Verge escapement

# Verge escapement

Discussion

Encyclopedia
The verge escapement is the earliest known type of mechanical escapement
Escapement
In mechanical watches and clocks, an escapement is a device that transfers energy to the timekeeping element and enables counting the number of oscillations of the timekeeping element...

, the mechanism in a mechanical clock
Clock
A clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a mechanism has traditionally been known as a timepiece...

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
History of technology
The history of technology is the history of the invention of tools and techniques, and is similar in many ways to the history of humanity. Background knowledge has enabled people to create new things, and conversely, many scientific endeavors have become possible through technologies which assist...

, because it made possible the development of all-mechanical clocks. This caused a shift from measuring time by continuous
Continuous function
In mathematics, a continuous function is a function for which, intuitively, "small" changes in the input result in "small" changes in the output. Otherwise, a function is said to be "discontinuous". A continuous function with a continuous inverse function is called "bicontinuous".Continuity of...

processes, such as the flow of liquid in water clocks, to repetitive, oscillatory processes, such as the swing of pendulum
Pendulum
A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced from its resting equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position...

s, which had the potential to be more accurate. Oscillating timekeepers are at the heart of every clock today.

## Verge and foliot clocks

The first 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 13th century, large tower clocks were built in European 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
Balance wheel
The balance wheel is the timekeeping device used in mechanical watches and some clocks, analogous to the pendulum in a pendulum clock. It is a weighted wheel that rotates back and forth, being returned toward its center position by a spiral spring, the balance spring or hairspring...

, 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.

The verge probably evolved from the alarum, which used the same mechanism to ring a bell and had appeared centuries earlier. However, it may never be known when the escapement was first used in clocks, 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. There is speculation that Villard de Honnecourt
Villard de Honnecourt
Villard de Honnecourt was a 13th-century artist from Picardy in northern France. He is known to history only through a surviving portfolio of 33 sheets of parchment containing about 250 drawings dating from the 1220s/1240s, now in the Bibliothèque Nationale, Paris...

invented the verge escapement in 1237 with an illustration of a questionable device. 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 13th century.

Actually, the earliest description of an escapement, in Richard of Wallingford
Richard of Wallingford
Richard of Wallingford was an English mathematician who made major contributions to astronomy/astrology and horology while serving as abbot of St Albans Abbey in Hertfordshire.-Biography:...

'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 one to two hours being mentioned. Early verge clocks were probably no more accurate than the previous water clock
Water clock
A water clock or clepsydra is any timepiece in which time is measured by the regulated flow of liquid into or out from a vessel where the amount is then measured.Water clocks, along with sundials, are likely to be the oldest time-measuring instruments, with the only exceptions...

s, but they 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.

## Verge pendulum clocks

Most of the gross inaccuracy of verge and foliot clocks was not due to the escapement itself, but to the foliot oscillator. The invention of the pendulum
Pendulum
A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced from its resting equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position...

around 1656 suddenly increased the accuracy of the verge clock from hours a day to minutes a day . Most clocks were rebuilt with their foliots replaced by pendula, to the extent that it is difficult to find original verge and foliot clocks intact today. A similar increase in accuracy in verge watches followed the introduction of the balance spring
Balance spring
A balance spring, or hairspring, is a part used in mechanical timepieces. The balance spring, attached to the balance wheel, controls the speed at which the wheels of the timepiece turn, and thus the rate of movement of the hands...

in 1658.

## How it works

The verge escapement consists of a wheel shaped like a crown, with sawtooth-shaped teeth protruding axially toward the front, and with its axis oriented horizontally. In front of it is a vertical rod, the verge, with two metal plates, the pallets, that engage the teeth at opposite sides of the crown wheel. The pallets are not parallel, but are oriented with an angle in between them so only one catches the teeth at a time. The balance wheel (or the pendulum) is mounted at the end of the verge rod. As the clock's gears turn the crown wheel, one of its teeth pushes on a pallet, rotating the verge in one direction, and rotating the second pallet into the path of the teeth on the opposite side of the wheel, until the tooth pushes past the first pallet. Then a tooth on the wheel's opposite side contacts the second pallet, rotating the verge back the other direction, and the cycle repeats. The result is to change the rotary motion of the wheel to an oscillating motion of the verge. Each swing of the foliot or pendulum thus allows the wheel train of the clock to advance by a fixed amount, moving the hands forward at a constant rate.

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
Isochronous
Isochronous : From Greek iso, equal + chronos, time. It literally means regularly, or at equal time intervals. In general English language, it refers to something that occurs at a regular interval, of the same duration; as opposed to synchronous which refers to more than one thing happening at the...

, 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.

