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21 juin 2012 4 21 /06 /juin /2012 03:41

John Harrison

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 orlongitude of a ship at sea, HP G61-440EL Keyboard 

thus revolutionising and extending the possibility of safe long distance sea travel in theAge of Sail. The problem was considered so intractable that the British Parliament offered a prize of £20,000 (comparable to £2.87 million in modern currency) for the solution.[1][2] HP G61-440SG Keyboard 

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] HP G61-440SS Keyboard 

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. HP G61-440ST Keyboard 

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

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. HP G61-445SI Keyboard 

Three of Harrison's early wooden clocks have survived; the first (1713) is at theWorshipful Company of Clockmakers' Collection in Guildhall; the second (1715), is in the Science Museum and the third (1717) is at Nostell Priory in Yorkshire, HP G61-448CA Keyboard 

the face bearing the inscription "John Harrison Barrow". The Nostell example, in thebilliards 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. HP G61-450EE Keyboard 

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. HP G61-450ES Keyboard 

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. HP G61-450SG Keyboard 

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. HP G61-451EE Keyboard 

No. 2 is in the collections of Leeds Museums and Galleries, West Yorkshire, United Kingdom. It is not on display but it is planned to put it on permanent display in the new Leeds City Museum some time in 2011. No. 3 is in the Worshipful Company of Clockmakers' collection. HP G61-452EE Keyboard 

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. HP G61-453EE Keyboard 

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, HP G61-454EE Keyboard

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, HP G61-455EE Keyboard

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 1728 to explain how his timekeeper worked. HP G61-456EE Keyboard

HP G61-465SL Keyboard

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. HP G61-455ET Keyboard

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. HP G61-511WM Keyboard

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 or place it on a map). HP G61-631NR Keyboard

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. HP G61-632NR Keyboard

HP G62 Keyboard

Harrison instead set out to solve the problem in a direct way: by producing a reliable clock. 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, HP G62-100 CTO Keyboard

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 Huygensdoubted that such a clock could ever be built and had more optimism for astronomical observations (such as the Method of Lunar Distances). HP G62-100EB Keyboard

Huygens ran trials using both a pendulum and a spiral balance spring clock as methods of determining longitude. Although both types showed some favourable results, they were both prone to fickleness. HP G62-100EE Keyboard

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 sought of watch can be found out. HP G62-100EJ Keyboard

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, HP G62-100SL Keyboard

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. HP G62-101TU Keyboard

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. HP G62-101XX Keyboard

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. HP G62-102TU Keyboard

On such voyages, cumulative errors in dead reckoningfrequently 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. HP G62-103XX Keyboard

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, HP G62-104SA Keyboard

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. HP G62-105SA Keyboard

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, HP G62-106SA Keyboard

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. HP G62-107SA Keyboard

The expiry of Arnold's patents at the end of the 1790s enabled many other watchmakers includingThomas 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. HP G62-110ED Keyboard

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. HP G62-110EE Keyboard

The first three marine timekeepers

The English clockmaker Henry Sully had already invented a marine clock to determine longitude accurately, a sophisticated pendulum clock.[6] He presented a firstMontre de la Mer in 1716 to the French Académie des Sciences.[7] and in 1726 published Une Horloge inventée et executée par M. Sulli.[7] However Sully's clock only kept accurate time in calm weather as it depended on the stable motion of the pendulum. HP G62-110EI Keyboard

HP G62-110EO Keyboard

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 toGeorge 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 marine clock. HP G62-110EY Keyboard

It took Harrison five years to build Harrison Number One or H1.[8] 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. HP G62-110SA Keyboard

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. HP G62-110SO Keyboard

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,[9] a more compact and rugged version. HP G62-110SS Keyboard

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. HP G62-110SW Keyboard

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 was granted another £500 by the Board while waiting for the war to end, HP G62-111EE Keyboard

which he used to work on H3.[10] 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. HP G62-112EE Keyboard

Certainly in this machine Harrison left the world two enduring legacies – thebimetallic strip and the caged roller bearing. HP G62-112SO Keyboard

