John Harrison Wooden Regulator The humble presentation of this longcase by Yorkshire carpenter John Harrison belies the fact that its development marked huge advances in horology. Harrison’s ground-breaking inventions – the temperature-compensating gridiron pendulum, roller pinion and the grasshopper escapement – made it the world's most accurate clock of the time and, indeed, for the next 150 years. This clock was not made in a busy workshop to sell for profit, nor was it commissioned by a well-to-do gentleman or royal patron. Rather, it was created in his own home by Harrison himself, assisted by his brother, James. Yet, this was no ordinary domestic clock. It was a precision instrument – a regulator – an accurate clock by which other clocks could be adjusted accordingly. Harrison’s Wooden Regulator, exhibited here on Clocktime, is one of a set of three identical wooden regulators that he completed between 1726 and 1728. With this design, Harrison succeeded in creating an entirely new kind of pendulum clock. What is more, the knowledge that he gained from this project lay the groundwork for the design of his marine timekeepers, the first of which was manufactured in 1730. Harrison’s endeavours in the area of marine timekeeping eventually culminated in the design of his break-through H4 chronometer in 1759. H4 was the first marine chronometer to enable ocean navigators to accurately determine longitude at sea. Thus, with H4, Harrison solved the greatest scientific problem of his day and lay the foundation for modern-day inventions like GPS (Global Positioning System). The major breakthrough that Harrison’s Wooden Regulator represents in terms of horological, navigational, and wider scientific development cannot be overstated. What follows is the story of this homemade, hand-painted clock, and how Harrison, one of the greatest figures in horological history, developed and realised its design. Carpenter turned clockmaker Harrison lived and worked in Barrow upon Humber, North Lincolnshire, making all his early wooden clocks there until he moved to London in 1730, after the successful calibration of this regulator and its identical mate. Harrison’s intimate knowledge of wood and its properties, and the degree of precision to which he could manipulate this material to solve problems and realise his designs, surely owes a great debt to his father, Henry Harrison (b. 1665, d. 1728). Henry was a carpenter who worked in Barrow upon Humber and trained his sons in the family joinery and woodworking business. From the time of his youth, it was already clear that John Harrison was endowed with a brilliant technical mind and an unrelenting creative drive. He cultivated an early interest in mechanics and music, and even learned how to conduct land surveys, for which purpose he made his own instruments, a plane table and compass.[1] Jonathan Betts, Curator Emeritus of Horology at the Royal Observatory at Greenwich, describes how the precocious young Harrison captured the attention of a visiting clergyman who, when John was a young man, lent him a copy of the celebrated lectures on mechanics by Nicolas Saunderson (1682 – 1739), Lucasian Professor of Mathematics at Cambridge. Harrison copied out the whole book. It remained a treasured possession and over the years he annotated it with copious notes.[2] Where or indeed whether Harrison received any training as a clockmaker is a mystery. It is certainly possible that he was an autodidact (self-taught) clockmaker, which could go towards explaining his unorthodox approaches to problem-solving when designing and constructing clocks. Whatever the case, his interest in horology took root early in his life because he managed to complete his first clock in 1713, at the age of 20. By the time this Wooden Regulator was created, Harrison was already a burgeoning clockmaker. However, he was still working full time as a joiner and making clocks on the side, with only the help of his brother, James. Thus, Harrison’s ability to innovate and invent is even more impressive: he conducted his experiments and perfected his methods in relative isolation, away from the collective knowledge, the resources, and hustle and bustle of London, then the clockmaking capital of the world. A new way of clockmaking: wooden movements It was in 1713 that Harrison made his first clock. This was made almost entirely from wood,[3] which was unheard of at the time.[4] He had constructed the wheels of the movement from sections of oak and assembled them in a way that mimicked the structure of a metal clock. The oak sections were cut along the grain and nested together like the pieces of a pie. This meant that each individual tooth was strong and did not go against the grain. Harrison then cut the gear teeth in a conventional tooth form, which enabled a ‘rolling’ friction. The wheels were then glued onto boxwood arbors with integral cut pinions and inserted steel ends. The frontplates and backplates that held the movement together were made of wood. The only metals that he used for the clock was limited to steel for the pallets and pallet arbor and the pivot ends, as well as brass for the bearings (inserted into the front- and backplates) and for the escape wheel. Harrison was clearly on to something, because, 300 years later, there is still virtually no wear on the clock’s bearings and remarkably little wear on its oak wheels and wood pinions.