When using sundials, people encountered several inconveniences, such as the inability of sundials to tell time on rainy days or at night. Therefore, the Egyptians devised water clocks (clepsydra) based on the water level in a transparent vessel.
The simplest water clock consists of a cylindrical vessel marked with gradations connected to a small tube. A container holds water with an outlet, allowing water to flow into the small tube at a constant rate. Once the cylindrical vessel is full, water automatically flows out through a U-shaped tube due to the principle of communicating vessels. The time is determined based on the water level rising in the cylindrical vessel. For example, if the water level reaches the seventh mark, the water clock indicates 7 o’clock.
Water clocks were gradually improved: a float on the water surface carries a wooden rod with a pointer that moves in front of a time display board. Later, the float’s shaft was connected to a gear wheel that moves the pointer across a graduated face.
Although water clocks were invented after sundials, they were used concurrently with sundials. On a wall in a tomb at a cemetery in Thebes, Egypt, the water clock of a priest and astronomer named Amenenhet, who died around 1550 BC, was discovered. Thanks to the water clock, Amenenhet observed that winter nights lasted 14 hours, while summer nights lasted only 12 hours, and throughout the year, the length of night changed from sunset to sunrise. Thus, sundials indicated the time, while water clocks were used to measure elapsed periods.
Around 250 BC, the Greek inventor Ctesibius created a highly ingenious water clock with a time recording pen. In this clock, water flowed into the cylindrical vessel in a more artistic manner: droplets of water from the top of a statue gradually fell into the vessel. As the water level rose, the float lifted a figure holding a pointer, and the end of this pointer was a pen that moved in front of a vertical column. After 24 hours, the water flowed out, falling onto a water wheel (roue à aubes), causing the vertical column to rotate slightly.
Later, the water clock was improved by adding a gear mechanism, turning it into an expensive machine that was still in demand among people living along the Mediterranean coast. In the 1st century BC, Pompey mandated the courts to use water clocks to prevent lawyers from “rambling.” Since orators needed ample time to defend their arguments, there was inevitably some dishonesty regarding timekeeping. Instances of bribing clock keepers to extend time were discovered, and notably, some lawyers poured muddy water into the clock to make it run slower than with clear water.
Another drawback of water clocks was that in cold weather, water would freeze. Julius Caesar encountered this issue when he led his soldiers to Britain. During his stay there, Caesar noticed that summer nights in Britain were shorter than those in Rome, and due to the lack of profound astronomical knowledge, he did not realize this was due to the difference in latitude.
In earlier times, the operation of water clocks was complex, so in 490 AD, when King Theodoric gifted a clock to King Gondebault of the Burgundians, he had to send along a person who knew how to operate it. The most famous clock of that era was sent by the Arab Prince Harun al-Rashid to Emperor Charlemagne in 809. This clock, made of brass, had 24 spherical brass pieces that gradually fell into a basin to indicate the hour.
There was also another type of water clock that operated very quietly, making it popular and widely used in the 17th century, during the era of pendulum clocks. This type consisted of a flattened cylindrical metal drum, divided inside into compartments a, b, c, d… These compartments were interconnected by small holes. Gravity caused water to flow from one compartment to another, gradually turning the metal drum. The axis of the drum descended and moved in front of a time display board. People only needed to look at the position of the horizontal axis to determine the time.
Pham Van Tuan
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