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. HP G62-113SO Keyboard

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, HP G62-115SE Keyboard

a tougher and more highly polished cylinder escapement to be produced.[11][12] 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. HP G62-115SO Keyboard

HP G62-117SO Keyboard

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 also probably the first to have both temperature compensation and Maintaining Power, HP G62-118EO Keyboard

enabling the watch to continue running while being wound. These features led to the very successful performance of this "Jefferys" watch and 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. HP G62-120EC Keyboard

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 for the first time, HP G62-120EE Keyboard

allowed a navigator to accurately assess his ship's position in longitude. Importantly, Harrison showed everyone that it could be done.[13] 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 1759. HP G62-120EG Keyboard

This first marine watch (or "Sea watch" as Harrison called it) is a 5.2" diameter watch in silver pair cases. The movementhas a novel type of escapement which can be classed as a frictional rest type, and superficially resembles the verge escapement with which it is often incorrectly associated. The pallets of this escapement are both made of diamond, HP G62-120EH Keyboard

a considerable feat of manufacture at the time. The balance spring is a flat spiral but for technical reasons the balance itself was made much larger than in a conventional watch of the period. The movement also has centre seconds motion with a sweep seconds hand. HP G62-120EK Keyboard

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. HP G62-120EL Keyboard

Temperature compensation is in the form of a 'compensation curb' (or 'Thermometer Kirb' as Harrison put it). This takes the form of a bimetallic strip mounted on the regulating slide, and carrying the curb pins at the free end. During development of No.4, Harrison dispensed with this regulation using the slide, but left its indicating dial or figure piece in place. HP G62-120EP Keyboard

H4 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, HP G62-120EQ Keyboard

or approximately one nautical mile.[14] When the ship returned, Harrison waited for the £20,000 prize but the Board believed the accuracy was just luck and demanded another trial. The Harrisons were outraged and demanded their prize, HP G62-120ER Keyboard

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 the Caribbean city of Bridgetown on the island of Barbados to settle the matter. HP G62-120ES Keyboard

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. HP G62-120ET Keyboard

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. HP G62-120EY Keyboard

During Harrison's second trial of "H4" the Reverend Nevil Maskelyne was asked to accompany HMS Tartar and test the Lunar Distances system. Once again "H4" 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).[14] HP G62-120SE Keyboard

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. HP G62-120SL Keyboard

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 H4 would have to be turned over to the Astronomer Royal for long-term on-land testing. HP G62-120SS Keyboard

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 H4 that was negative, claiming that the "going rate" of the clock, HP G62-120SW Keyboard

the amount of time it gained or lost per day, was actually an inaccuracy, and refused to allow it to be factored out when measuring longitude. Consequently, the H4 failed the needs of the Board despite the fact that it actually succeeded in two previous trials. HP G62-121EE Keyboard

Harrison began working on his H5 while the H4 testing was conducted, with H4 being effectively 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. HP G62-125EK Keyboard

He obtained an audience by the King, who was extremely annoyed with the Board. King George tested 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. HP G62-125EL Keyboard

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. HP G62-125EV Keyboard

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.[15]HP G62-125SL Keyboard

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. HP G62-130EG Keyboard

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.[16] K1 was made by Larcum Kendall, who had been apprenticed to John Jefferys. HP G62-130EK Keyboard

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. HP G62-130ET Keyboard

HP G62-130EV Keyboard

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.[17][18] HP G62-130SD Keyboard

Many historians point to relatively low production volumes over time as evidence that the chronometers were not widely used. However, Landes[17] 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.[19][20] HP G62-130SL Keyboard

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. HP G62-134CA Keyboard

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. HP G62-135EV Keyboard

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 andThomas Tompion, HP G62-140EF Keyboard

"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. HP G62-140EL Keyboard

The Corpus Clock in Cambridge, unveiled in 2008, is an homage to Harrison's work. Harrison's grasshopper escapement – sculpted to resemble an actual grasshopper – is the clock's defining feature, HP G62-140ES Keyboard

even though the appellation 'grasshopper' is a romantic 19th century conceit. Harrison himself probably would have accurately defined it as an "isochronal anchor escapement with pivoted pallets".HP G62-140EQ Keyboard

