Bunch-Hellemans, Inventing the clock

In ancient time-measurement systems, including those of both Egypt and China, daylight and nighttime were each given 12 hours. This was convenient for use with sundials, which are known from Egypt as early as 1500 BCE, although telling time “by the Sun” probably predates the first humans. Because the length of daylight and nighttime varies with the season, so did the length of the Egyptian hour.

When water clocks came into use, shortly after sundials, a conflict between the two forms of measurement was apparent. A water clock works because water from a container flows through an opening at a nearly steady rate. The amount of water in another container is used to move an indicator of some kind –– in simplest form, the level of water in a container. Since the water flows at an almost steady rate, the indicator shows the hours as it moves along a marked face. But when the length of the hour changed from season to season, a different water clock was needed for each month.

Ancient peoples solved this problem in various ways, such as by having different marked faces for each month. In that way the water clock was never far out of line with the sundial, which also remained in use. Later, instead of modifying water clocks to change with the seasons, sundials were constructed to show hours of the same length all year.

In the eighth century CE, the Chinese began to fashion water clocks with primitive escapements. The escapement is a ratchet that causes a wheel to move only so far and then stop, so that there is no runaway action when the clock is fully loaded with water. Continuous motion is replaced with discrete “ticks.” By the beginning of the 14th century, the concept of an escapement was known in Europe.

The escapement was used to slow down the motion of a falling weight attached to it by a cord or chain. This motion could then be converted with gears to turn the dial of a clock. Mechanical clocks using escapements and weights were gradually improved and put in towers all over Europe.