Harrison Number Four

Harrison Number Four

For other people named John Harrison, see John Harrison (disambiguation).

John Harrison
P.L. Tassaert's half-tone print of Thomas King's original 1767 portrait of John Harrison, located at the Science and Society Picture Library, London
Born (1693-03-24)24 March 1693
Foulby, near Wakefield, West Yorkshire
Died 24 March 1776(1776-03-24) (aged 83)
London
Residence Red Lion Square
Nationality United Kingdom
Fields Horology
Known for Marine chronometer
Notable awards Copley Medal

John Harrison (24 March 1693 – 24 March 1776) was a self-educated English carpenter and later a 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 of Sail. The problem was considered so intractable that the British Parliament offered a prize of £20,000 (comparable to £2.66 million/$4.25 million US in modern currency) for the solution.[1][2]

Harrison came 39th in the BBC's 2002 public poll of the 100 Greatest Britons.[3]

Early life

John Harrison was born in Foulby, near Wakefield in West Yorkshire, the first of five children in his family. His father worked as a carpenter at the nearby Nostell Priory estate. A house on the site of what may have been the family home bears a blue plaque.[4]

Around 1700, the family moved to the North Lincolnshire village of Barrow upon Humber. Following his father's trade as a carpenter, Harrison built and repaired clocks in his spare time. Legend has it that at the age of six while in bed with smallpox he was given a watch to amuse himself, spending hours listening to it and studying its moving parts.

He also had a fascination for music, eventually becoming choirmaster for Barrow parish church.[5]

Career

Harrison built his first longcase clock in 1713, at the age of 20. The mechanism was made entirely of wood, which was a natural choice of material for a joiner. Three of Harrison's early wooden clocks have survived: the first (1713) is at the Worshipful Company of Clockmakers' Collection in Guildhall; the second (1715), is in the Science Museum; and, the third (1717) is at Nostell Priory in Yorkshire, the face bearing the inscription "John Harrison Barrow". The Nostell example, in the billiards room of this fine stately home, has a Victorian outer case, which has been thoughtfully provided with small glass windows on each side of the movement so that the wooden workings may be inspected. In the early 1720s Harrison was commissioned to make a new turret clock at Brocklesby Park, North Lincolnshire. The clock still operates and like his previous clocks has a wooden movement, made of oak and lignum vitae. Unlike his early clocks it incorporates some original features to improve timekeeping, for example the grasshopper escapement. Between 1725 and 1728 John and his brother James, also a skilled joiner, made at least three precision pendulum-clocks, again with oak and lignum vitae movements and longcase. The grid-iron pendulum was developed during this phase. These precision pendulum-clocks are thought by some to have been the most accurate clocks in the world at the time, and significantly are the direct link to the sea clocks. No. 1, now in a private collection was in the collections of the Time Museum, USA, until that museum closed in 2000 and its collection dispersed at auction in 2004. No. 2 is in the collections of Leeds Museums and Galleries, West Yorkshire, United Kingdom. It is not usually on display but it will form part of a 2012 exhibition at York.[6][dated info] No. 3 is in the Worshipful Company of Clockmakers' collection.

He was a man of many skills and used these to systematically improve the performance of pendulum clocks. He invented the gridiron pendulum, consisting of alternating brass and iron rods assembled so that the different expansions and contractions cancel each other out. Another example of his inventive genius was the grasshopper escapement – a control device for the step-by-step release of a clock's driving power. Developed from the anchor escapement, it was almost frictionless, requiring no lubrication because the pallets were made from the wood lignum vitae. This was an important advantage at a time when lubricants and their degradation were little understood. It is not often recognized that in his earlier work on the "Sea clocks" Harrison was continually assisted both financially and in many other ways by George Graham, the watchmaker and instrument maker who lent him a large sum on the basis of trust even after Harrison's first visit to Graham in 1730 to explain how his timekeeper worked. Harrison was introduced to Graham by the Astronomer Royal Edmond Halley who also championed Harrison and his work. This support was important as Harrison is reputed to have found it difficult to communicate his ideas in a coherent manner.