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, HP G62-140ET Keyboard

repairing and restoring them without being compensated for his efforts.[21] 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, HP G62-140SF Keyboard

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, HP G62-140SS Keyboard

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. HP G62-140US Keyboard

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, HP G62-143CL Keyboard

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. HP G62-144DX Keyboard

In the final years of his life, John Harrison wrote about his research into musical tuning and manufacturing methods forbells. His tuning system, (a meantone system derived from pi), is described in his book Concerning Such Mechanism ........ (CSM). HP G62-145NR Keyboard

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, HP G62-147NR Keyboard

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.[1] HP G62-149WM Keyboard

In television & drama

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: HP G62-150EE Keyboard

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 andMaskelyne, her book became the first ever popular bestseller with a theme focused HP G62-150EF Keyboard

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. HP G62-150EQ Keyboard

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 a certain marine time-keeper that was lost for centuries, HP G62-150ET Keyboard

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. HP G62-150EV Keyboard


A clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately (via Dutch, Northern French, and Medieval Latin) from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a mechanism has traditionally been known as a timepiece.[1]HP G62-150SE Keyboard

In general usage today a "clock" refers to any device for measuring and displaying the time. Watches and other timepieces that can be carried on one's person are often distinguished from clocks.[2] HP G62-150SF Keyboard

The clock is one of the oldest human inventions, meeting the need to consistently measure intervals of time shorter than the natural units: the day; the lunar month; and the year. Devices operating on several different physical processes have been used over the millennia, culminating in the clocks of today. HP G62-150SL Keyboard

The study of timekeeping is known as horology.

The sundial, which measures the time of day by using the sun casting a shadow onto a cylindrical stone, was widely used in ancient times. A well-constructed sundial can measure local solar time with reasonable accuracy, HP G62-153CA Keyboard

and sundials continued to be used to monitor the performance of clocks until the modern era. However, its practical limitations—it requires the sun to shine and does not work at all during the night—encouraged the use of other techniques for measuring time. HP G62-154CA Keyboard

class="MsoNormalCxSpMiddle"> Candle clocks and sticks of incense that burn down at approximately predictable speeds have also been used to estimate the passing of time. In an hourglass, fine sand pours through a tiny hole at a constant rate and indicates a predetermined passage of an arbitrary period of time. HP G62-165SL Keyboard

Water clocks, also known as clepsydrae (sg: clepsydra), along with the sundials, are possibly the oldest time-measuring instruments, with the only exceptions being the vertical gnomon and the day-counting tally stick.[3]HP G62-166SB Keyboard

Given their great antiquity, where and when they first existed is not known and perhaps unknowable. The bowl-shaped outflow is the simplest form of a water clock and is known to have existed in Babylon and in Egypt around the 16th century BC. HP G62-200XX Keyboard

Other regions of the world, including India and China, also have early evidence of water clocks, but the earliest dates are less certain. Some authors, however, write about water clocks appearing as early as 4000 BC in these regions of the world.[4] HP G62-201TU Keyboard

Greek astronomer Andronicus of Cyrrhus supervised the construction of the Tower of the Winds in Athens in the 1st century B.C.[5] HP G62-201XX Keyboard

The Greek and Roman civilizations are credited for initially advancing water clock design to include complexgearing,[6][dead link] which was connected to fanciful automata and also resulted in improved accuracy. HP G62-208CA Keyboard

These advances were passed on through Byzantium and Islamic times, eventually making their way back to Europe. Independently, the Chinese developed their own advanced water clocks(水鐘)in 725 A.D., passing their ideas on to Korea and Japan. HP G62-219CA Keyboard

Some water clock designs were developed independently and some knowledge was transferred through the spread of trade. Pre-modern societies do not have the same precise timekeeping requirements that exist in modern industrial societies, HP G62-219WM Keyboard

where every hour of work or rest is monitored, and work may start or finish at any time regardless of external conditions. Instead, water clocks in ancient societies were used mainly for astrological reasons. HP G62-220CA Keyboard

These early water clocks were calibrated with a sundial. While never reaching the level of accuracy of a modern timepiece, the water clock was the most accurate and commonly used timekeeping device for millennia, until it was replaced by the more accurate pendulum clock in 17th century Europe. HP G62-220US Keyboard

