The Evolution of Timekeeping: Water Clocks in China and Mechanical Clocks in Europe
The Evolution of Timekeeping: Water Clocks in China and Mechanical Clocks in Europe
Early in history, humans sought methods to tell time. A concept rather than a physical entity, time eluded accurate measurement for many centuries. One of the first successful timekeeping devices was the water clock, which was perfected in China in the eighth century. It wasn't until nearly seven centuries later that mechanical clocks began to make their appearance. Mechanical clocks not only made timekeeping much more precise, which was important for scientific purposes, but also introduced it to the masses when centrally located clock towers equipped with bells loudly struck the hour.
One solar day spans one rotation of the earth on its axis. This natural unit of time is still the basic unit of timekeeping. For a variety of reasons, however, humans from past to present have desired smaller increments for determining the time. Thousands of years ago, humans began to separate the day into sections. At first, they assigned such broad categories as late morning or early afternoon, or identified the time of day by its association to mealtimes. By 2100 b.c., Egyptians had begun dividing the day and night each into 12 parts. Derived from the Greek word hora, an hour denoted the interval between the rising of specific stars at night. Since the period from dawn to dusk and from dusk to day were not identical—changing from season to season and even day to day—the length of an hour changed accordingly. As the days become longer or shorter, the time covered by these so-called temporal hours varied. For example, 12 daytime temporal hours in the summer might cover 14 hours of daylight, whereas the following 12 nighttime temporal hours would be crowded into the remaining 10-hour period.
Many societies used ancient sundials to measure time intervals. Originally employed to identify the changing of seasons, they were further developed to measure increments within a day. Sundials rely on the sun to cast a shadow onto a marked platform. As the sun moves across the sky, the shadow advances across the platform and denotes the temporal hour.
Chinese inventors developed the first method for measuring time consistently and without reliance on sunlight, day length, or star movement. Since about 3000 b.c., the Chinese used water clocks to gauge the passage of time. Water clocks are also known as clepsydrae, the Greek word for "water thief." A simple water clock is an apparatus that slowly drips or runs water from a small hole in one vessel into another that is stationed below it. By marking the water level in the lower vessel after a day had passed and then dividing it into equal portions, the clockmaker could use the device to tell time fairly accurately. Tests have indicated that early water clocks were correct to within 15 minutes each day.
Water clocks in China continued to progress into more sophisticated and accurate devices. Their development took a leap forward in the eighth century during the K'ai-Yuan reign when a Buddhist monk named I-Hsing (I-Xing) along with Liang Lin-Tsan, an engineer and member of the crown prince's bodyguard, began work on a clock escapement to control the speed and regularity of the clock's movements. The clock, a bronze model of the celestial sphere (a representation of how the stars appear from Earth), used drops of water to move the driving-wheel mechanism, and keep track of hours, days, and years. The clock also connected to a bell and drum, to provide a sound alert every 15 minutes.
Another notable clock in Chinese history was the astronomical clock of Chang Ssu-Hsün in 1092. Built into a 33-foot (10-m)-tall tower, the clock used water to power a complicated escapement mechanism that was similar in appearance to a Ferris wheel with water buckets in the place of seats. A water tank dripped water into one bucket at a time. As the bucket filled, it became heavy enough to trip a lever and rotate the wheel. When the wheel rotated, the next bucket moved under the water tank for filling. Chang also included in his clock design an armillary sphere, which consisted of rings to mimic planet orbits. In addition, the clock mechanism triggered 12 jacks, or puppets, to appear in sequence to ring bells and hit a drum to announce the time.
The next major advancement in timekeeping came with the development of mechanical clocks, probably in the late thirteenth century. These clocks depended on neither the sun nor water to keep time. Some used pendulums, while other smaller clocks relied on repeated winding to run. English records indicate that a mechanical clock was operating in a Bedfordshire church in 1283. Similar reports refer to five other mechanical clocks in English churches before 1300. Within the next 50 years, the mechanical clocks became common throughout Europe.
While temporal hours and early timekeeping methods were sufficient for many societal uses, humans continued their quest for better modes of telling time. Early astronomers and mathematicians in particular needed accurate time increments that remained static from day to day and season to season. Without precise measurements they could not determine speed, which was crucial for navigational and astronomical observations and applications.
The advent of the water clock did much to change the way humans viewed time. Now the time of day did not depend on whether the sun was sufficiently able to penetrate the clouds and cast a shadow onto a sundial or whether the night sky was dark enough to view the stars' positions. An hour could now represent a constant length of time, and could be further divided into smaller fragments. When I-Hsing and Liang invented the escapement, they greatly refined clock performance. Chang then took I-Hsing and Liang's contribution to the next level by making an even more intricate escapement, which was named the "heavenly scale." Water clocks continued to be popular in China and many other countries well into the fourteenth century. (Currently, The Children's Museum of Indianapolis boasts the largest water clock in North America with a 26.5-foot [8-m]-tall device.)
Despite improvements to the mechanism over the centuries, water clocks never attained perfection. They repeatedly needed resetting to the correct time, as well as near-constant maintenance. Winter was particularly trying. During these colder months, if the water was not replaced with mercury or some other liquid with a lower freezing point, the water would turn to ice and the clocks would stop.
In Europe, the development of clocks took a different turn. Instead of looking to water as a power source, Europeans took another path. According to History of the Hour: Clocks and Modern Temporal Orders, "The principle of the Chinese escapement is pivoting balance levers that stabilized a stop-and-go motion. The principle of the European escapement, which employs the centrifugal force of an oscillating inert mass, does not resemble it in any way whatsoever."
These weight-driven mechanical clocks injected time into European society. Clock towers sprung up in cities and loudly rang the hour for all the residents to hear. The earliest tower clocks were rather inaccurate—they lost or gained up to two hours each day—and had only one hand to denote the general time of day. For years, clockmakers struggled to regulate the mechanism's oscillation without much success. This problem did not deter the public from demanding better timekeeping devices. By 1500, clockmakers found a way to make the mechanism small enough so that the wealthy could purchase models for their homes. These clocks, many of which were used as alarm clocks, kept time by springs that were wound about once a day. The clocks kept time fairly well, although hours went by more and more slowly as the spring unwound.
With timekeeping becoming commonplace, societal dependence grew. Meetings, church services, and other appointments could now be scheduled at certain hours, instead of general times of day. Scientists could now begin to make much more accurate time measurements, physicians could do simple diagnostic tests as determining pulse rate, and navigators could use time to determine their position at sea. As time became more important, people began demanding more accurate clocks. Despite persistent attempts to perfect mechanical clocks, it wasn't until 1656 when Dutch mathematician Christiaan Huygens (1629-1695) used pendulums as a timekeeping mechanism, that clocks were able to tick off minutes accurately. Huygens's original design was correct to within a minute a day. By 1670 William Clement of London had refined the pendulum clock to keep time to within a second each day.
These improvements set the stage for later advancements that by 1761 had generated John Harrison's (1693-1776) marine chronometer accurate to 0.2 seconds per day, and by 1889 Siegmund Riefler's pendulum clock was true to 0.01 seconds a day. High-performance quartz-crystal clocks appeared in the 1930s, followed by the atomic clocks of more recent years.
LESLIE A. MERTZ
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National Institute of Standards and Technology. "A Revolution in Timekeeping." http://physics.nist.gov/GenInt/Time/revol.html.
National Institute of Standards and Technology. "Earliest Clocks." http://physics.nist.gov/GenInt/Time/early.html.