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 allowing the gear train to advance at regular intervals or 'ticks'. Verge escapements were used from the late 13th century until the mid 19th century in clocks and . 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 , which had the potential to be more accurate.
, p.31 Oscillating timekeepers keep time for all modern clocks.
The verge escapement probably evolved from an alarm mechanism to ring a bell which had appeared centuries earlier.
, p.103-104 There has been speculation that Villard de Honnecourt invented the verge escapement in 1237 with an illustration of a strange mechanism to turn an angel statue to follow the sun with its finger,MS. 19093, folio 44, French Collection, Bibliothèque Nationale, Paris (No. 1104 Library of Saint-Germain-des Prés until c.1800). Villard_de_Honnecourt_-_Sketchbook_-_44.jpg is an image of the page on Wikimedia Commons but the consensus is that this was not an escapement., footnote 7, footnote 3It is believed that sometime in the late 13th century the verge escapement mechanism was applied to turret clock, creating the first mechanical escapement clock. In spite of the fact that these clocks were celebrated objects of civic pride which were written about at the time, it may never be known when the new escapement was first used. This is because it has proven difficult to distinguish from the meager written documentation which of these early tower clocks were mechanical, and which were water clocks; the same Latin word, horologe, was used for both.White 1966, p.124 None of the original mechanisms have survived unaltered. 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 Dunstable Priory clock in Bedfordshire, England built in 1283, because accounts say it was installed above the rood screen, where it would be difficult to replenish the water needed for a water clock.
Another is the clock built at the Palace of the Visconti, Milan, Italy, in 1335., p.196 Astronomer Robertus Anglicus wrote in 1271 that were trying to invent an escapement, but hadn't been successful yet.White, 1966, pp. 126-127. However, there is agreement that mechanical clocks existed by the late 13th century.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 more usual verge in clocks.For the first two hundred years or so of the mechanical clock's existence, the verge, with foliot or balance wheel, was the only escapement used in mechanical clocks. In the sixteenth century alternative escapements started to appear, but the verge remained the most used escapement for 350 years until mid-17th century advances in mechanics, resulted in the adoption of the pendulum, and later the anchor escapement.
Since clocks were valuable, after the invention of the pendulum many verge clocks were rebuilt to use this more accurate timekeeping technology, so very few of the early verge and foliot clocks have survived unaltered to the present day.How accurate the first verge and foliot clocks were is debatable, with estimates of one to two hours error per day
being mentioned, although modern experiments with clocks of this construction show accuracies of minutes per day were achievable with enough care in design and maintenance.W. Houtkooper "The Accuracy of the Foliot" Antiquarian Horology Vol. 20 No. One, Spring 1992M. Maltin "Some notes on the Medieval Clock in Salisbury Cathedral" Antiquarian Horology Vol. 20 No. 5, Spring 1993 Early verge clocks were probably no more accurate than the previous , but they did not require water to be manually hauled to fill the reservoir, did not freeze in winter, and were a more promising technology for innovation. By the mid-17th century, when the pendulum replaced the foliot, the best verge and foliot clocks had achieved an accuracy of 15 minutes per day.
In a verge pendulum clock (see picture) which appeared after the pendulum was invented in 1656, the escapement was turned 90° so the verge rod was horizontal, while the escape wheel's axis was vertical, located under the verge rod. In the first pendulum clocks the pendulum was attached to the end of the verge rod instead of the balance wheel or foliot. In later pendulum clocks the pendulum was suspended by a short straight spring of metal ribbon from the clock frame, and a vertical arm attached to the end of the verge rod ended in a fork which embraced the pendulum rod; this avoided the friction of suspending the pendulum directly from the pivoted verge rod. Each swing of the pendulum released an escape wheel tooth.
The escape wheel must have an odd number of teeth for the escapement to function. With an even number, two opposing teeth will contact the pallets at the same time, jamming the escapement. 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, upward 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., p.391-392
In , 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 relatively inexpensive verge fusee watches continued to be produced until the mid 19th century, when the lever escapement took over. These later verge watches were colloquially called 'turnips' because of their bulky build.
The verge was only used briefly in before it was replaced by the anchor escapement, invented around 1660 probably by Robert Hooke, and widely used beginning in 1680.
The problem with the verge was that it required the pendulum to swing in a wide arc of 80° to 100°. Christiaan Huygens in 1674 showed that a pendulum swinging in a wide arc is an inaccurate timekeeper, because its period of swing is sensitive to small changes in the drive force provided by the clock mechanism.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 diamond 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.
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