Islamic civilization is credited with further advancing the accuracy of clocks with elaborate engineering. In 797 (or possibly 801), the Abbasid caliph of Baghdad, Harun al-Rashid, presented Charlemagne with an Asian Elephant named Abul-Abbas together with a "particularly elaborate example" of a water[7] clock. HP G62-221CA Keyboard

In the 13th century, Al-Jazari, a Kurdish Muslim engineer from Mesopotamia (lived 1136-1206) who worked for Artuqid king of Diyar-Bakr, Nasir al-Din, made numerous clocks of all shapes and sizes. The book described 50 mechanical devices in 6 categories, including water clocks. HP G62-222US Keyboard

The most reputed clocks included the Elephant, Scribe and Castle clocks, all of which have been successfully reconstructed. As well as telling the time, these grand clocks were symbols of status, grandeur and wealth of the Urtuq State.[citation needed] HP G62-223CA Keyboard

None of the first clocks survived from 13th century Europe, but various mentions in church records reveal some of the early history of the clock. HP G62-223CL Keyboard

The word horologia (from the Greek ρα, hour, and λέγειν, to tell) was used to describe all these devices,[9] but the use of this word (still used in several Romance languages) for all timekeepers conceals from us the true nature of the mechanisms. For example, there is a record that in 1176 Sens Cathedral installed a ‘horologe’ [10]HP G62-224CA Keyboard

but the mechanism used is unknown. According to Jocelin of Brakelond, in 1198 during a fire at the abbey of St Edmundsbury (now Bury St Edmunds), the monks 'ran to the clock' to fetch water, indicating that their water clock had a reservoir large enough to help extinguish the occasional fire.[11] HP G62-224HE Keyboard

A new mechanism

The word clock (from the Latin words clocca and clogan, both meaning "bell"), which gradually supersedes "horologe", suggests that it was the sound of bells which also characterized the prototype mechanical clocks that appeared during the 13th century in Europe. HP G62-225DX Keyboard

Outside Europe, the escapement mechanism had been known and used in medieval China, as the Song Dynasty horologist and engineer Su Song (1020–1101) incorporated it into his astronomical clock-tower of Kaifeng in 1088.[12][page needed] HP G62-225NR Keyboard

However, his astronomical clock and rotating armillary sphere still relied on the use of flowing water (i.e. hydraulics), while European clockworks of the following centuries shed this old method for a more efficient driving power of weights, in addition to the escapement mechanism. HP G62-226NR Keyboard

A mercury clock, described in the Libros del saber, a Spanish work from 1277 consisting of translations and paraphrases of Arabic works, is sometimes quoted as evidence for Muslim knowledge of a mechanical clock. The first mercury powered automata clock was invented by Ibn Khalaf al-Muradi[13][14] HP G62-227CA Keyboard

Between 1280 and 1320, there is an increase in the number of references to clocks and horologes in church records, and this probably indicates that a new type of clock mechanism had been devised. Existing clock mechanisms that used water power were being adapted to take their driving power from falling weights. HP G62-227CL Keyboard

This power was controlled by some form of oscillating mechanism, probably derived from existing bell-ringing or alarm devices. This controlled release of power - theescapement - marks the beginning of the true mechanical clock. HP G62-228CA Keyboard

These mechanical clocks were intended for two main purposes: for signalling and notification (e.g. the timing of services and public events), and for modeling the solar system. The former purpose is administrative, HP G62-228CL Keyboard

the latter arises naturally given the scholarly interest in astronomy, science, astrology, and how these subjects integrated with the religious philosophy of the time. The astrolabe was used both by astronomers and astrologers, and it was natural to apply a clockwork drive to the rotating plate to produce a working model of the solar system. HP G62-228NR Keyboard

Simple clocks intended mainly for notification were installed in towers, and did not always require faces or hands. They would have announced the canonical hours or intervals between set times of prayer. Canonical hours varied in length as the times of sunrise and sunset shifted. HP G62-229NR Keyboard

