Key innovation: the escapement

In general terms, the medieval European mechanical clock consisted of four components: (1) a weight drive harnessing gravitational energy; (2) a mechanism (known as an ‘escapement’) controlling the release of that energy; (3) a geared mechanism for transmitting that controlled energy through a gear train; and (4) an indicating mechanism to mark out intervals of time (either by turning a dial or sounding a bell). 

As noted above, the third and fourth of these components were already in use in the inflow water clocks used in Hellenistic antiquity. Escapements of a different kind had also been used in the most sophisticated water-clocks in China and the Islamic world. But using weights (rather than water) to drive the mechanism required the invention of a new kind of escapement, which constitutes the key innovation in the invention of the mechanical clock.

The descent of a heavy weight needed to be rhythmically interrupted at uniform intervals which could be used to mark out time. This was the purpose of the ‘escapement’. The animation below is designed to explain how the earliest form of escapement functioned (with terminology identified in the diagram above). 

In the video, a suspended weight (not shown) forces the wheel to turn in a clockwise direction. The wheel, fitted with teeth, resembles a crown (from which the term ‘crown-wheel escapement’ derives). The teeth on the ‘crown-wheel’ engage with the two ‘pallets’ on the ‘verge’. The ‘pallets’ are attached to the ‘verge’ at 90 degrees to one another so that they regulate the turning of the wheel. As one tooth on the turning wheel forces the upper ‘pallet’ aside, the ‘verge’ twists so that the lower ‘pallet’ engages with the opposite tooth to restrict the rotation of the wheel. When the wheel ‘escapes’ this lower ‘pallet’, the upper one re-engages the next tooth in the wheel. In this way, the steady descent of the weight is divided into identical temporal units. The length of these units can be adjusted by moving the weights on the ‘foliot’ (also attached to the ‘verge’) in or out: moving the weights outward increases the duration of each oscillation; moving the weights inward decreases these intervals. These carefully calibrated, regular oscillations are conveyed through the drive train to the dial (or bell) which indicates the time. 

From the mid-seventeenth century onward, many other kinds of escapement were devised for use in watches as well as clocks; but this ‘verge-and-foliot escapement’ or ‘crown-wheel escapement’, invented in the early fourteenth century, dominated the first three centuries of European clockmaking.

Commentary. Philipp Nothaft (May-June 2019), enlarged by Howard Hotson (May 2024)