Longitude

Overview of the problem

A longitude describes the location of a place on Earth east or west of a north-south line called the Prime Meridian. Longitude is given as an angular measurement ranging from 0° at the Prime Meridian to +180° eastward and −180° westward. Many solutions were proposed for how to determine longitude at the end of an exploratory sea voyage and hence the longitude of the place that was visited (in case one would want to revisit it, place it on a map, or more urgently, avoid known marine hazards). The practical methods relied on a comparison of local time with the time at a given place (such as Greenwich or Paris). Many of these methods relied on astronomical observations relying on the predictable, "clockwork" nature of motions of heavenly bodies. By measuring locations of the fixed stars, local time could be reckoned. The remaining problem was to accurately estimate the time at the given (now distant) place.

Harrison set out to solve the problem in a direct way: by producing a reliable clock that could keep the time of the given place across a long sea journey. The theory was simple and had been first proposed by Frisius. The difficulty, however, was in producing a clock which could maintain accurate time on a lengthy, rough sea voyage with widely varying conditions of temperature, pressure and humidity. Frisius had realized that to determine longitude, a clock would have to be “of great exactness”. Many leading scientists including Newton and Huygens doubted that such a clock could ever be built and had more optimism for astronomical observations (such as the Method of Lunar Distances). Huygens ran trials using both a pendulum and a spiral balance spring clock as methods of determining longitude. Although both types showed some favorable results, they were both prone to fickleness. Newton observed that “A good watch may serve to keep a reckoning at sea for some days and to know the time of a celestial observation; and for this end a good Jewel may suffice till a better sort of watch can be found out. But when longitude at sea is lost, it cannot be found again by any watch.” However, if such a clock were built and set at noon in London at the start of a voyage, it would subsequently always tell you how far from noon it was in London at that second, regardless of where you had traveled. By referring to the clock when it is noon locally (i.e. the Sun is at its highest in the sky where you are) you can read, almost directly from the clock face, how far around the world you are from London. For instance, if the clock shows that it is midnight in London when it is noon locally, then you are half way round the world, (e.g. 180 degrees of longitude) from London.

This is so because the earth is constantly rotating, and therefore knowing the time while making an altitude measurement to a known heavenly body such as the sun, provided critical data for a ship's position east-west—a necessary capability for re-approaching land after voyages over medium and long distances. On such voyages, cumulative errors in dead reckoning frequently led to shipwrecks and lost lives. Avoiding maritime tragedies became an imperative in Harrison's lifetime because this was an era when trade and navigation were on an explosive increase around the globe due to the maturing of other technologies, and also due to geo-political circumstances.

Knowing such measurements without an accurate time could only show position in latitude which was a trivial problem in comparison. Such a maritime clock had to be not only highly accurate over long time intervals, but relatively impervious to corrosion in salt air, able to tolerate wide variations in temperature and humidity and in general durable while able to function at the odd angles and pitch and yaw typical of decks under strong waves and storm tossed conditions.

Yet the timekeeping device with such accuracy would eventually also allow the determination of longitude accurately, making the device a fundamental key to the modern age. Following Harrison, the marine timekeeper was reinvented yet again by John Arnold who while basing his design on Harrison's most important principles, at the same time simplified it enough for him to produce equally accurate but far less costly marine chronometers in quantity from around 1783. Nonetheless, for many years even towards the end of the 18th century, chronometers were expensive rarities, as their adoption and use proceeded slowly due to the precision manufacturing necessary and hence high expense. The expiry of Arnold's patents at the end of the 1790s enabled many other watchmakers including Thomas Earnshaw to produce chronometers in greater quantities at less cost even than those of Arnold's. By the early 19th century, navigation at sea without one was considered unwise to unthinkable. Using a chronometer to aid navigation simply saved lives and ships—the insurance industry, exercise of self-interest, and common sense did the rest in making the device a universal tool of maritime trade.