The more sophisticated astronomical clocks would have had moving dials or hands, and would have shown the time in various time systems, including Italian hours, canonical hours, and time as measured by astronomers at the time. Both styles of clock started acquiring extravagant features such as automata. HP G62-229WM Keyboard

In 1283, a large clock was installed at Dunstable Priory; its location above the rood screen suggests that it was not a water clock[citation needed]. In 1292,Canterbury Cathedral installed a 'great horloge'. Over the next 30 years there are brief mentions of clocks at a number of ecclesiastical institutions in England, Italy, HP G62-231NR Keyboard

and France. In 1322, a new clock was installed in Norwich, an expensive replacement for an earlier clock installed in 1273. This had a large (2 metre) astronomical dial with automata and bells. The costs of the installation included the full-time employment of two clockkeepers for two years[citation needed]. HP G62-233NR Keyboard

Early astronomical clocks

Besides the Chinese astronomical clock of Su Song in 1088 mentioned above, in Europe there were the clocks constructed byRichard of Wallingford in St Albans by 1336, and by Giovanni de Dondi in Padua from 1348 to 1364. HP G62-234DX Keyboard

They no longer exist, but detailed descriptions of their design and construction survive, [15][16] and modern reproductions have been made.[16] They illustrate how quickly the theory of the mechanical clock had been translated into practical constructions, and also that one of the many impulses to their development had been the desire of astronomers to investigate celestial phenomena. HP G62-236NR Keyboard

Wallingford's clock had a large astrolabe-type dial, showing the sun, the moon's age, phase, and node, a star map, and possibly the planets. In addition, it had a wheel of fortune and an indicator of the state of the tide at London Bridge. Bells rang every hour, the number of strokes indicating the time.[15] HP G62-237CA Keyboard

Dondi's clock was a seven-sided construction, 1 metre high, with dials showing the time of day, including minutes, the motions of all the known planets, an automatic calendar of fixed and movable feasts, and an eclipse prediction hand rotating once every 18 years.[16] HP G62-237US Keyboard

It is not known how accurate or reliable these clocks would have been. They were probably adjusted manually every day to compensate for errors caused by wear and imprecise manufacture.

Water clocks are sometimes still used today, and can be examined in places such as ancient castles and museums. HP G62-238CA Keyboard

The Salisbury Cathedral clock, built in 1386, is considered to be the world's oldest surviving mechanical clock that strikes the hours.[17]

Clockmakers developed their art in various ways. Building smaller clocks was a technical challenge, as was improving accuracy and reliability. Clocks could be impressive showpieces to demonstrate skilled craftsmanship, or less expensive, mass-produced items for domestic use. The escapement in particular was an important factor affecting the clock's accuracy, so many different mechanisms were tried. HP G62-238NR Keyboard

Spring-driven clocks appeared during the 15th century,[18][19][20] although they are often erroneously credited to Nurembergwatchmaker Peter Henlein (or Henle, or Hele) around 1511.[21][22][23] The earliest existing spring driven clock is the chamber clock given to Peter the Good, Duke of Burgundy, around 1430, now in the Germanisches Nationalmuseum.[19]HP G62-244CA Keyboard

Spring power presented clockmakers with a new problem: how to keep the clock movement running at a constant rate as the spring ran down. This resulted in the invention of the stackfreed and the fusee in the 15th century, and many other innovations, down to the invention of the modern going barrel in 1760. HP G62-251TU Keyboard

Early clock dials did not use minutes and seconds. A clock with a dial indicating minutes was illustrated in a 1475 manuscript by Paulus Almanus,[24] and some 15th-century clocks in Germany indicated minutes and seconds.[25HP G62-251XX Keyboard

]An early record of a second hand on a clock dates back to about 1560 on a clock now in the Fremersdorf collection.[citation needed] However, this clock could not have been accurate, and the second hand was probably for indicating that the clock was working. HP G62-252TU Keyboard

During the 15th and 16th centuries, clockmaking flourished, particularly in the metalworking towns of Nuremberg and Augsburg, and in Blois, France. Some of the more basic table clocks have only one time-keeping hand, HP G62-253TU Keyboard

with the dial between the hour markers being divided into four equal parts making the clocks readable to the nearer 15 minutes. Other clocks were exhibitions of craftsmanship and skill, incorporating astronomical indicators and musical movements. HP G62-264CA Keyboard

