Science Art: Verge Watch Escapement, from The Cyclopaedia, or Universal Dictionary, Vol. 2, 1820.

Scientific illustration of a watch escapement

Scientific illustration of a watch escapement

A horological device called a “verge escapement” (on the bottom) with a balance wheel (on the top) from a pocketwatch.

An “escapement” is the thing that makes a watch tick – it advances in set intervals, then pauses, then advances, then pauses again.

From the image description on Wikimedia Commons:

The verge escapement, the oldest mechanical escapement, was used from the 13th century, and was the first escapement used in clocks and watches. It consisted of a vertical rod, the “verge”, with angled flat plates called “pallets” (e,f) attached. The pallets are pushed by the teeth projecting from a vertical wheel (c,d), turning the balance wheel alternately clockwise and counterclockwise, against the restoring force of the balance spring i. The verge was replaced by better escapements beginning around 1700 but verge watches continued to be made until the mid 1800s.