Water clocks were among the earliest timekeepers
that didn't depend on the observation of celestial bodies. One of the
oldest was found in the tomb of the Egyptian pharaoh Amenhotep I,
buried around 1500 BCE. Later named clepsydras ("water
thieves") by the Greeks, who began using them about 325 BCE, these were
stone vessels with sloping sides that allowed water to drip at a nearly
constant rate from a small hole near the bottom. Other clepsydras were
cylindrical or bowl-shaped containers designed to slowly fill with
water coming in at a constant rate. Markings on the inside surfaces
measured the passage of "hours" as the water level reached them. These
clocks were used to determine hours at night, but may have been used in
daylight as well. Another version consisted of a metal bowl with a hole
in the bottom; when placed in a container of water the bowl would fill
and sink in a certain time. These were still in use in North Africa in
the 20th century.
More elaborate and impressive mechanized water clocks were developed
between 100 BCE and 500 CE by Greek and Roman horologists and
astronomers. The added complexity was aimed at making the flow more
constant by regulating the pressure, and at providing fancier displays
of the passage of time. Some water clocks rang bells and gongs; others
opened doors and windows to show little figures of people, or moved
pointers, dials, and astrological models of the universe.
A Macedonian astronomer, Andronikos, supervised the construction of his Horologion,
known today as the Tower of the Winds, in the Athens marketplace in the
first half of the first century BCE. This octagonal structure showed
scholars and shoppers both sundials and mechanical hour indicators. It
featured a 24 hour mechanized clepsydra and indicators for the eight
winds from which the tower got its name, and it displayed the seasons
of the year and astrological dates and periods. The Romans also
developed mechanized clepsydras, though their complexity accomplished
little improvement over simpler methods for determining the passage of
time.
In the Far East, mechanized astronomical/astrological clock
making developed from 200 to 1300 CE. Third-century Chinese clepsydras
drove various mechanisms that illustrated astronomical phenomena. One
of the most elaborate clock towers was built by Su Sung and his
associates in 1088 CE. Su Sung's mechanism incorporated a water-driven
escapement invented about 725 CE. The Su Sung clock tower, over 30 feet
tall, possessed a bronze power-driven armillary sphere for
observations, an automatically rotating celestial globe, and five front
panels with doors that permitted the viewing of changing manikins which
rang bells or gongs, and held tablets indicating the hour or other
special times of the day.
Since the rate of flow of water is very difficult to control
accurately, a clock based on that flow could never achieve excellent
accuracy. People were naturally led to other approaches.
By - Joseph Letzelter
Posted at 01:08 pm by dravid