[5] Harrison used the same methods for the manufacture of all his early clocks, three of which have survived. In his book John Harrison and the Quest for Longitude, Betts describes in detail the design of each of Harrison’s early wooden clocks.[6] Each of these was designed and completed by the young clockmaker while he was living and working full-time as a joiner in Barrow-upon Humber. For his next project, he continued to experiment with wood in the same vein. Lignum vitae, the tree of life Around 1722, Harrison was commissioned by Sir Charles Pellam to build a turret clock for Brocklesby Hall.[7] This was the first major clockmaking project undertaken by the young clockmaker, and he enlisted the help of his brother James. While working on the project, Harrison struggled with the limitations of clock oils. This was not unusual. During the 1700s, clock oils were typically derived from animal and vegetable sources. These were (and still are) difficult to work with because they are prone to thickening, becoming acidic, and even evaporating with age. They also, over time, tend to spread away from where they are needed. All these properties can wreak havoc with the smooth running of a clock, meaning that their mechanisms required frequent cleaning and re-oiling. Clockmakers tried to find ways around these problems and were constantly at pains to improve the quality of the clock oils used to grease the mechanism. Unfortunately, however, their efforts achieved only limited success. Instead of attempting to improve clock oils, Harrison took a radical approach: he set about designing a clock that did not need oil. In essence, he turned the usual approach to solving the problem on its head! He did so by experimenting with different woods for the turret clock’s movement, trying to find one that could provide an effective oil-free bearing surface. After having achieved some success with boxwood, he found the definitive solution in a wood called lignum vitae, the name of which literally translates to ‘tree of life’ in Latin. Expansion of trade routes, as well as the exploitation of exotic resources by colonial powers, made Harrison’s access to this type of wood possible. Lignum vitae is a hard oily wood from trees of the genus Guaiacum, which are indigenous to the Caribbean and the northern coast of South America. It had been exported to Europe since the early 1500s. The wood is olive green and famous for its resistance to rot. It is also widely regarded as being the heaviest and hardest wood in the world. Harrison also used lignum vitae not only for the movement but also for the turret clock’s escapement, replacing its original anchor escapement. In so doing, he entirely rethought and streamlined the design of the anchor escapement. Harrison’s new kind of escapement became known as the ‘grasshopper escapement’ because the movement of its pallets resembled the movement of the hind legs of a grasshopper.[8] While other escapements moved with a sliding action, the wooden grasshopper escapement ‘jumps’ after each impulse. This makes the movement virtually frictionless, so no oiling is required. Betts perfectly sums up the elegant simplicity of Harrison’s solution: ‘Harrison’s victory over the problem of lubrication by eliminating the problem itself was ingenious...’[9] The Brocklesby project was a huge success for Harrison. It is also a testament to the integrity of his design that, roughly 300 years later, the Brocklesby Turret Clock is still running and keeping time in its original setting at Brocklesby Park without need of lubrication. The project also played a crucial role in Harrison’s development as a clockmaker. At a practical level, the solutions that he discovered informed the design of his next project: the making of his twin precision wooden regulators. These were basically smaller, longcase versions of the Brocklesby Turret Clock.[10] And looking at the bigger picture, the knowledge and experience gained from the Brocklesby project must have given Harrison the confidence to turn his mind to finding solutions to even greater problems. Longitude beckons Navigation of the open sea had always been a dangerous business, and this danger was purely down to navigational errors.[11] While determining latitude was relatively easy, early ocean navigators had to rely on dead reckoning to estimate longitude. This was especially inaccurate on long voyages without sight of land and could sometimes lead to calamity. From the 1400s, transoceanic travel had only grown in significance due to the expansion of empires and trade routes. By Harrison’s time, the problem of determining longitude at sea was seen as the greatest scientific challenge of its day. This was because there were fortunes to be made from the accumulation of resources from the ‘New World’. Thus, reliable navigation at sea was crucial to the financial success of European colonial empires such as Britain, France, Portugal and Spain. European lives and fortunes were at stake. Scientific thinkers had been working on the ‘problem of longitude’ for a long time. Most believed that the solution would be an astronomical