The first three marine timekeepers

The English clockmaker Henry Sully had already invented a marine clock to determine longitude accurately; this was in the form of a small clock with a large balance wheel vertically mounted on friction rollers and impulsed by a frictional rest Debaufre type escapement. Very unconventionally, the balance oscillations were controlled not by a spring but by a weight at the end of a pivoted horizontal lever attached to the balance by a cord, a solution which rather neatly avoided temperature error, a problem which affects steel balance springs. Sully's clock was not successful as a marine timekeeper, however. It only kept accurate time in calm weather, as the balance oscillations were affected by the pitching and rolling of the ship. However Sully's clocks are of importance as they were amongst the first serious attempts to find the Longitude by using a clock. It is perhaps not coincidental that Harrison's machines, though much larger, are of similar layout. For example his third sea clock (H3) also has a vertically mounted balance wheel and is linked to another the same size. This arrangement eliminates the problems arising from pitching and rolling motions of the ship.[7] Sully presented a first Montre de la Mer in 1716 to the French Académie des Sciences.[8] and in 1726 published Une Horloge inventée et executée par M. Sulli.[8]


In 1730 Harrison created a description and drawings for a proposed marine clock to compete for the Longitude Prize and went to London seeking financial assistance. He presented his ideas to Edmond Halley, the Astronomer Royal. Halley referred him to George Graham, the country's foremost clockmaker. He must have been impressed by Harrison, for Graham personally loaned Harrison money to build a model of his 'Sea clock' as Harrison called it.

This clock was essentially an attempt to make a seagoing version of his wooden pendulum clocks which had performed so exceptionally well. Thus the use of wooden wheels, roller pinions and a version of the 'grasshopper' escapement. Instead of a pendulum he used two dumbbell balances linked together,

It took Harrison five years to build his first Sea Clock (or H1).[9] He demonstrated it to members of the Royal Society who spoke on his behalf to the Board of Longitude. The clock was the first proposal that the Board considered to be worthy of a sea trial. In 1736, Harrison sailed to Lisbon on HMS Centurion and returned on HMS Orford. On their return, both the captain and the sailing master of the Orford praised the design. The master noted that his own calculations had placed the ship sixty miles east of its true landfall which had been correctly predicted by Harrison using H1.

This was not the transatlantic voyage demanded by the Board of Longitude, but the Board was impressed enough to grant Harrison £500 for further development. Harrison moved on to develop H2,[10] a more compact and rugged version. In 1741, after three years of building and two of on-land testing, H2 was ready, but by then Britain was at war with Spain in the War of Austrian Succession and the mechanism was deemed too important to risk falling into Spanish hands. In any event, Harrison suddenly abandoned all work on this second machine when he discovered a serious design flaw in the concept of the bar balances. He had not recognized that the period of oscillation of the bar balances could be affected by the pitching action of the ship. It was this that led him to adopt circular balances in the Third Sea Clock (H3). The Board granted him another £500, and while waiting for the war to end, he proceeded to work on H3.[11]

Harrison spent seventeen years working on this third 'sea clock' but despite every effort it seems not to have performed exactly as he would have wished. Despite this, it had proved a very valuable experiment. Certainly in this machine Harrison left the world two enduring legacies – the bimetallic strip and the caged roller bearing. The failure of the sea clocks 1, 2 and 3 were due mainly to the fact that their balances though large, did not vibrate quickly enough to confer the property of stability on the timekeeping. Around 1750 Harrison had also come to this conclusion and abandoned the idea of the 'Sea clock' as a timekeeper, realizing that a watch sized timekeeper would be more successful as it could incorporate a balance which though smaller, oscillated at a much higher speed. A watch would also be more practicable, another factor required by the Longitude Act of 1714.

The longitude watches

After steadfastly pursuing various methods during thirty years of experimentation, Harrison moved to London in late 1758 where to his surprise he found that some of the watches made by Graham's successor Thomas Mudge kept time just as accurately as his huge sea clocks. It is possible that Mudge was able to do this after the early 1740s thanks to the availability of the new "Huntsman" or "Crucible" steel produced by Benjamin Huntsman sometime in the early 1740s which enabled harder pinions but more importantly, a tougher and more highly polished cylinder escapement to be produced.[12][13] Harrison then realized that a mere watch after all could be made accurate enough for the task and was a far more practical proposition for use as a marine timekeeper. He proceeded to redesign the concept of the watch as a timekeeping device, basing his design on sound scientific principles.