As might be expected from its early invention, the verge is the most inaccurate of the widely-used escapements. It suffers from these problems:
• Verge clocks are sensitive to changes in the drive force; they slow down as the mainspring
Mainspring
A mainspring is a spiral torsion spring of metal ribbon that is the power source in mechanical watches and some clocks. Winding the timepiece, by turning a knob or key, stores energy in the mainspring by twisting the spiral tighter. The force of the mainspring then turns the clock's wheels as it...

unwinds. This is called lack of isochronism
Isochronous
Isochronous : From Greek iso, equal + chronos, time. It literally means regularly, or at equal time intervals. In general English language, it refers to something that occurs at a regular interval, of the same duration; as opposed to synchronous which refers to more than one thing happening at the...

. It was much worse in verge and foliot clocks due to the lack of a balance spring, but is a problem in all verge movements. In fact, the standard method of adjusting the rate of early verge watches was to alter the force of the mainspring. The cause of this problem is that the crown wheel teeth are always pushing on the pallets, driving the pendulum (or balance wheel) throughout its cycle; it is never allowed to swing freely. All verge watches and spring driven clocks required fusees to equalize the force of the mainspring to achieve even minimal accuracy.
• The escapement has "recoil", meaning that the momentum of the foliot or pendulum pushes the crown wheel backward momentarily, causing the clock's wheel train
Wheel train (horology)
In horology, a wheel train is the gear train of a mechanical watch or clock. Although the term is used for other types of gear trains, the long history of mechanical timepieces has created a traditional terminology for their gear trains which is not used in other applications of gears.Watch...

to move backward, during part of its cycle. This increases friction and wear, resulting in inaccuracy. One way to tell whether an antique watch has a verge escapement is to observe the second hand closely; if it moves backward a little during each cycle, the watch is a verge. This is not necessarily the case in clocks, as there are some other pendulum escapements which exhibit recoil.
• In pendulum clocks, the wide pendulum swing angles of 80°-100° required by the verge cause an additional lack of isochronism due to circular error
Pendulum
A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced from its resting equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position...

.
• The wide pendulum swings also cause a lot of air friction
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...

, reducing the accuracy of the pendulum, and requiring a lot of power to keep it going, increasing wear. So verge pendulum clocks had lighter bobs, which reduced accuracy.
• Verge timepieces tend to accelerate as the crown wheel and the pallets wear down. This is particularly evident in verge watches from the mid-18th century onwards. It is not in the least unusual for these watches, when run today, to gain many hours per day, or to simply spin as if there were no balance present. The reason for this is that as new escapements were invented, it became the fashion to have a thin watch. To achieve this in a verge watch requires the crown wheel to be made very small, magnifying the effects of wear.

## Decline

Verge escapements were used in virtually all clocks and watches for 400 years. Then the increase in accuracy due to the introduction of the pendulum and balance spring in the mid 17th century focused attention on error caused by the escapement. By the 1820s, the verge was superseded by better escapements, though many examples of mid 19th century verge watches exist, as they were much cheaper by this time. These later verge watches were colloquially called 'turnips' because of their bulky build.

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 mid 19th century, when the lever escapement
Lever escapement
The lever escapement is a key component of the typical movement found in most mechanical wristwatches, pocket watches and many small mechanical non-pendulum clocks....

took over.

The verge was only used briefly in pendulum clock
Pendulum clock
A pendulum clock is a clock that uses a pendulum, a swinging weight, as its timekeeping element. The advantage of a pendulum for timekeeping is that it is a resonant device; it swings back and forth in a precise time interval dependent on its length, and resists swinging at other rates...

s before it was replaced by the anchor escapement
Anchor escapement
In horology, the recoil or anchor escapement is a type of escapement used in pendulum clocks. An escapement is the mechanism in a mechanical clock that maintains the swing of the pendulum and allows the clock's wheels to advance a fixed amount with each swing, moving the hands forward...

, invented around 1660 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 chronometer
Marine chronometer
A marine chronometer is a clock that is precise and accurate enough to be used as a portable time standard; it can therefore be used to determine longitude by means of celestial navigation...

s, H4 and H5, made by John Harrison
John Harrison
John Harrison was a self-educated English clockmaker. He invented the marine chronometer, a long-sought device in solving the problem of establishing the East-West position or longitude of a ship at sea, thus revolutionising and extending the possibility of safe long distance sea travel in the Age...

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.

• Galileo's escapement
Galileo's escapement
Galileo's escapement is a design for a clock escapement, invented by Galileo Galilei.While observing, and timing with his pulse, the swinging of lamps in the cathedral at Pisa, Galileo realized that their apparent isochronicity could be used for timekeeping...

- a proposed replacement of the Verge Escapement by Galileo Galilei
Galileo Galilei
Galileo Galilei , was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism...

• Salisbury cathedral clock
Salisbury cathedral clock
The Salisbury Cathedral clock, a large iron-framed clock without a dial located in the aisle of Salisbury Cathedral. Supposedly dating from about 1386, it is claimed to be the oldest working clock in the world....

—oldest known operating clock, with verge and foliot escapement