The verge (or crown wheel) escapement is the earliest known type of mechanical escapement, the mechanism in a mechanical clock that controls its rate by advancing the gear train at regular intervals or 'ticks'. Its origin is unknown. Verge escapements were used from the 14th century until about 1800 in clocks and pocketwatches. The name verge comes from the Latin virga, meaning stick or rod.
Its invention is important in the history of technology, because it made possible the development of all-mechanical clocks. This caused a shift from measuring time by continuous processes, such as the flow of liquid in water clocks, to repetitive, oscillatory processes, such as the swing of pendulums, which had the potential to be more accurate. Oscillating timekeepers are at the heart of every clock today.
In China, escapement-like devices were employed in a water-powered celestial globe built by Yi Xing in 725 CE and Su Song's astronomical clock-tower in 1088. However, these were not true mechanical escapements as employed later in mechanical clocks, since the time was still measured by flowing water:
The first hard evidence of the verge escapement dates from 14th century Europe, where its invention led to the development of the first all-mechanical clocks. Starting in the 1200s, large tower clocks began appearing in town squares and cathedrals. They kept time by using the verge escapement to drive a horizontal bar with weights on the ends called the foliot, a primitive type of balance wheel, to oscillate back and forth. The rate of the clock could be adjusted by sliding the weights in or out on the foliot bar.
There is speculation that Villard de Honnecourt invented the verge escapement in 1237 with an illustration of a questionable device. It probably evolved from the alarum, invented centuries earlier, which used the same mechanism to ring a bell. However, it may never be known when the escapement was first used, because it has proven impossible to distinguish from existing documentation which of these early tower clocks were mechanical, and which were water clocks . The same Latin word, horologe, was used for both. Sources differ on which was the first clock 'known' to be mechanical, depending on which manuscript evidence they regard as conclusive. One candidate is the clock built at the Palace of the Visconti, Milan, Italy, in 1335. However, there is agreement that mechanical clocks existed by the late 1200s.
Actually, the earliest description of an escapement, in Richard of Wallingford's 1327 manuscript Tractatus Horologii Astronomici on the clock he built at the Abbey of St. Albans, was not a verge, but a variation called a 'strob' escapement. It consisted of a pair of escape wheels on the same axle, with alternating radial teeth. The verge rod was suspended between them, with a short crosspiece that rotated first in one direction and then the other as the staggered teeth pushed past. Although no other example is known, it is possible that this design preceded the verge in clocks.
How accurate these early verge and foliot clocks were is debatable, with estimates of two hours to 15 minutes per day being mentioned. Early verge clocks were probably no more accurate than the previous water clocks, but they did not freeze in winter and were a more promising technology for innovation.
The crown wheel must have an odd number of teeth for the escapement to function. The usual angle between the pallets was 90° to 105°, resulting in a foliot or pendulum swing of around 80° to 100°. In order to reduce the pendulum's swing to make it more isochronous, the French used larger pallet angles, upwards of 115°. This reduced the pendulum swing to around 50° and reduced recoil (below), but required the verge to be located so near the crown wheel that the teeth fell on the pallets very near the axis, reducing initial leverage and increasing friction, thus requiring lighter pendulums.
In pendulum clocks, it was replaced by the anchor escapement, invented around 1670. By locating the pallets farther from the axis of rotation, the anchor escapement reduced the pendulum swing from around 100° in verge clocks to 4°-6°. In addition to eliminating circular error, this allowed room in the clock case for longer and slower pendulums, reducing wear. In England the anchor escapement took over, but the French continued to use verge escapements until about 1800. Many verge clocks were rebuilt with anchor escapements.
In pocketwatches, besides its inaccuracy, the vertical orientation of the crown wheel and the need for a bulky fusee made the verge movement unfashionably thick. French watchmakers adopted the thinner cylinder escapement, invented in 1695. In England, high end watches went to the duplex escapement, developed in 1782, but inexpensive verge fusee watches continued to be produced until the early 1800s, when the lever escapement took over.
Although the verge is not known for accuracy, it is capable of it. The first successful marine chronometers, H4 and H5, made by John Harrison in 1759 and 1770, used verge escapements with ruby pallets. In trials they were accurate to within a fifth of a second per day.
Today the verge is seen only in antique or antique-replica timepieces. Many original bracket clocks have their Victorian-era anchor escapement conversions undone and the original style of verge escapement restored. Clockmakers call this a verge reconversion.