The 'Jefferys' watch

He had already in the early 1750s designed a precision watch for his own personal use, which was made for him by the watchmaker John Jefferys c. 1752 – 53. This watch incorporated a novel frictional rest escapement and was not only the first to have a compensation for temperature variations but also contained the first 'going fusee' of Harrison's design which enabled the watch to continue running whilst being wound. These features led to the very successful performance of the "Jefferys" watch so therefore Harrison incorporated them into the design of two new timekeepers which he proposed to build. These were in the form of a large watch and another of a smaller size but of similar pattern. However only the larger No. 1 (or "H4" as it sometimes called) watch appears ever to have been finished. (See the reference to "H6" below) Aided by some of London's finest workmen, he proceeded to design and make the world's first successful marine timekeeper that allowed a navigator to accurately assess his ship's position in longitude. Importantly, Harrison showed everyone that it could be done by using a watch to calculate longitude.[14] This was to be Harrison's masterpiece – an instrument of beauty, resembling an oversized pocket watch from the period. It is engraved with Harrison's signature, marked Number 1 and dated AD 1759.


Harrison's First Marine Watch

This watch was referred to by Harrison as his first "Sea watch" but denoted "H4" by Gould in order to differentiate it from the first 'Sea clock'. Of large size, it is housed in silver pair cases some 5.2 inches (13 cm) in diameter. The movement is for the period highly complex but resembling a conventional movement of this period but of much larger size. It has a novel type of 'vertical' escapement, which due to its appearance is often incorrectly associated with the 'verge' escapement which it superficially resembles. However, its action is completely different to the verge as being a frictional rest escapement it enables the balance to have a large arc. This is quite unlike the verge which is a different escapement altogether having recoil with a limited balance arc, and moreover sensitive to variations in driving torque.

The D shaped pallets of Harrison's escapement are both made of diamond, a considerable feat of manufacture at the time. For technical reasons the balance was made much larger than in a conventional watch of the period, and the vibrations controlled by a flat spiral steel spring. The movement also has centre seconds motion with a sweep seconds hand. The Third Wheel is equipped with internal teeth and has an elaborate bridge similar to the pierced and engraved bridge for the period. It runs at 5 beats (ticks) per second, and is equipped with a tiny remontoire. A balance-brake stops the watch half an hour before it is completely run down, in order that the remontoire does not run down also. Temperature compensation is in the form of a 'compensation curb' (or 'Thermometer Kirb' as Harrison called it). This takes the form of a bimetallic strip mounted on the regulating slide, and carrying the curb pins at the free end. During its initial testing, Harrison dispensed with this regulation using the slide, but left its indicating dial or figure piece in place.

This first watch took six years to construct and Harrison, by then 68 years old, sent it on its transatlantic trial in the care of his son, William, in 1761. When HMS Deptford reached Jamaica, the watch was 5 seconds slow, corresponding to an error in longitude of 1.25 minutes, or approximately one nautical mile.[15] When the ship returned, Harrison waited for the £20,000 prize but the Board were persuaded that the accuracy was just luck and demanded another trial. The Harrisons were outraged and demanded their prize, a matter that eventually worked its way to Parliament, which offered £5,000 for the design. The Harrisons refused but were eventually obliged to make another trip to Bridgetown on the island of Barbados to settle the matter.

At the time of the trial, another method for measuring longitude was ready for testing: the Method of Lunar Distances. The moon moves fast enough, some thirteen degrees a day, to easily measure the movement from day to day. By comparing the angle between the moon and the sun for the day one left for Britain, the "proper position" (how it would appear in Greenwich, England, at that specific time) of the moon could be calculated. By comparing this with the angle of the moon over the horizon, the longitude could be calculated.

During Harrison's second trial of his 'Sea watch' (H4) the Reverend Nevil Maskelyne was asked to accompany HMS Tartar and test the Lunar Distances system. Once again the watch proved extremely accurate, keeping time to within 39 seconds, corresponding to an error in the longitude of Bridgetown of less than 10 miles (16 km).[15] Maskelyne's measures were also fairly good, at 30 miles (48 km), but required considerable work and calculation in order to use. At a meeting of the Board in 1765 the results were presented, but they again attributed the accuracy of the measurements to luck. Once again the matter reached Parliament, which offered £10,000 in advance and the other half once he turned over the design to other watchmakers to duplicate. In the meantime Harrison's watch would have to be turned over to the Astronomer Royal for long-term on-land testing.

