horloge with Eau, functioning on the same principle as the Sablier.

History

The word clepsydre comes from the Greek “κλεψύδρα” (will klepsudra) and taken again by Latin “will clepsydra”. This name was made starting from κλεπτείν (kleptein), which means to conceal, and of ΰδωρ, ΰδατος (hudôr, hudatos), which mean water. It would be also associated with the Clepsydre source whose water fed the clepsydre of the Agora in Athens which had two superimposed cylinders and was emptied in 24:00.

Oldest clepsydre discovered is that of Karnak, in 1904 (it is exposed today to the Musée of Cairo). It goes back to approximately -1400 and was carried out under the reign of Aménophis III. It is appeared as a simple conical bowl, having an opening at its base, being used for the water run-off. The measurement of time was done using the graduations being inside the bowl. It is estimated that the first clepsydres were created in Egypt towards -1600. This kind of clepsydre was with single filling and its precision of about 5 to 10 minutes.

The principle of the clepsydre was also used by the Amerindian S.

In fact the Greek improved the precision of the clepsydre towards -270. Because of the fall of the level of water, the Pression on the outlet side of the bowl was reduced and the Débit with it. That caused a loss of precision. The Egyptians cured that by consequently graduating the bowls according to the level. They had also used bowls in form of cone, to attenuate the problem of the pressure. But the precision was still not good enough. To maintain the precision, it is necessary that the flow at exit is constant. For that, the Greek inventor Ctésibios imagined a system using the principle of the communicating vases and the valve. He thus succeeds in maintaining the level of water constant.

The most sophisticated clepsydres were those carried out by the Arab and the Chinese. In 807, the Caliph of Baghdad offered to Charlemagne a clepsydre setting in motion automats. This kind of clepsydre had a decorative vocation more than utility. And in 1088, the Engineer Su Song made build a clepsydre of more than 10 meters in height in Kaifeng, China.

Uses

The clepsydre was used to measure short periods, like:

duration of a speech or a pleading in Greece;
durations of the guards in the Roman Legion;
duration of short moments during experiments, as that of Galileo in 1610 on the fall of the bodies.

The clepsydre was also used to measure time when it grew dark, or when the weather conditions did not allow the use of the sundials.

Principle

The clepsydre functions on the same principle as the sand glass. It is the flow of a certain quantity of water which fixes the past duration.

The first clepsydres are appeared as a bowl, with a hole at their point low, allowing the water run-off. It is using the graduations inside the bowl that past time is given.

The problem is that as the bowl is emptied, the flow of the flow becomes lower. The quantity of water run out for the same duration is thus different when the bowl is full and when it is almost empty, which poses problems of exactitude. The Egyptians partly cured that by using bowls in the shape of cone and modifying the graduations, consequently.

This problem in the constancy of the flow is due to the fact that the pressure of water at the bottom of the bowl decreases, because the quantity of water is less. To maintain the same pressure, Ctésibios had the idea to maintain the water level constant.

For that it used three tanks (has, B and C). The tank has contains an important quantity of water and its goal is always to feed out of water the tank B. the level of B is maintained thanks to a hole close to its top, which is charged to empty the too full of one water (it is a valve). The water of B runs out normally by a small hole at its base. The flow by this hole is constant, because the water level in B does not vary, the pressure at the exit thus remains it also constant. The water of B running out by the hole at the base, goes in the tank C, which is graduated to him. It is by observing the level of filling of C that one determines past time.

In spite of this notable improvement, such clocks remain not very precise. Several factors can influence over the measured duration, like:

the Atmospheric pressure
impurities in water
the temperature of water

Moreover, the manufacture of a clepsydre request a high degree of accuracy, in particular on the realization of the weep hole of water.

See too

Bernard Gitton, an artist who uses the clepsydre.

External bonds

the clepsydre
the clepsydre

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