A series of features identifying the most extraordinary mechanical masterpieces in history, blending precision, innovation, and craftsmanship. We all have our favourite timepieces either in our collection or those incredible horological masterpieces that have been invented or created through the ages. This series will showcase examples from the previous centuries up to the present day and look at the importance and impact on modern day timekeeping.
John Harrison’s H4 Chronometer was a groundbreaking timepiece that solved the longitude problem, a major challenge for navigation during the 18th century. Before Harrison’s work, sailors had no reliable way to determine their precise east-west position at sea, leading to deadly navigation errors. The British government, through the Longitude Act of 1714, offered a large prize to anyone who could develop a practical solution.
The Catalyst for the Longitude Act
In the early 18th century, determining longitude at sea was one of the greatest challenges in navigation. While sailors could measure latitude using the position of the sun or stars, longitude required precise timekeeping—something that was nearly impossible with the unreliable clocks of the time. The inability to determine longitude accurately led to deadly shipwrecks and massive losses in trade and military operations. The Scilly Isles naval disaster of the 22nd October 1707 was the catalyst for the government passing the Longitude Act of 1714.
The Isles of Scilly shipwreck was one of the British Navy’s greatest tragedies, in which four ships and 1,300 men were lost. The Admiral of the Fleet, Sir Cloudesley Shovell (above) had a long and illustrious service career in the Royal Navy. On that fateful day he was on his way back from Toulon in his flagship, the Association, accompanied by 20 other navy vessels. It should have been a routine voyage, but due to an error in navigation the ships ended up on dangerous rocks and reefs surrounding the Isles of Scilly.
The Longitude Prize
The need to be able to calculate longitude at sea became so acute by 1714, that the British
Government offered a prize of £20,000 (£3 million today) for a solution. An accurate marine clock was thought by some to be the answer, and the prize precipitated a surge of interest in scientific horology throughout Europe. The solution was found in London by the end of the century.
Mariners had no means of finding their longitude at sea. The main contenders were those who thought that longitude could safely be found only by astronomy and those who believed that clocks could provide the answer. In theory, if a clock is set to local time at a
home port and maintained accurately, a navigator simply must check the time where he is and compare it with home time. The time difference will reveal the number of degrees east or west of home the ship had travelled.
In practice, the best land-based timekeeper, the pendulum, wouldn’t work at sea. Many other problems related to temperature, motion, corrosion, friction and lubrication had to be overcome. This was imperative as for every second you were out in your calculations, you were four miles off course.
John Harrison, whose lifetime of experimentation brought him from Lincolnshire to live in London, eventually won the prize. His fourth marine timekeeper and his fifth (below) changed the world.
Early Life and Education
Harrison was born on the 24th March 1693 in Foulby near Wakefield in Yorkshire. He had a modest upbringing and was the eldest of five children. His father, Henry Harrison, was a carpenter, and John followed in his footsteps, developing skills in woodworking and mechanical craftsmanship from an early age.
Despite having no formal education in science or mathematics, he was naturally curious about mechanics and timekeeping. He reportedly developed a fascination with clocks after being bedridden with smallpox as a child—spending his time studying a pocket watch that had been given to him by his father.
By his late teens, Harrison had already built his first wooden clock, using the skills he learned from his father. He constructed these early clocks with wooden gears, as metalworking was not yet his expertise. By the 1720s, Harrison and his younger brother, James, had built a series of precision longcase clocks that demonstrated remarkable accuracy for their time (see movement below).
The Long Journey to Determine Longitude At Sea
The Board of Longitude was officially established by the Longitude Act of 1714, passed by the British Parliament. The Act outlined a substantial reward for anyone who could devise a practical method for determining longitude at sea. The board was composed of a mix of government officials, scientists, and naval officers who were responsible for evaluating proposed solutions. This was one of the largest scientific prizes ever offered and led to a race between astronomers, mathematicians, and clockmakers to find the best method.
The exact list of the first members has not always clearly been documented, but the Longitude Act specified that the board should include the following individuals:
- The Lord High Admiral
- The Speaker of the House of Commons
- The First Commissioner of the Navy
- The Admirals of the Royal Navy
- The Master of the Trinity House
- The President of the Royal Society
- Regius Professors of Mathematics from Oxford and Cambridge
- Other Scientists and Experts as Needed
While individual names may have varied, some key figures associated with the board’s early work included Sir Isaac Newton (President of the Royal Society); Edmond Halley (Astronomer and naval expert, later Astronomer Royal), Samuel Clarke (theologian and mathematician), and James Brydges, 1st Duke of Chandos (a politician involved in scientific funding).