Unfortunately, Nevil Maskelyne had been appointed Astronomer Royal on his return from Barbados, and was therefore also placed on the Board of Longitude. He returned a report of the watch that was negative, claiming that its "going rate" (the amount of time it gained or lost per day) was due to inaccuracies cancelling themselves out, and refused to allow it to be factored out when measuring longitude. Consequently, this first Marine Watch of Harrison's failed the needs of the Board despite the fact that it had succeeded in two previous trials.

Harrison began working on his second 'Sea watch' (H5) while testing was conducted on the first, which Harrison felt was being held hostage by the Board. After three years he had had enough; Harrison felt "extremely ill used by the gentlemen who I might have expected better treatment from" and decided to enlist the aid of King George III. He obtained an audience with the King, who was extremely annoyed with the Board. King George tested the watch No.2 (H5) himself at the palace and after ten weeks of daily observations between May and July in 1772, found it to be accurate to within one third of one second per day. King George then advised Harrison to petition Parliament for the full prize after threatening to appear in person to dress them down. Finally in 1773, when he was 80 years old, Harrison received a monetary award in the amount of £8,750 from Parliament for his achievements, but he never received the official award (which was never awarded to anyone). He was to survive for just three more years.

In total, Harrison received £23,065 for his work on chronometers. He received £4,315 in increments from the Board of Longitude for his work, £10,000 as an interim payment for H4 in 1765 and £8,750 from Parliament in 1773.[16] This gave him a reasonable income for most of his life (equivalent to roughly £45,000 per year in 2007, though all his costs, such as materials and subcontracting work to other horologists, had to come out of this). He became the equivalent of a multi-millionaire (in today's terms) in the final decade of his life.


Captain James Cook used K1, a copy of H4, on his second and third voyages, having used the lunar distance method on his first voyage.[17] K1 was made by Larcum Kendall, who had been apprenticed to John Jefferys. Cook's log is full of praise for the watch and the charts of the southern Pacific Ocean he made with its use were remarkably accurate. K2 was on HMS Bounty, was recovered from Pitcairn Island, and then passed through several hands before reaching the National Maritime Museum in London.

Initially, the cost of these chronometers was quite high (roughly 30% of a ship's cost). However, over time, the costs dropped to between £25 and £100 (half a year's to two years' salary for a skilled worker) in the early 19th century.[18][19] Many historians point to relatively low production volumes over time as evidence that the chronometers were not widely used. However, Landes[18] points out that the chronometers lasted for decades and did not need to be replaced frequently – indeed the number of makers of marine chronometers reduced over time due to the ease in supplying the demand even as the merchant marine expanded.[20][21] Also, many merchant mariners would make do with a deck chronometer at half the price. These were not as accurate as the boxed marine chronometer but were adequate for many. While the Lunar Distances method would complement and rival the marine chronometer initially, the chronometer would overtake it in the 19th century.

Memorials

Harrison died on his eighty-third birthday and is buried in the graveyard of St John's Church, Hampstead along with his second wife Elizabeth and their son William. His tomb was restored in 1879 by the Worshipful Company of Clockmakers even though Harrison had never been a member of the Company.



Harrison's last home was in Red Lion Square in London, now a short walk from the Holborn Underground station. There is a plaque dedicated to Harrison on the wall of Summit House in the south side of the square. A memorial tablet to Harrison was unveiled in Westminster Abbey on 24 March 2006 finally recognising him as a worthy companion to his friend George Graham and Thomas Tompion, "The Father of English Watchmaking", who are both buried in the Abbey. The memorial shows a meridian line (line of constant longitude) in two metals to highlight Harrison's most widespread invention, the bimetallic strip thermometer. The strip is engraved with its own longitude of 0 degrees, 7 minutes and 35 seconds West.

The Corpus Clock in Cambridge, unveiled in 2008, is an homage by the designer to Harrison's work but is of an electro mechanical design. It features Harrison's grasshopper escapement but in appearance only, the 'pallet frame' being sculpted to resemble an actual grasshopper. This is the clock's defining feature. It should be pointed out the Harrison did not use the term 'grasshopper escapement'. The term appears first in The Horological Journal in the late 19th century.

