The verge escapement

An escapement lets a clock's wheels turn in small, regular steps. In the verge, a crown-shaped wheel pushes in turn on two small plates (pallets) set on a rod, the verge, so that the rod swings back and forth.^1, 6

In early clocks the rod carried a foliot, a bar with movable weights at its ends that set the rate; later a wheel-shaped balance did the same job, and this balance is what went into watches.^1, 4

The fault lay mostly in the oscillator, not the escapement: a foliot or unsprung balance has no natural period, so its rate follows the driving force. The pendulum (1656) and the balance spring (1675) cured that while keeping the verge.^1, 4, 7

Nobody knows who devised it. Dohrn-van Rossum's study finds no record of the invention, in contrast to the striking clock, which contemporaries noticed at once.^1, 2

Wikipedia says it probably grew out of earlier alarm mechanisms that rang a bell; a 1237 drawing by Villard de Honnecourt was once taken for an escapement but is generally not, and in 1271 Robertus Anglicus wrote that clockmakers were still trying to make one.¹

Wikipedia calls Richard of Wallingford's treatise of 1327 the earliest written description of an escapement; it describes not a verge but a variant with two escape wheels, the 'strob'. Giovanni Dondi's treatise on his astrarium (built 1348-1364) is one of the few other detailed descriptions of a 14th-century clock.^1, 8, 9

Possible Chinese roots are argued over: Su Song's tower of 1088 used a water-wheel escapement descended from one of 725, and Derek de Solla Price held that this spread west, but no transmission route is documented.^{2, 10, 11}

Wikipedia notes that how accurate verge-and-foliot clocks were is debated: estimates of one to two hours a day are quoted, while modern trials suggest minutes a day with care; the best were within about 15 minutes a day by the mid-17th century.¹

For about 350 years the verge had no rival: it drove the tower clocks at Salisbury (about 1386) and Wells (1386-1392), the 16th-century watches of Nuremberg and Augsburg, and English watches such as Robert Grinkin's of about 1610-1620.^{1, 12, 13}

Few early verge-and-foliot clocks survive unaltered, because most were converted to pendulums after 1656.^1, 12

Watches kept the verge until the 18th and early 19th centuries, when the cylinder and lever escapements replaced it in quality work.^3, 15