one rather than a horological one. For example, many thought that the reliable calculation of longitude could be achieved by measuring the position of the navigator relative to celestial bodies such as Jupiter’s moons, which had been discovered by Galileo Galilei in 1610. The first Royal Astronomer, John Flamsteed, shared this belief with his royal patron, King Charles II. In fact, the King’s worry that the French were close to a solution, compelled him to establish the Royal Observatory by royal warrant in 1675 to further Flamsteed’s pursuit of a solution to the problem. Then, in 1707, the Scilly naval disaster occurred. This was a wreck in which four ships and around 2000 souls were lost, making it one of the worst naval disasters in British naval history. Because poor navigation contributed to the catastrophe, this and the desire to capitalise on global trade (specifically the trade of goods, raw materials and enslaved Africans that were trafficked by way of the transatlantic slave trade route) helped spur the government into providing a formal response to the problem of longitude. In May 1714, a petition calling for endeavours to find an adequate solution to the longitude problem was presented to the Palace of Westminster (where the British Parliament convenes). Soon after, in July 1714, Parliament passed the Longitude Act. This included the establishment of the Board of Longitude and offered a prize of £20,000 (worth an equivalent of £5 million today) for solving the problem of longitude. At the time, it was widely assumed that astronomy held the key to unlocking the solution. Up to that point, no clockmaker had been able to create a clock that could keep time accurately and reliably at sea. Existing clocks were simply unable to meet the Longitude Prize’s accuracy requirements, and the pendulum clock technology of the time was virtually useless at sea. Pendulum clocks were far too sensitive to movement and temperature fluctuations, and moisture wreaked havoc with the regularity of their back-and-forth swing. Because of these considerations, the possibility of a horological solution was rejected by all the London clockmakers of the day. Clockmakers assumed that they stood no chance of winning the prize, so they did not try to. Harrison thought otherwise, however, thanks to his success with the Brocklesby project. He believed that he could provide a horological solution – a scientific fete that would bring him riches and fame. Upon completion of the Brocklesby Turret Clock project, Harrison immediately took steps towards the design of a sea clock accurate enough to be used to reliably calculate longitude at sea. He knew that this would not be possible without an accurate land clock against which he could test his sea clocks, and that the existence of such a land clock required an entirely new design. Thus, in 1725, he set to work on a revolutionary new kind of pendulum land clock.[12] Harrison’s Wooden Regulator: an entirely new kind of pendulum clock As he did with his earlier wooden clocks, Harrison produced the Clocktime Wooden Regulator at his home in Yorkshire and in relative isolation, with only the help of his brother James.[13] The brothers used homemade instruments and unorthodox methods, pushing the capabilities of Harrison’s design every step of the way. At the time, Harrison was still working full time as a joiner and carpenter. Astoundingly, in 1726 (just a year after finishing the Brocklesby project), the brothers completed and successfully calibrated this Wooden Regulator and its two identical mates.[14] Harrison claimed that it was the most accurate clock in the world. Inner beauty In many respects, the outer appearance of Harrison’s Wooden Regulator makes it the most understated of the timekeepers exhibited on Clocktime. This is because it is a calibration instrument: its presentation (case and dial) favours function over form. It also looks very much like the earlier longcases that Harrison made.[15] However, its unpretentious presentation manages to quietly signal the beauty and significance of what lies within. The entirety of the longcase’s large soft-wood frame is ebonised, and it is possible that Harrison concocted his own finish. There is an almost complete absence of metal adornment, excepting the silver chapter ring. All other decorative elements are made of wood or have been hand painted onto the case itself. They include gilt wood finials atop the domed-top caddy case, wooden pilasters with gilt wood capitals on either side of the dial, and the gilt wood lenticle inset in the trunk door. The remaining decoration consists of understated stylised flowers – all delicately hand painted onto the case by the Harrisons, exercising balanced restraint upon the hood, dial, trunk and base, and using a minimal colour pallette of gilt and red. The contrast provided by the red and gilt flowers against the ebonised wood is quietly striking and impactful. Gilt floral motifs also cover the wooden break-arch dial plate. Additionally, there is a gilt-painted putti (representation of naked cherub) at the dial’s centre. On other clocks, these areas would typically be occupied by gilt-cast spandrels and a gilt metal centre disc with engraving. On this longcase, however, the hand-painted