Water clock.

When using a fund, people encounter many inconveniences such as a fund that cannot tell time on rainy days or at night. So the Egyptians thought of water clocks (clepsydres) based on the water level in a transparent vase.

The simplest water meter consists of a graduated cylinder with a small tube. A water tank has a drain, keeping the water flowing into the small tube with a certain flow. After the cylindrical flask is full, the water automatically flows out through a U-shaped tube thanks to the principle of bottom-bottoming. People base on the rising water level in the cylindrical tank to know the time. For example, the water level reaches the 7th line, at that time, the water meter is only 7 hours.

The water meters are gradually improved: a float floats on the water with a wooden stick with a pointer and the needle moves before a board with the time. Later on the shaft of the floating buoy is attached to a cog wheel, which causes the needle movement to be indicated before a graduated surface.

The water meter, though invented after the sundial, is used simultaneously with the sundial. On the wall in the tomb in a graveyard of Thèbes province, Egypt, people also found the water clock of the monk and astronomer named Amenenhet, died around 1550 before the Western Calendar. Thanks to the water meter, Amenenhet commented that the winter night was 14 hours long, while in the summer, the night was only 12 hours and during the year, the night had changed if the sun had set until the sun rose. Thus the sundial indicates the time and the water meter is used to measure the time elapsed.

Around 250 BC, the Greek Ctésibius had come up with a very ingenious water meter because there was a pen that indicated the time . In this watch, water flows into the cylinder in a more poetic form: the tears from the head of the statue have gradually fallen into the vase. When the water rose, the float floated up a figure of a person holding a pointer and this needle was a pen moving in front of a vertical cylinder. After 24 hours, the water flows out, falls on a water wheel (roue à aubes) and this wheel makes the vertical cylinder turn away a bit.

Later, the water meter was improved by adding the serrated parts and the water meter became a mechanical device but expensive but still required by people living on the Mediterranean coast. In the first century before Western Calendar, Pompée forced the Courts to use water meters to avoid lawyers saying 'babbling'. Because the orators need a lot of time to defend a reason, it is not possible to avoid cheating on time issues. People have discovered cases of bribing watchmakers to extend the time and have the best reputation, and a lawyer has poured all the muddy water into the watch, making the clock run slower than the water. in.

Another disadvantage of water meters is the cold season, the water is solid. Jules César encountered this problem when leading soldiers to England. During the garrison at this place, César found that the English summer night was shorter than in Rome and because the General's astronomical knowledge was not profound, he did not know that it was due to latitude difference.

In previous times, the movement of the water meter was complicated, so in 490 when King Théodoric presented the clock to King Gondebault of Burgondes, the King had to send the watchman to the watch. The most famous watch of the time was the Royal Arabian Haroum-al-Raschid, who gave the Emperor Charlemagne in 809. This watch is made of brass, with 24 rounds of bronze also falling into a basin to only hours.

Another type of water meter that runs very quietly should be used by many people and therefore, very popular in the 17th century, the period of pendulum clocks. This type includes a flat cylindrical metal drum, which is divided into walls a, b, c, d. . . and these are connected by small holes. Gravity caused water to flow from one base to the other and gradually rotate the metal drum. The axis of the drum descends and moves in front of a time table. One just needs to see which figure the horizontal axis is located to know what time it is.

Pham Van Tuan

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