The cross-beat escapement was invented in 1584 by Jost Bürgi, who also developed the remontoire. Bürgi's clocks were a great improvement in accuracy as they were correct to within a minute a day.[26][27] These clocks helped the 16th-century astronomer Tycho Brahe to observe astronomical events with much greater precision than before. HP G62-318CA Keyboard

A mechanical weight-driven astronomical clock with a verge-and-foliot escapement, a striking train of gears, an alarm, and a representation of the moon's phases was described by the Ottoman engineer Taqi al-Din in his book, HP G62-320CA Keyboard

The Brightest Stars for the Construction of Mechanical Clocks (Al-Kawākib al-durriyya fī wadh' al-bankāmat al-dawriyya), published in 1556-1559.[28]Similarly to earlier 15th-century European alarm clocks,[29][30] it was capable of sounding at a specified time, HP G62-323CA Keyboard

achieved by placing a peg on the dial wheel. At the requested time, the peg activated a ringing device. The clock had three dials which indicated hours, degrees and minutes. He later made an observational clock for the Istanbul observatory of Taqi al-Din(1577–1580), HP G62-325CA Keyboard

describing it as "a mechanical clock with three dials which show the hours, the minutes, and the seconds." This was an important innovation in 16th-century practical astronomy, as at the start of the century clocks were not accurate enough to be used for astronomical purposes.[31] HP G62-327CA Keyboard

The next development in accuracy occurred after 1656 with the invention of the pendulum clock. Galileo had the idea to use a swinging bob to regulate the motion of a time-telling device earlier in the 17th century. HP G62-337NR Keyboard

christiaan Huygens, however, is usually credited as the inventor. He determined the mathematical formula that related pendulum length to time (99.38 cm or 39.13 inches for the one second movement) and had the first pendulum-driven clock made. HP G62-339WM Keyboard

In 1670, the English clockmaker William Clement created the anchor escapement[32],[citation needed] an improvement over Huygens' crown escapement[citation needed]. Within just one generation, minute hands and then second hands were added. HP G62-340US Keyboard

In the late 17th and 18th Centuries, equation clocks were made, which allowed the user to see or calculate apparent solar time, as would be shown by a sundial. Before the invention of the pendulum clock, sundials were the only accurate timepieces. HP G62-341NR Keyboard

When good clocks became available, they appeared inaccurate to people who were used to trusting sundials. The annual variation of the equation of time made a clock up to about 15 minutes fast or slow, relative to a sundial, depending on the time of year. HP G62-343NR Keyboard

Equation clocks satisfied the demand for clocks that always agreed with sundials. Several types of equation clock mechanism were devised. which can be seen in surviving examples, mostly in museums. HP G62-346NR Keyboard

A major stimulus to improving the accuracy and reliability of clocks was the importance of precise time-keeping for navigation. The position of a ship at sea could be determined with reasonable accuracy if a navigator could refer to a clock that lost or gained less than about 10 seconds per day. This clock could not contain a pendulum, HP G62-347CL Keyboard

which would be virtually useless on a rocking ship. Many European governments offered a large prize for anyone who could determine longitude accurately; for example, HP G62-348CA Keyboard

Great Britain offered 20,000 pounds, equivalent to millions of dollars today. The reward was eventually claimed in 1761 by John Harrison, who dedicated his life to improving the accuracy of his clocks. His H5 clock was in error by less than 5 seconds over 10 weeks.[33] HP G62-347NR Keyboard

The excitement over the pendulum clock had attracted the attention of designers, resulting in a proliferation of clock forms. Notably, the longcase clock (also known as the grandfather clock) was created to house the pendulum and works. HP G62-348NR Keyboard

The English clockmaker William Clement is also credited with developing this form in 1670 or 1671. It was also at this time that clock cases began to be made of wood and clock faces to utilize enamel as well as hand-painted ceramics. HP G62-352CA Keyboard

On November 17, 1797, Eli Terry received his first patent for a clock. Terry is known as the founder of the American clock-making industry. HP G62-352TU Keyboard