floral motifs stand in as substitutes for these traditional, usually costly adornments. The dialplate itself fits directly onto the frame – a design detail that allowed Harrison to forgo the use of dial pillars (which were compulsory for clocks with metal dial plates). The restored dial features a silver chapter ring with Roman numerals, a calendar aperture above the VI, and a seconds aperture below XII. There is also a maintaining power lever at the top corner on the side of the III. In the trunk door is a gilt wood lenticle which provides a glimpse of the (then) cutting-edge pendulum swinging back and forth within. The trunk has been constructed with raised panels to allow room for the oscillation of the gridiron pendulum’s bob. Just above the lenticle, behind a replica glazed panel, is an Equation of Time table written in Harrison’s hand. James Harrison’s main responsibility was to help his brother with the creation of the clock cases. His name, James Harrison Barrow, is signed in gold powder on the dial plate just above XII. John’s signature, Jon Harrison Barrow, is engraved on the chapter ring below, around the VI. Pushing the capabilities Housed within this longcase are several innovations, each of which represents a horological breakthrough in its own right. They include a wooden movement with a new kind of escapement, maintaining power and anti-friction rollers, as well as the gridiron pendulum, a new kind of temperature-compensating pendulum. Expanding upon what he had learned from the Brocklesby Turret Clock project, Harrison harnessed the potential of different woods for the construction of the Wooden Regulator’s movement while retaining the use of brass for the escape wheel. He used oak for the wheels and the self-lubricating hardwood lignum vitae for the bearings and bushes. He also used lignum vitae for his roller pinions, a design detail that further reduced friction. The Wooden Regulator’s movement represents the culmination of Harrison’s bold experimentation with wood for his clock movements – a wholly new approach to clockmaking in of itself. Harrison also redesigned the wooden grasshopper escapement that he had created for the Brocklesby Turret Clock. For this scaled-down version, he altered the design by using only one gold pin for the two pivoting wooden pallets and their two damper weights. However, it is Harrison’s pendulum that most distinguishes the Wooden Regulator from his previous clocks. For this project, Harrison invented a new kind of pendulum – one that could compensate for fluctuations in temperature. Clockmakers had long been plagued by the effects of temperature on the length of the pendulum. Typically, pendulums were made of a single metal, usually brass. When it got hotter, the metal would expand in length; conversely, when it got colder, the metal would contract and therefore shorten. To solve this problem, Harrison first gauged the temperature compensation of a traditional brass pendulum by observing the effects of warmth and cold on his two identical Wooden Regulators, which he had placed in adjacent rooms. To differentiate the temperature in each room, he banked up the fire and closed the windows in one room, while in the other room, he let the fire go out and opened all the windows to the cold, frosty air. Next, he observed how the metal of the brass pendulum expanded in the clock in the heated room and contracted in the clock in the cold room. He then reversed these effects by heating up the contracted pendulum and freezing the expanded one. In this way, he was able to observe and precisely measure the effects of the change in temperature upon the metallic properties of the brass pendulum.[16] Harrison then studied the expansion properties of steel and was able to work out that these could counter the contraction properties of brass. For instance, he observed that as the temperature increased, steel would expand proportionately less than brass. At the time, there was a general awareness of the expansion properties of metals but little to no knowledge about the exact expansion properties of different kinds of metals in comparison with one another. By precisely measuring the expansion properties of steel and brass, Harrison had made a scientific breakthrough. Harrison then set about designing a solution that could prevent or offset changes in the properties of the metal of the pendulum. To do this, he constructed a new type of pendulum made of more than one metal. Instead of a single brass rod, this new pendulum had a symmetrical system of five metal rods, comprising a central steel rod with two brass rods on one side and two steel rods on the other. The overall length of the pendulum in relation to the bob remained constant because the three steel rods under tension were countered by the upward movement of the two brass rods under compression. Harrison’s new pendulum represented the first use of bi-metals in any mechanism ever. Because it resembled a gridiron, it became known as the gridiron pendulum. To see it in action, watch the video for this exhibit, in which horologist Dr John C. Taylor OBE explains how the pendulum was made, as a computer animation reveals how it is put together and functions. Testing by way of a chimney stack How did Harrison test the long-term accuracy of his twin Wooden Regulators? The first step was to access an accurate time base to calibrate the clocks, which leads to the question, ‘How did Harrison figure out the correct and exact time?’ Nowadays, we take our access to the correct time for granted. We can look at our phones or find a running clock or digital time display almost anywhere in our homes or in public spaces. Harrison did not have this luxury. In Harrison’s day, sundials were still relied upon as the primary source for accurate time setting. Even if a person had been fortunate enough to be able to afford a (then) cutting-edge pendulum clock made by one of the leading London clockmakers of the day, it would still have had to be set locally by reference to a sundial. However, access to sundials was limited, and their time is only readable on a clear, sunny day. They were also tricky to read because the time indicated on the dial represents solar time. Therefore, an Equation of Time had to be used to calculate the difference between solar and local mean time[17] To establish an accurate time base, Harrison and his brother looked to the stars in the night sky. Instead of using a telescope, they used a simple but effective homemade astronomical tracking instrument to pinpoint the stars’ positions: the border of a windowpane and the silhouette of the neighbour’s chimney stack. Harrison knew that one day on earth measured 23 hours, 56 minutes and 4 seconds long – a sidereal day. He also knew that because of the earth’s rotation, a star should transit exactly 3 minutes and 56 seconds earlier than the night before. In her book Longitude, science writer Dava Sobel recounts that ‘Night after night, they [the Harrisons] marked the clock hour when given stars exited their field of view behind the chimney”[18] By conducting these late-night tests, the Harrisons were able to gauge the time to within a fraction of a second. They were also able to determine that their temperature-compensating gridiron pendulums never erred more than a single second in a whole month. This was an astonishing level of accuracy for the time. The world’s most accurate clock When Harrison’s Wooden Regulator was made, it became the world’s most accurate clock, and it remained so for the next 150 years. With this clock, Harrison also created a whole new kind of pendulum clock – one that could run for another 40–50 years with no maintenance, thanks to its low friction, maintaining power, newly refined grasshopper escapement, and gridiron pendulum.[19] Moreover, his design utterly surpassed the capabilities of timekeepers made by the best and brightest of his contemporaries in London. What is even more remarkable is that he accomplished all this using homemade instruments, and without access to resources such as materials, machinists and labour – all of which were readily available to clockmakers in London. He also accomplished all that he did in relative isolation in Yorkshire, quite removed from the beating heart of the clockmaking world, London. Many horologists believe that this isolation was key to Harrison’s early success. Essentially, his location and lone status as an outsider gave him the creative space to problem solve and experiment with new methods without interference, fear of judgement, and the financial pressure to turn a profit – all part and parcel of operating as a clockmaker in London. Following the successful completion of testing, Harrison kept his set of two Wooden Regulators, including the one exhibited here on Clocktime, in his possession, and used them as standards against which to test his other clocks.[20] Towards a horological solution With the design of his Wooden Regulators in 1726, Harrison single-handedly ushered in a horological revolution, dramatically driving horological, navigational and wider scientific development. He also established the principles that lay the groundwork for his next and most important phase of work: the design of his sea clocks and marine chronometers. Within four years of completing and successfully calibrating his Wooden Regulators, in 1730, he completed the design of his first sea clock, H1. Soon after, in 1736, he moved to London to secure financing for the further development of his sea clocks. Over the next 23 years, he produced two more sea clocks, H2 and H3. Then, in 1759, his quest to find a horological solution to determining longitude at sea was finally realised with the production of his H4 chronometer, which represented a radical rethink and improvement upon his earlier designs. It was not until 1773 that Harrison, by then an old man, was finally given his due and awarded the Longitude Prize. Finally, please spare Harrison a thought the next time you Google directions on your smart phone, as his designs also paved the way for inventions like GPS. To learn more about Harrison, visit related exhibits here on Clocktime. Harrison's quest to solve the problem of determining longitude at sea is also well documented. You can read more about his designs, trials and tribulations in Longitude: The true story of a lone genius who solved the greatest scientific problem of his time by Dava Sobel; John Harrison and the Quest for Longitude by Jonathan Betts; and John Harrison: The man who found