Subsequent history

After World War I, Harrison's timepieces were rediscovered at the Royal Greenwich Observatory by retired naval officer Lieutenant Commander Rupert T. Gould. They were in a highly decrepit state, and Gould then spent many years documenting, repairing and restoring them without being compensated for his efforts.[22] It was Gould, not Harrison, who gave them the designations H1 to H5, although initially he called them simply No.1 to No.5. Unfortunately Gould made some of his own modifications and repairs to these machines that would not pass today's standards of good museum conservation practice, although most Harrison scholars give Gould some credit for having ensured the survival of the historical artifacts as working mechanisms to the present time. Gould is the author of the book The Marine Chronometer, covering the history of chronometers from the Middle Ages through to the 1920s. It includes detailed descriptions of Harrison's work and the subsequent evolution of the chronometer. It remains the authoritative work on the marine chronometer.

Today the restored H1, H2, H3 and H4 can be seen on display in the National Maritime Museum at the Royal Observatory, Greenwich. H1, 2 and 3 are still running; H4 is kept in a stopped state because, unlike the first three, it requires oil for lubrication and will degrade as it runs. H5 is owned by the Worshipful Company of Clockmakers of London and is on display at the Clockmakers' Museum in the Guildhall, London, as part of the Company's collection.

In the final years of his life, John Harrison wrote about his research into musical tuning and manufacturing methods for bells. His tuning system, (a meantone system derived from pi), is described in his pamphlet A Description Concerning Such Mechanism ........ (CSM). This system challenges the traditional view that "harmonics" occur at integer frequency ratios, and in consequence all music using this tuning produces low frequency beating. In 2002, Harrison's last manuscript, A true and short, but full Account of the Foundation of Musick, or, as principally therein, of the Existence of the Natural Notes of Melody was rediscovered in the US Library of Congress. His theories on the mathematics of bell manufacturing (using "Radical Numbers") are yet to be clearly understood.[23]

In literature, television, drama & music

In 1995, following a major Symposium on the Longitude Problem organized by the National Association of Watch and Clock Collectors (NAWCC) at Harvard University, Dava Sobel wrote a book chronicling the history of John Harrison's invention entitled Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time. Although horological historians are of the opinion that Sobel over-dramatised events such as the struggle between Harrison and Maskelyne, her book became the first ever popular bestseller with a theme focused on horology.

An illustrated volume co-written with William J. H. Andrewes was printed in 1998: The Illustrated Longitude.

Sobel's book was dramatised for UK television by Charles Sturridge in a Granada Productions film for Channel 4 in 1999 under the title Longitude and was broadcast in the US later that same year by co-producer A&E. The production starred Michael Gambon as Harrison and Jeremy Irons as Gould.

Sobel's book was also the basis for a PBS NOVA episode entitled Lost at Sea: The Search for Longitude.

Harrison's marine time-keepers were an essential part of the plot in the 1996 Christmas special of long-running British sitcom Only Fools And Horses entitled "Time On Our Hands". Del Boy happens to be the owner of Harrison's Lesser Watch which was lost for centuries, which eventually fetches them £6.2 million at auction at Sotheby's. Harrison's notes and drawings suggest that H6 was built but it has never been found. It looked like an overgrown pocket watch and Harrison scholars still dream of finding it in an attic.

A song, John Harrison's Hands, written by Brian McNeill and Dick Gaughan, appeared on Gaughan's 2001 album Outlaws & Dreamers. The song has also been covered by Steve Knightley, appearing on his album 2011 Live In Somerset.

See also

References

Further reading

  • The Man Who Made Time Travel by Lasky, Kathryn (Book – 2003)
  • Wolfendale (ed) - Harrison in the Abbey ; Published in Honour of John Harrison on the Occasion of the Unveiling of his Memorial in the Abbey on 24 March 2006 - London, Worshipful Company of Clockmakers, 2006

External links

  • John Harrison and the Longitude Problem, at the National Maritime Museum site
  • John 'Longitude' Harrison and musical tuning
  • Excerpt from: Time Restored: The Story of the Harrison Timekeepers and R.T. Gould, ‘The Man who Knew (almost) Everything’
  • UK Telegraph: 'Clock from 1776 just goes on and on'
  • The Independent, 31 May 2009
  • Excellent accounting of John Harrison and his H1,H2,H3 Achievements
  • Harrison's precision pendulum-clock No. 2, 1727, on the BBC's "A History of the World" website
  • Leeds Museums and Galleries "Secret Life of Objects" blog, John Harrison's precision pendulum-clock No. 2
  • , January 17, 1999.