John Harrison

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	3 April [O.S. 24 March] 1693 Foulby, near Wakefield, West Riding of Yorkshire
Died	24 March 1776(1776-03-24) (aged 82) London, England
Residence	Red Lion Square
Nationality	United Kingdom
Fields	Horology
Known for	Marine chronometer
Notable awards	Copley Medal (1749)

John Harrison (3 April [O.S. 24 March] 1693 – 24 March 1776) was a self-educated English carpenter and clockmaker who invented the marine chronometer, a long-sought-after device for solving the problem of calculating longitude while at sea. His solution revolutionized navigation and greatly increased the safety of long-distance sea travel. The problem he solved was considered so important following the Scilly naval disaster of 1707 that the British Parliament offered financial rewards of up to £20,000 (equivalent to £2.84 million today) under the 1714 Longitude Act.^[1] Harrison came 39th in the BBC's 2002 public poll of the 100 Greatest Britons.^[2]

Early life

John Harrison was born in Foulby, near Wakefield in the West Riding of 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.^[3]

In around 1700, the Harrison family moved to the 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 and he spent hours listening to it and studying its moving parts.

He also had a fascination for music, eventually becoming choirmaster for Barrow parish church.^[4]

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 in the Worshipful Company of Clockmakers' collection previously in the Guildhall in London, and since 2015 on display in the Science Museum. The second (1715) is also in the Science Museum in London; 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 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 works, and like his previous clocks has a wooden movement 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 longcase clocks, again with the movements and longcase made of oak and lignum vitae. The grid-iron pendulum was developed during this period. These precision clocks are thought by some to have been the most accurate clocks in the world at the time. They are the direct link to the Harrison's sea clocks. Number 1, now in a private collection, belonged to the Time Museum, USA, until the museum closed in 2000 and its collection was dispersed at auction in 2004. Number 2 is in the Leeds City Museum. It forms the core of a permanent display dedicated to John Harrison's achievements, "John Harrison: The Clockmaker Who Changed the World" and had its official opening on 23 January 2014, the first longitude-related event marking the tercentenary of the Longitude Act. Number 3 is in the Worshipful Company of Clockmakers' collection.

Harrison was a man of many skills and he used these to systematically improve the performance of the pendulum clock. He invented the gridiron pendulum, consisting of alternating brass and iron rods assembled so that the thermal expansions and contractions essentially 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 wood. This was an important advantage at a time when lubricants and their degradation were little understood.

In his earlier work on sea clocks, Harrison was continually assisted, both financially and in many other ways, by George Graham, the watchmaker and instrument maker. Harrison was introduced to Graham by the Astronomer Royal Edmond Halley, who championed Harrison and his work. This support was important to Harrison, as he is supposed to have found it difficult to communicate his ideas in a coherent manner.

Overview of the problem of determining longitude

Longitude fixes the location of a place on Earth east or west of a north-south (imaginary and arbitrary) line called the prime meridian. It is given as an angular measurement that ranges from 0° at the prime meridian to +180° eastward and −180° westward. A ship's east-west position was essential when approaching land. After a long voyage, cumulative errors in dead reckoning frequently led to shipwrecks and a great loss of life. Avoiding such disasters became vital in Harrison's lifetime, in an era when trade and navigation were increasing dramatically around the world.

Many ideas were proposed for how to determine longitude during a sea voyage. Earlier methods attempted to compare local time with the known time at a reference place, such as Greenwich or Paris, based on a simple theory that had been first proposed by Gemma Frisius. The methods relied on astronomical observations that were themselves reliant on the predictable nature of the motions of different heavenly bodies. Such methods were problematic because of the difficulty in accurately estimating the time at the reference place.

Harrison set out to solve the problem directly, by producing a reliable clock that could keep the time of the reference place. His difficulty was in producing a clock that was not affected by variations in temperature, pressure or humidity, remained accurate over long time intervals, resisted corrosion in salt air, and was able to function on board a constantly-moving ship. Many scientists, including Isaac Newton and Christiaan Huygens, doubted that such a clock could ever be built and favoured other methods for reckoning longitude, 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, with both types producing inconsistent results. 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".