The board played a crucial role in evaluating proposals like those from John Harrison, whose marine chronometers ultimately provided the best solution for measuring longitude accurately at sea.
There were two primary approaches that emerged, namely, (1) Astronomical Method (Lunar Distance Method) – Championed by astronomers such as Nevil Maskelyne, this method involved calculating longitude by measuring the position of the moon relative to stars. It required complex calculations but was supported by the Royal Navy. (2) Timekeeping Method (Marine Chronometer) for which Harrison and others such as Henry Scully, took a different approach: building a highly accurate marine chronometer that could keep time at sea, allowing sailors to calculate longitude by comparing local time to a fixed reference point.
John Harrison’s Breakthrough
Harrison spent decades perfecting his designs, creating a series of four timepieces (H1 to H4). Marine timekeeper, H1 was the first experimental marine timekeeper made by John Harrison in Barrow-on-Humber between 1730 and 1735. Marine timekeeper, H2 was made between 1737 and 1739, a larger and more solidly built version of H1. Marine timekeeper H3 was started in 1740, and this third timekeeper took Harrison nearly 19 years to build and adjust.
His final model, the H4 chronometer (1761), proved remarkably accurate, losing only 5.1 seconds over 81 days at sea. Despite this, the Board of Longitude was reluctant to grant him the full prize. Harrison had to fight for recognition and, with King George III’s support, eventually received £8,750 (a partial payout valued at £1.3 million today) in 1773. It took Harrison over 40-years to solve the Longitude problem and didn’t receive this payment until 60-years after the Longitude Act came to pass.
Inspiration For H4
Harrison commissioned John Jeffreys to make him a pocket watch that was completed in 1753. The watch contains all but one of the refinements included in Harrison’s prizewinning marine timekeeper H4 and represents the turning point in his horological career. This was Harrison’s personal pocket watch.
The watch contains the first maintaining power and first temperature compensation ever fitted to a watch which inspired Harrison to go on to include them in H4, thus ensuring its accuracy whilst travelling through different climates. This is the original drawing by Harrison of the layout for the iconic H4 timekeeper.
It is lavishly jewelled which helps to reduce friction significantly. Signed ‘John Jefferys, London A.D. 1753,’ it was severely damaged by enemy action in World War II which left it with a blackened dial and scorched movement. It is shown in his hand in the portrait below.
Harrison was helped by eminent horologist George Graham and inventor of the deadbeat escapement. He played a crucial role in helping Harrison, through financially supporting him and offering technical advice but he did not directly work on the marine chronometer itself. Graham was a highly respected English clockmaker, inventor, and scientist, and his support was essential in Harrison’s early efforts to develop a solution for the longitude problem.
Harrison was disliked by the establishment because he was an outsider challenging the scientific elite, was seen as difficult and uncooperative, and his solution threatened astronomers’ preferred methods. His battle with the Board of Longitude became a symbol of institutional resistance to innovation.
Positive Impact of the Longitude Act
The Longitude Act led to the adoption of marine chronometers, which dramatically improved safe navigation and global exploration. It standardised timekeeping for navigation, eventually leading to the adoption of Greenwich Mean Time (GMT) as a reference. It laid the foundation for modern navigation, GPS, and timekeeping technology.
Though the Longitude Act was repealed in 1828, its influence continues to shape navigation today. Harrison’s marine chronometer remains one of the greatest engineering feats in history, solving a problem that had baffled the world for centuries.
Legacy of Harrison’s Marine Chronometers
While Harrison’s H4 proved that accurate marine timekeeping was possible, it was later refinements (especially by Thomas Earnshaw and John Arnold) that made chronometers practical and widely adopted in the 19th century. These innovations transformed global navigation, making long voyages far safer and more efficient.
John Harrison’s marine chronometers transformed navigation forever, making sea travel safer and more precise. His contributions earned him posthumous recognition, and his original timepieces are displayed at the Royal Observatory in Greenwich (H1 to H4) and at the Clockmakers’ Museum at the Science Museum in London (H5), a testament to his genius.
Andrew Canter
2025-10-27 14:00:00