John Harrison's H4 Chronometer (1761) and the Longitude Prize

John Harrison’s H4 is the marine timekeeper that proved, in sea trials between 1761 and 1764, that a clock could keep accurate enough time on a ship to find its longitude. Its success answered one of the great scientific challenges of the age, set by the British Longitude Act of 1714. Harrison, a self-taught carpenter, spent decades and a bitter dispute with the Board of Longitude before being rewarded.

The longitude problem

A ship’s position is given by latitude, its distance north or south of the equator, and longitude, its distance east or west. Latitude could be found fairly easily by measuring the height of the Sun at noon or of the Pole Star. Longitude was far harder, and navigators relied on dead reckoning: estimating position from speed, course and time.

Errors could be fatal. In October 1707 a British fleet under Admiral Sir Cloudesley Shovell struck rocks off the Isles of Scilly, and four ships were lost with an estimated 1,400 to 2,000 men. The disaster was due to several navigational errors, not longitude alone, but it became a symbol of the problem.

In principle the solution was known. The Earth turns 360 degrees in 24 hours, or 15 degrees every hour. If a navigator knew the time at a home port, such as Greenwich, and compared it with local time measured from the Sun, each hour of difference would equal 15 degrees of longitude. The difficulty was a clock that could keep time on a rolling ship, through changes of temperature and humidity, for weeks or months.

The Longitude Act of 1714

In 1714, after a petition from the mathematicians William Whiston and Humphry Ditton, Parliament passed the Longitude Act. Isaac Newton, then President of the Royal Society, gave evidence to the committee and doubted that a clock could ever be accurate enough.

The Act offered rewards for a “practicable and useful” method tested on a voyage to the West Indies: 10,000 pounds for accuracy within one degree of longitude, 15,000 pounds within two-thirds of a degree, and 20,000 pounds within half a degree. Half a degree at the equator is about 30 nautical miles, which meant a clock could lose or gain no more than about two minutes over a voyage of six weeks. Commissioners, later known as the Board of Longitude, judged the claims.

Harrison’s early clocks

John Harrison was born in March 1693 at Foulby in Yorkshire and grew up in Barrow upon Humber in Lincolnshire. Trained as a carpenter by his father, he taught himself clockmaking and in the 1720s built precision longcase clocks, partly of wood, with innovations such as the gridiron pendulum to cancel the effect of temperature.

In 1730 he went to London and won the support of the astronomer royal, Edmond Halley, and the clockmaker George Graham. His first marine timekeeper, H1, completed in 1735, was a large brass machine with balances linked by springs instead of a pendulum. Tested on a voyage to Lisbon in 1736, it performed well enough for the Board to fund further work.

H2, completed around 1739, was never tested at sea. Harrison spent about 19 years on H3, inventing along the way the bimetallic strip and the caged roller bearing, both still used today. But he came to realise that a smaller, faster-beating watch might work better than a large clock.

H4

Harrison completed H4 in 1759, when he was 66. It looks like a very large pocket watch, about 13 centimetres in diameter and weighing around 1.45 kilograms, in a silver case. Its key features were a fast-beating balance, a mechanism to keep the driving force constant, compensation for temperature and jewelled bearings that reduced friction.

The balance beat five times a second, far faster than in ordinary watches, which made it less sensitive to the motion of a ship. A small spring mechanism called a remontoire was rewound every few seconds so that the force reaching the balance stayed almost constant. Harrison himself believed that H4 was the most beautiful mechanical object he had made, and modern horologists still regard it as a masterpiece.

The sea trials

On 18 November 1761 H4 sailed from Portsmouth aboard HMS Deptford, bound for Jamaica, in the care of Harrison’s son William. The ship reached Port Royal on 19 January 1762. After allowing for the watch’s known daily rate, H4 had lost only about 5 seconds, a remarkable result, though the Board argued about how the rate had been set and how longitude in Jamaica had been checked.

A second trial was arranged. In 1764 William Harrison took H4 to Barbados aboard HMS Tartar. After 47 days its error was 39.2 seconds, equivalent to about 10 miles of longitude, roughly three times better than the Act required for the top prize.

Dispute with the Board

The Board accepted that H4 was accurate but insisted on proof that it could be reproduced. One of its members, Nevil Maskelyne, became astronomer royal in 1765 and was a leading advocate of the rival lunar distance method, which found time at Greenwich by measuring the Moon’s position against the stars. Harrison came to regard him as an enemy, though historians now see the Board’s caution as understandable.

In 1765 Harrison explained H4’s workings to a panel of experts and handed over his drawings and the watches. He received 10,000 pounds but was told the rest would follow only when copies had proved themselves. The watchmaker Larcum Kendall produced an exact copy, K1, completed in 1769. James Cook took K1 on his second voyage of 1772–1775 and praised its performance highly.

Harrison, by now in his late seventies, built another watch, H5. In 1772 King George III had it tested at his private observatory at Kew, where it performed excellently. With the king’s support, Harrison petitioned Parliament, which in 1773 granted him 8,750 pounds. He died in London on 24 March 1776.

Legacy

Harrison’s work showed that the chronometer method was practical, but it took decades for chronometers to become cheap enough for general use. Simpler designs by makers such as John Arnold and Thomas Earnshaw were produced in large numbers from the 1780s, and by the early 19th century the Royal Navy routinely carried them. Lunar distances, published in the Nautical Almanac from 1767, remained in use alongside them for many years.

The use of Greenwich time for both methods strengthened the role of the Royal Observatory at Greenwich, which became the site of the prime meridian in 1884. Harrison’s clocks were restored by Rupert Gould from the 1920s and are displayed there today. Dava Sobel’s bestseller Longitude (1995) made Harrison widely known.

Timeline

• 1693, March: John Harrison is born at Foulby, Yorkshire.
• 1707, 22 October: Shovell’s fleet is wrecked off the Isles of Scilly.
• 1714, July: The Longitude Act becomes law.
• 1736: H1 is tested on a voyage to Lisbon.
• 1759: Harrison completes H4.
• 1761, 18 November: H4 sails from Portsmouth for Jamaica aboard HMS Deptford.
• 1764: Second trial of H4 on a voyage to Barbados.
• 1765: Harrison discloses H4’s design and receives 10,000 pounds.
• 1773: Parliament awards Harrison a further 8,750 pounds.
• 1776, 24 March: Harrison dies in London.

Why it matters

H4 turned the marine timekeeper from a dream into reality. Reliable longitude made sea travel safer, improved charts and supported the expansion of trade, exploration and naval power, while Harrison’s inventions influenced precision engineering long after his death.