The first three marine timekeepers

In the 1720s, the English clockmaker Henry Sully invented a marine clock that was designed to determine longitude: this was in the form of a clock with a large balance wheel that was vertically mounted on friction rollers and impulsed by a frictional rest Debaufre type escapement. Very unconventionally, the balance oscillations were controlled by a weight at the end of a pivoted horizontal lever attached to the balance by a cord. This solution avoided temperature error due to thermal expansion, a problem which affects steel balance springs. Sully's clock only kept accurate time in calm weather, because the balance oscillations were affected by the pitching and rolling of the ship. However his clocks were amongst the first serious attempts to find longitude in this way. Harrison's machines, though much larger, are of similar layout: H3 has a vertically mounted balance wheel and is linked to another wheel of the same size, an arrangement that eliminates problems arising from the ship's motion.^[5]

In 1716, Sully presented his first Montre de la Mer to the French Académie des Sciences^[6] and in 1726 he published Une Horloge inventée et executée par M. Sulli.^[6]

In 1730, Harrison designed a marine clock to compete for the Longitude Prize and travelled to London, seeking financial assistance. He presented his ideas to Edmond Halley, the Astronomer Royal, who in turn referred him to George Graham, the country's foremost clockmaker. Graham must have been impressed by Harrison's ideas, for he loaned him money to build a model of his "Sea clock". As the clock was an attempt to make a seagoing version of his wooden pendulum clocks, which performed exceptionally well, he used 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).^[7] 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 under the command of Captain George Proctor and returned on HMS Orford after Proctor died at Lisbon on 4 October 1736. The clock lost time on the outward voyage. However, it performed well on the return trip: 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,^[8] 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 yawing action of the ship (when the ship turned such as 'coming about' while tacking). 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.^[9]

Harrison spent seventeen years working on this third 'sea clock', but despite every effort it did not perform exactly as he would have wished. The problem was that, because Harrison did not fully understand the physics behind the springs used to control the balance wheels, the timing of the wheels was not isochronous, a characteristic that affected its accuracy. The engineering world was not to fully understand the properties of springs for such applications for another two centuries. Despite this, it had proved a very valuable experiment as much was learnt from its construction. Certainly in this machine Harrison left the world two enduring legacies – the bimetallic strip and the caged roller bearing.

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.^[10] 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. 1752–1753. 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, which Harrison incorporated 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.^[11] 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.

H4

Harrison's first "Sea watch" (now known as H4) is housed in silver pair cases some 5.2 inches (13 cm) in diameter. The clock's movement is highly complex for that period, resembling a larger version of the then-current conventional movement. It has a novel type of 'vertical' escapement, which is often incorrectly associated with the 'verge' escapement, which it superficially resembles. However, the action of the frictional rest escapement enables the balance to have a large arc. In comparison, the verge's escapement has a recoil with a limited balance arc and is 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, following which the Board of Longitude determined to trial it on a voyage from Portsmouth to Kingston, Jamaica. For this purpose it was placed aboard the 50-gun HMS Deptford, which set sail from Portsmouth on 18 November 1761.^[12]:13–14 Harrison, by then 68 years old, sent it on this transatlantic trial in the care of his son, William. The watch was tested before departure by Robertson, Master of the Academy at Portsmouth, who reported that on 6 November 1761 at noon it was 3 seconds slow, having lost 24 seconds in 9 days on mean solar time. The daily rate of the watch was therefore fixed as losing 24/9 seconds per day.^[13]

When Deptford reached its destination, after correction for the initial error of 3 seconds and accumulated loss of 3 minutes 36.5 seconds at the daily rate over the 81 days and 5 hours of the voyage,^[13] the watch was found to be 5 seconds slow compared to the known longitude of Kingston, corresponding to an error in longitude of 1.25 minutes, or approximately one nautical mile.^[14]:56 Harrison returned aboard the 14-gun HMS Merlin, reaching England on 26 March 1762 to report the successful outcome of the experiment.^[12] Harrison senior thereupon waited for the £20,000 prize, but the Board were persuaded that the accuracy could have been just luck and demanded another trial. The board were also not convinced that a timekeeper which took six years to construct met the test of practicality required by the Longitude Act. 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 this second 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).^[14]:60 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.^[15] 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.^[16] 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 loaned to Lieutenant William Bligh, commander of HMS Bounty but it was retained by Fletcher Christian following the infamous mutiny. It was not recovered from Pitcairn Island until 1840, 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.^[17][18] 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] 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.

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. 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, self-interest, and common sense did the rest in making the device a universal tool of maritime trade.

Memorials

Harrison died on his eighty-third birthday and was buried in the graveyard of St John's Church, Hampstead, in north London, along with his second wife Elizabeth and later 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 a homage by the designer to Harrison's work but is of an electromechanical design. In appearance it features Harrison's grasshopper escapement, the 'pallet frame' being sculpted to resemble an actual grasshopper. This is the clock's defining feature.

In 2014, Northern Rail renamed its train that runs between Barton and Cleethorpes as the John 'Longitude' Harrison.^[21][22]

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.

The timepieces were in a highly decrepit state and Gould spent many years documenting, repairing and restoring them, without compensation for his efforts.^[23] Gould was the first to designate the timepieces from H1 to H5, initially calling them No.1 to No.5. Unfortunately, Gould made modifications and repairs that would not pass today's standards of good museum conservation practice, although most Harrison scholars give Gould credit for having ensured that the historical artifacts survived as working mechanisms to the present time. Gould wrote The Marine Chronometer published in 1923, which covered the history of chronometers from the Middle Ages through to the 1920s, and which included detailed descriptions of Harrison's work and the subsequent evolution of the chronometer. The book remains the authoritative work on the marine chronometer.