longitude by Humphrey Quill (1966). Dr Kristin Leith, Senior Curator of Clocktime June 2024 End Notes [1] Betts 2023, 34–35. [2] Betts 2023, 34. [3] The movement of Harrison’s first clock is on display at the Science Museum, London, as part of the Worshipful Company of Clockmakers Collection (Object Number L2015-3435). Also see Carter 2021, 218. [4] Betts (2023, 36) suggests that Harrison may have seen ‘continental precedents’ for wooden clocks, because there were a few coming out of southern Germany at the time, and ‘one or two characteristics of Harrison’s early work suggest the influence of Continental clockwork’. [5] Betts 2023, 36. [6] Betts 2023, 36–39; King forthcoming. [7] Betts 2023, 40–42; Garnier and Hollis 2018, 381, Catalogue No. 119; Sobel 2011, 68–70, 109. [8] Betts 2023, 40–42; Quill 1971. [9] Betts 2023, 44; Garnier and Hollis 2018, 381, Catalogue No. 119. [10] Betts 2023, 43. [11] Sobel 2011, 1–33. [12] When Harrison began work on his precision Wooden Regulator Longcase in 1725, a lauded astronomer by the name of Nevil Maskelyne occupied the prestigious post of Astronomer Royal at the Royal Observatory in Greenwich. Maskelyne, the fifth Astronomer Royal, adamantly adhered to his predecessors’ belief that the solution to the problem of longitude would be an astronomical one and particularly favoured a solution using the lunar distance method. Maskelyne was also on the Board of Longitude. The astronomer did not support Harrison’s approach and used his position and influence to undermine the clockmaker’s efforts. You can read a full account of Harrison and Maskelyne’s feud in Sobel 2011. [13] Betts 2023, 43–45; Garnier and Hollis 2018, 381, Catalogue No. 119; Taylor et al. 2020, 1; Matthew King forthcoming. Note that Betts and Sobel focus on the development and testing of Harrison’s chronometers, providing only summary detail about the earlier development (prior to 1730) of Harrison’s Wooden Regulators. Matthew King’s forthcoming book, Clocking On with John Harrison (working title), will go into comparatively more detail about the design and testing of Harrison’s wooden clocks made between 1713 and 1728. It will also draw upon King’s 40 years of experience with woodworking and 20 years of experience working with Harrison clocks and researching the clockmaker’s methods. King is a widely known and respected maker of Harrison replica clocks. [14] Garnier and Hollis 2018, 381. The gridiron pendulum from the second in Harrison’s set of three wooden regulators was eventually removed and sold, later ending up in the Leeds City Museum, Leeds, UK. The third regulator, which was made in 1728, is displayed in the Clockmakers' Museum at the Science Museum in London (Object Number: L2015-3438). John's brother James made its case, which is signed Barrow, James Harrison in the arch of the dial and 'James Harrison 3rd 1728 Barrow on the calendar wheel. John made its movement, and there is an equation of time table written in his own hand that is displayed within the door of the regulator's case. The table is corrected for the change to the Gregorian calendar in 1752. [15] Betts 2023, 43. [16] Note that simple properties such as length, volume, density, spring stiffness, strength of material and viscosity of lubricants are all affected and can affect the accuracy of a clock. [17] You can read more about sundials in the Clocktime article The first timekeepers: Telling time before the pendulum clock. [18] Sobel 2011, 72. Thanks to precision timekeepers, we know now that one day on Earth is actually 23 hours, 56 minutes and 4.09 seconds long, thus 0.09 seconds longer than was known in Harrison’s day. [19] Garnier and Hollis 2018, 381, Catalogue No. 119; Sobel 2011, 70–73. [20] Garnier and Hollis 2018, 381. The gridiron pendulum from the other precision longcase in Harrison’s set of two was eventually removed and sold, later ending up in the Leeds City Museum, Leeds, UK. References Betts, J. 2023. John Harrison and the Quest for Longitude (2nd edition). Greenwich, London: National Maritime Museum. Carter, J. 2021. The John C Taylor Collection: Part II (Selling Exhibition Catalogue, Carter Marsh & Co.). Winchester: Carter Marsh & Co. Garnier, R. and L. Hollis. 2018. Innovation & Collaboration: The early development of the pendulum clock in London. Isle of Man: Fromanteel Ltd. King, M. Forthcoming. Clocking on with John Harrison [working title]. Quill, H. 1966. John Harrison: The man who found longitude. New York, NY: Humanities Press. Sobel, D. 2011. Longitude: The true story of a lone genius who solved the greatest scientific problem of his time. London: Harper Perennial. Taylor, J. C. and K. Leith (with contributions from T. Phillipson and K. Neate). 2020. The Luxury of Time: Clocks from 1500–1800. Isle of Man: Fromanteel Ltd. Further Reading Andrews, W. (editor). The Quest for Longitude (Proceedings of the Harvard University Longitude Conference of 1993). Cambridge, MA: Harvard University Press. Betts, J. 2006. Time Restored. Oxford/Greenwich, London: Oxford University Press/National Maritime Museum. Quill, H. 1971. ‘The grasshopper escapement’ in AHJ VII/4: 288–296. Roberts, D. 2003. Precision Pendulum Clocks: The question for accurate timekeeping in England. West Chester, PA: Schiffer Publishing.
