Escapement

Lore & Background

The earliest liquid-driven escapement was described by the Greek engineer Philo of Byzantium in the 3rd century BC in chapter 31 of his technical treatise Pneumatics, as part of a washstand. A counterweighted spoon, supplied by a water tank, tips over in a basin when full, releasing a spherical piece of pumice in the process. Once the spoon has emptied, it is pulled up again by the counterweight, closing the door on the pumice by the tightening string. Philo's comment that 'its construction is similar to that of clocks' indicates that such escapement mechanisms were already integrated in ancient water clocks. In China, the Tang dynasty Buddhist monk Yi Xing, along with government official Liang Lingzan, made in 723 (or 725) AD the escapement for the workings of a water-powered armillary sphere and clock drive, which was the world's first clockwork escapement. Song dynasty horologists Zhang Sixun and Su Song duly applied escapement devices for their astronomical clock towers in the 10th century, where water flowed into a container on a pivot. However, the technology later stagnated and retrogressed. According to historian Derek J. de Solla Price, the Chinese escapement spread west and was the source of Western escapement technology. According to Ahmad Y. Hassan, a mercury escapement described in a Spanish document for Alfonso X in 1277 can be traced to earlier Arabic sources. Knowledge of these mercury escapements may have spread through Europe with translations of Arabic and Spanish texts. However, none of these were true mechanical escapements, since they still depended on the flow of liquid through a hole to measure time.

Reader's Guide

The invention of the escapement was an important step in the history of technology, as it made the all-mechanical clock possible. The first all-mechanical escapement, the verge escapement, was invented in 13th-century Europe. It allowed timekeeping methods to move from continuous processes such as the flow of water in water clocks, to repetitive oscillatory processes such as the swing of pendulums, enabling more accurate timekeeping. Oscillating timekeepers are the controlling devices in all modern clocks. The verge became the standard escapement used in all other early clocks and watches, and remained the only known escapement for 400 years. Its performance was limited by friction and recoil, but most importantly, the early balance wheels used in verge escapements, known as the foliot, lacked a balance spring and thus had no natural 'beat', severely limiting their timekeeping accuracy. A great leap in the accuracy of escapements happened after 1657, due to the invention of the pendulum and the addition of the balance spring to the balance wheel, which made the timekeepers in both clocks and watches harmonic oscillators. The resulting improvement in timekeeping accuracy enabled greater focus on the accuracy of the escapement. The next two centuries, the 'golden age' of mechanical horology, saw the invention of over 300 escapement designs, although only about ten of these were ever widely used in clocks and watches. The invention of the crystal oscillator and the quartz clock in the 1920s, which became the most accurate clock by the 1930s, shifted technological research in timekeeping to electronic methods, and escapement design ceased to play a role in advancing timekeeping precision.

Did You Know?

• The first mechanical escapement, the verge escapement, was invented in 13th-century Europe.
• The earliest liquid-driven escapement was described by Greek engineer Philo of Byzantium in the 3rd century BC.
• Manual typewriters used escapements to step the carriage as each letter or space was typed.
• The sudden stopping of the escapement's tooth generates the characteristic 'ticking' sound in mechanical clocks and watches.