Starting in the U.S. in early decades of the 19th century, clocks were one of the first items to be mass produced and also to use interchangeable parts.[34][35] HP G62-352US Keyboard

Alexander Bain, Scottish clockmaker, patented the electric clock in 1840. The electric clock's mainspring is wound either with an electric motor or with an electro-magnet and armature. In 1841, he first patented the electromagnetic pendulum. HP G62-353NR Keyboard

The development of electronics in the 20th century led to clocks with no clockwork parts at all. Time in these cases is measured in several ways, such as by the vibration of a tuning fork, the behaviour of quartz crystals, HP G62-353TU Keyboard

or the quantum vibrations of atoms. Even mechanical clocks have since come to be largely powered by batteries, removing the need for winding. HP G62-353TX Keyboard

How clocks work

The invention of the mechanical clock in the 13th century initiated a change in timekeeping methods from continuousprocesses, HP G62-354TU Keyboard

such as the motion of the gnomon's shadow on a sundial or the flow of liquid in a water clock, to repetitiveoscillatory processes, like the swing of a pendulum or the vibration of a quartz crystal, which were more accurate.[36] All modern clocks use oscillation. HP G62-354CA Keyboard

Although the methods they use vary, all oscillating clocks, mechanical and digital and atomic, work similarly and can be divided into analogous parts.[37][38][39] They consist of an object that repeats the same motion over and over again, anoscillator, HP G62-355CA Keyboard

with a precisely constant time interval between each repetition, or 'beat'. Attached to the oscillator is acontroller device, which sustains the oscillator's motion by replacing the energy it loses to friction, and converts its oscillations into a series of pulses. HP G62-355DX Keyboard

The pulses are then added up in a chain of some type of counters to express the time in convenient units, usually seconds, minutes, hours, etc. Then finally some kind of indicator displays the result in a human-readable form. HP G62-355TU Keyboard

The advantage of a harmonic oscillator over other forms of oscillator is that it employs resonance to vibrate at a precise natural resonant frequency or 'beat' dependent only on its physical characteristics, and resists vibrating at other rates. HP G62-357CA Keyboard

The possible precision achievable by a harmonic oscillator is measured by a parameter called its Q,[42][43] or quality factor, which increases (other things being equal) with its resonant frequency.[44] This is why there has been a long term trend toward higher frequency oscillators in clocks. HP G62-357TX Keyboard

Balance wheels and pendulums always include a means of adjusting the rate of the timepiece. Quartz timepieces sometimes include a rate screw that adjusts a capacitor for that purpose. Atomic clocks are primary standards, and their rate cannot be adjusted. HP G62-358CA Keyboard

Analog clocks usually indicate time using angles. The most common clock face uses a fixed numbered dial or dials and moving hand or hands. It usually has a circular scale of 12 hours, which can also serve as a scale of 60minutes, HP G62-358NR Keyboard

and 60 seconds if the clock has a second hand. Many other styles and designs have been used throughout the years, including dials divided into 6, 8, 10, and 24 hours. The only other widely used clock face today is the24 hour analog dial, because of the use of 24 hour time in military organizations and timetablesHP G62-359CA Keyboard

The 10-hour clock was briefly popular during the French Revolution, when the metric system was applied to time measurement, and an Italian 6 hour clock was developed in the 18th century, presumably to save power (a clock or watch striking 24 times uses more power). HP G62-359TU Keyboard

Another type of analog clock is the sundial, which tracks the sun continuously, registering the time by the shadow position of its gnomon. Because the sun does not adjust to daylight savings times, users must add an hour during that time. HP G62-361TX Keyboard

Corrections must also be made for the equation of time, and for the difference between the longitudes of the sundial and of the central meridian of the time zone that is being used (i.e. 15 degrees east of the prime meridian for each hour that the time zone is ahead of GMT). HP G62-362TU Keyboard

Sundials use some or part of the 24 hour analog dial. There also exist clocks which use a digital display despite having an analog mechanism—these are commonly referred to as flip clocks.

