Elements of a Clock
Before we continue describing the evolution of ways to mark the passage of time, perhaps we should broadly define what constitutes a clock. All clocks must have two basic components:
The history of timekeeping is the story of the search for ever more consistent actions or processes to regulate the rate of a clock.
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.