The Longitude Prize and the Harrison Chronometer

One of the greatest problems that marine navigators face is working out where they are when out of sight of land. Knowing one’s latitude (i.e. how far north or south) is not too difficult, because the height of the Sun in the sky will tell one this, but navigation also relies on pinpointing one’s longitude, or position east or west, which is harder to determine.

In order to determine longitude, there are two possible methods. One is to use the night sky, including the position of the Moon, as a kind of celestial clock. This is the “lunar distance” method, but it has the obvious disadvantage that measurements can only be made at night, and is not particularly accurate. The other is to have on board a clock that is set to the time at some predetermined place, such as one’s home port, that can be compared to local time.

It is not difficult to work out the current local time, based on the Sun’s position, but the problem is knowing what the time is at the port that could have been left weeks or months before. In the early 18^th century there was no clock available that could be relied upon to be accurate enough, especially on a ship at sea that was subject to being tossed about by wind and waves.

The Royal Observatory in London had been established in 1675 with the sole purpose of solving the problem of finding longitude at sea, but by 1714 it had produced nothing better than the lunar distance method. The British Government therefore passed the Longitude Act which offered a prize of 20,000 pounds (several million in modern money) to anyone who could devise a timepiece that could operate with accuracy at sea. The size of the reward shows just how serious this issue was. Great Britain was now a maritime nation that wished to “rule the waves”, but the huge losses of ships at sea, caused by navigational errors, presented a severe handicap to this ambition.

Enter John Harrison

The man who solved the problem was John Harrison (1693-1776), a carpenter’s son from Lincolnshire with no formal education but with an interest in clocks. Although he had only built a few wooden clocks before seeking the longitude prize, he had made several important advances in their accuracy and believed that he had the answer.

He heard about the yet-to-be-claimed prize in 1726, and in 1730 had designed a portable version of his best long-case clock. He showed his drawings to Edmond Halley, the Astronomer Royal, who advised him to consult a well-known clockmaker named George Graham. Graham was impressed by the design and lent Harrison the money to build a prototype clock.

This clock, now referred to as “H1” was completed by 1735. Although portable by the standards of the day, it still weighed 72 pounds. Halley and Graham recommended that it should be tested at sea, and this was done in 1736 on a voyage to Lisbon. Harrison’s clock was accurate enough to correct the ship’s reckoning by one and a half degrees, which was sufficient to persuade the “Board of Navigation” to make Harrison an award of 500 pounds to allow him to make an improved prototype.

The next two prototypes, H2 and H3, were even heavier than H1, and beset with various technical problems, but the real breakthrough came with H4, which was built to a different specification altogether.

This was a large pocket-watch, more than five inches in diameter but only weighing three pounds. Harrison had intended to use this only as a means of “transferring” time from land to sea, so that the sea clock could be set accurately before a ship left port, but he found that H4 worked far better than expected and made the heavy sea clock unnecessary.

How John Harrison eventually won his reward

The terms of the prize were that the timepiece should be sent on a voyage to the West Indies (a regular route at the time of the slave trade), and the amount of the award would depend on the degree of accuracy of the clock or watch. The full 20,000 pounds would be paid if the longitude obtained was correct to within 30 miles, but if this was only 60 miles the prize would reduce to 10,000 miles.

When tested in 1761, the watch lost only 5.1 seconds over the 81 days of the round voyage, although this figure was arrived at by making an allowance, or “rate”, for the known performance of the timepiece over that length of time. Unfortunately, this was not made clear by Harrison at the outset, and the discrepancy nullified the trial. As a result, he was only awarded 2,500 pounds, and this would only be paid if the result was confirmed by a second trial.

This second trial took place in 1764, with a gain of one second per day. On the outer voyage of 47 days, the watch allowed computation of the longitude to within 10 miles, which was three times better than the maximum requirement of the test and should have been enough to land Harrison the full 20,000 pound prize.

However, the Board of Longitude refused to believe that the watch was that accurate and made all sorts of stipulations before they would agree to hand over the money. Harrison was required to make two more watches, and to hand over the original watch so that it could be dismantled and examined by a committee. If an independent craftsman could replicate the watch, Harrison would be awarded the balance of 10,000 pounds, with the remaining 10,000 pounds only being payable if the two extra watches were produced.

When the committee met in August 1765 and examined the H4 watch in Harrison’s presence they were sufficiently impressed to pay him the money, but it was still only half of what had originally been promised. Harrison was determined to win the full amount.

When H4 was copied by a master watchmaker, Larcum Kendall, in 1769, it was found to be of such excellent craftsmanship that it was taken by Captain Cook on his second and third voyages of discovery and used to map the South Pacific Ocean.

Before Harrison could produce another watch, mariners were able to make full use of another invention, namely the sextant, which could be used to make much more accurate calculations of local time and thus render the rival lunar distance method more workable. Harrison therefore had to produce something that was even more accurate than H4, and he was not even allowed access to his own invention when building the new watch, which was labelled H5.

In order to get H5 tested, and to claim the rest of the 20,000 pounds, Harrison was forced to appeal to the King, and in 1772 H5 was tested by the Royal Observatory and found to keep time to within a third of a second a day. Nevertheless, the Board refused to acknowledge the test and it was only when Harrison appealed to Prime Minister (Lord North), and a further Act of Parliament was passed in 1773, that the full prize was finally awarded.

However, Harrison was by now an old man, and he only had three years left in which to bask in the recognition that he so fully deserved. He died in 1776 on what was believed to be his 83^rd birthday.

One has to suppose that the Board of Navigation never really believed that anyone would meet the full terms of the prize, which had been unclaimed since 1714, and was always going to be reluctant to award it to a man whose background was in joinery and was, to all intents and purposes, an amateur when in came to clocks and watches. However, John Harrison was an extremely clever and inventive man who was prepared to spend many years on getting something as good as he could get it.

One innovation that Harrison incorporated was the bimetallic strip, being a strip of two metals fixed together such that changes in temperature would be compensated due to the different expansion coefficients of the two metals. This is the principle used in many later inventions, including the electric toaster. In clocks and watches, the mechanism will not be subject to warping as the temperature rises and falls, thus affecting the accuracy of the timepiece.

The modern marine chronometer, developed from Harrison’s watches, enabled the British Navy to explore and chart the world’s oceans for the next 200 years, and helped Great Britain to become a major world power due to its dominance of the sea.

Of course, the advent of satellites has revolutionised navigation and made much of Harrison’s work redundant. That should not, however, diminish the credit that Harrison deserved. Countless lives must have been saved thanks to his hard work and dedication.

© John Welford