Alternative systems have been proposed. HP G62-363NR Keyboard

For example, the Twelv clock indicates the current hour using one of twelve colors, and indicates the minute by showing a proportion of a circular disk, similar to a moon phase. HP G62-363TU Keyboard

Most digital clocks use an LCD, LED, or VFD display; many other display technologies are used as well (cathode ray tubes,nixie tubes, etc.). After a reset, battery change or power failure, digital clocks without a backup battery or capacitoreither start counting from 12:00, or stay at 12:00, HP G62-363TX Keyboard

often with blinking digits indicating that the time needs to be set. Some newer clocks will reset themselves based on radio or Internet time servers that are tuned to national atomic clocks. Since the advent of digital clocks in the 1960s, the use of analog clocks has declined significantly. HP G62-364DX Keyboard

Some clocks, usually digital ones, include an optical projector that shines a magnified image of the time display onto a screen or onto a surface such as an indoor ceiling or wall. The digits are large enough to be easily read, without using glasses, HP G62-364TU Keyboard

by persons with moderately imperfect vision, so the clocks are convenient for use in their bedrooms. Usually, the timekeeping circuitry has a battery as a backup source for an uninterrupted power supply to keep the clock on time, HP G62-365CA Keyboard

while the projection light only works when the unit is connected to an A.C. supply. Completely battery-powered portable versions resembling flashlights are also available. HP G62-365TU Keyboard

Multi-display clocks

Some clocks have several displays driven by a single mechanism, and some others have several completely separate mechanisms in a single case. Clocks in public places often have several faces visible from different directions, HP G62-365TX Keyboard

so that the clock can be read from anywhere in the vicinity. Of course, all the faces show the same time. Other clocks show the current time in several time-zones. Watches that are intended to be carried by travellers often have two displays, HP G62-366TU Keyboard

one for the local time and the other for the time at home, which is useful for making pre-arranged phone calls. Some equation clocks have two displays, one showing mean time and the other solar time, HP G62-366TX Keyboard

as would be shown by a sundial. Some clocks have both analog and digital displays. Clocks with Braille displays usually also have conventional digits so they can be read by sighted people. HP G62-367DX Keyboard

Auditory and projection clocks can be used by people who are blind or have limited vision. There are also clocks for the blind that have displays that can be read by using the sense of touch. Some of these are similar to normal analog displays, HP G62-367TX Keyboard

but are constructed so the hands can be felt without damaging them. Another type is essentially digital, and uses devices that use a code such as Braille to show the digits so that they can be felt with the fingertips. HP G62-368TX Keyboard

The recurrent, periodic process (e.g. a metronome) is an oscillator and typically generates a clock signal. Sometimes that signal alone is (confusingly) called "the clock", but sometimes "the clock" includes the counter, its indicator, and everything else supporting it. HP G62-369TX Keyboard

This definition can be further improved by the consideration of successive levels of smaller and smaller error tolerances. While not all physical processes can be surveyed, the definition should be based on the set of physical processes which includes all individual physical processes which are proposed for consideration. HP G62-370TX Keyboard

Since atoms are so numerous and since, within current measurement tolerances they all beat in a manner such that if one is chosen as periodic then the others are all deemed to be periodic also, it follows that atomic clocks represent ideal clocks to within present measurement tolerances and in relation to all presently known physical HP G62-371DX Keyboard

However, they are not so designated by fiat. Rather, they are designated as the current ideal clock because they are currently the best instantiation of the definition. HP G62-371TX Keyboard

Navigation by ships and planes depends on the ability to measure latitude and longitude. Latitude is fairly easy to determine through celestial navigation, but the measurement of longitude requires accurate measurement of time. HP G62-372TX Keyboard

This need was a major motivation for the development of accurate mechanical clocks. John Harrison created the first highly accurate marine chronometer in the mid-18th century. The Noon gun in Cape Town still fires an accurate signal to allow ships to check their chronometers. HP G62-372US Keyboard

Many buildings near major ports used to have (some still do) a large ball mounted on a tower or mast arranged to drop at a pre-determined time, for the same purpose. HP G62-373DX Keyboard

HP G62-373TX Keyboard

HP G62-374CA Keyboard

HP G62-374TX Keyboard

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