Today the restored H1, H2, H3 and H4 timepieces can be seen on display in the Royal Observatory at Greenwich. H1, H2 and H3 still work: H4 is kept in a stopped state because, unlike the first three, it requires oil for lubrication and so will degrade as it runs. H5 is owned by the Worshipful Company of Clockmakers of London, and was previously on display at the Clockmakers' Museum in the Guildhall, London, as part of the Company's collection; since 2015 the collection has been displayed in the Science Museum, London.

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).^[24] This system challenged 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.^[25]

One of the controversial claims of his last years was that of being able to build a land clock more accurate than any competing design. Specifically, he claimed to have designed a clock capable of keeping accurate to within one second over a span of 100 days.^[24]:25–41 At the time, such publications as The London Review of English and Foreign Literature ridiculed Harrison for what was considered an outlandish claim. Harrison drew a design but never built such a clock himself, but in 1970 Martin Burgess, a Harrison expert and himself a clockmaker, studied the plans and endeavored to build the timepiece as drawn. He built two versions, dubbed Clock A and Clock B. Clock A became the Gurney Clock which was given to the city of Norwich in 1975, while Clock B lay unfinished in his workshop for decades until it was acquired in 2009 by Donald Saff. The completed Clock B was submitted to the National Maritime Museum in Greenwich for further study. It was found that Clock B could potentially meet Harrison's original claim, so the clock's design was carefully checked and adjusted. Finally, over a 100-day period from 6 January to 17 April 2015, Clock B was secured in a transparent case in the Royal Observatory and left to run untouched, apart from regular winding.. Upon completion of the run, the clock was measured to have lost only 5/8 of a second, meaning Harrison's design was fundamentally sound. In fact, if this clock had been built in 1762, the date of Harrison's testing of his H4, and run continuously since then without correction, it would now (August 2017) be slow by just 9 minutes and 43 seconds. Guinness World Records has declared the Martin Burgess' Clock B the "most accurate mechanical clock with a pendulum swinging in free air."^[26]

In literature, television, drama and music

In 1995, inspired by a Harvard University symposium on the longitude problem organized by the National Association of Watch and Clock Collectors, Dava Sobel wrote a book on Harrison's work. Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time became the first popular bestseller on the subject of horology. The Illustrated Longitude, in which Sobel's text was accompanied by 180 images selected by William J. H. Andrewes, appeared in 1998. The book was dramatised for UK television by Charles Sturridge in a Granada Productions film for Channel 4 in 1999, under the title Longitude. It 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". The plot concerns the discovery and subsequent sale at auction of Harrison's Lesser Watch H6.

The song "John Harrison's Hands", written by Brian McNeill and Dick Gaughan, appeared on the 2001 album Outlaws & Dreamers. The song has also been covered by Steve Knightley, appearing on his album 2011 Live In Somerset. It was further covered by the British band Show of Hands and appears on their 2016 album The Long Way Home.

British composer Peter Graham's piece Harrison's Dream is about Harrison's forty-year quest to produce an accurate clock. Graham worked simultaneously on the brass band and wind band versions of the piece, which received their first performances just four months apart, in October 2000 and February 2001 respectively.^[27]

See also

• History of longitude
• Lunar distance (navigation)
• Marine chronometer

References

Further reading

• Lasky, Kathryn (2003). The Man Who Made Time Travel. Farrar, Straus and Giroux. ISBN 978-0-374-34788-8. 
• North, Thomas (1882). The Church Bells of the County and City of Lincoln. Leicester: Samuel Clark. pp. 60–61. 
• Sobel, Dava (1995). Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time. New York: Penguin. ISBN 978-0-8027-1312-4. 
• Sobel, Dava; Andrewes, Willam J.H. (1998). The Illustrated Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time. New York: Walker Publishing Co. ISBN 0-8027-1344-0. 
• Wolfendale, Arnold, ed. (2006). Harrison in the Abbey. London: Worshipful Company of Clockmakers. Published in Honour of John Harrison on the Occasion of the Unveiling of his Memorial in the Abbey on 24 March 2006 

External links

Wikimedia Commons has media related to John Harrison.

• John Harrison and the Longitude Problem, at the National Maritime Museum site
• PBS Nova Online: Lost at Sea, the Search for Longitude
• 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'
• Andrew Johnson, Longitude pioneer was not a 'lone genius', 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
• Booknotes interview with Dava Sobel on Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time, January 17, 1999.
• Account of John Harrison and his chronometer at Cambridge Digital Library
• Building an Impossible Clock Shayla Love, Jan 19, 2016, The Atlantic