The measurement and keeping of time was, for centuries, one of the most important functions of an observatory. Thus it was necessary to ensure that observatories possessed the most accurate clocks available.
In the 18th century the art of making pendulum clocks improved remarkably and one of the most outstanding clock makers at that time was Thomas Earnshaw of London who is known as the ‘father of the chronometer’. He was principally known as a watchmaker, and when asked by Nevil Maskelyne if he would make a clock, for Armagh said he did not even know how many wheels were in one. In fact he produced a masterpiece, which is recognised by horologists today as one of the world’s most important clocks.
It incorporated Earnshaw’s new design of escapement and had a number of novel features including its air-tight case (designed to reduce dust and draughts). It was highly praised by Thomas Romney Robinson in the 19th century who at that time believed it to be the most accurate clock in the world. Its purchase price was 100 pounds in 1794 and Earnshaw charged 100 pounds to travel with it to Armagh and set it up in the new Observatory. Partly as a result of the excellent performance of this clock, its maker, Earnshaw was awarded a prize of 3000 pounds by the government. The Observatory also purchased Earnshaw’s second clock which was operated at sideral rate with the Troughton Equatorial Telescope.
By the late eighteenth century the basic principles of how to determine latitude and longitude from observations of the Sun and stars were well understood. The latitude, in the northern hemisphere at least, could be easily established by measuring the altitude of the pole star above the horizon, which with a small correction, gave the latitude directly. The determination of longitude is more difficult as it requires comparison of local time with the time at Greenwich. It is from the difference in local time, say midday as determined from the Sun’s highest point, from the Greenwich time at that instant, that ships were able to measure their longitude.
The most serious problem with this procedure is that it required a ship to carry a clock which could be relied upon to keep accurate Greenwich time for the many months, even years, it took for a voyage around the world. At that time the only reasonably accurate clocks were regulated by pendulums and these were notoriously unstable at sea due to the rocking of the ships on which they were carried. Many ingenious devices were tried to stabilize pendulum clocks but the problem proved intractable and to encourage inventors the British Government, in 1714, offered a prize of 20,000 pounds to the first person to develop a clock which, after a voyage lasting six weeks, allowed the ship’s position to be determined better than 30 miles. It is quoted that ‘the prize at once became the immediate and accessible target of every crank, swindler, fanatic, enthusiast and lunatic in or out of Bedlam’ but in addition, for over 50 years, the prize eluded many serious clockmakers as well.
Eventually after several attempts, with ever more complicated machines, half of the prize was awarded to John Harrison, a Yorkshire man, with little or no formal education. The investigating body, the Board of Longitude, were eventually forced into accepting his claim after the intervention of George III, who had taken a personal interest in testing Harrison’s time pieces at his own observatory in Kew. Annoyed by the intervention of the King and Parliament on Harrison’s behalf, the Board of Longitude denied Harrison the right to the other half of his prize under the pretext that his clock could not be copied. A further prize was announced for the construction of a simple chronometer that could be cheaply made. Two famous London clockmakers vied with each other for this prize; John Arnold, and Thomas Earnshaw the maker of two of the Armagh Observatory clocks. Two clocks by Earnshaw’s arch-rival, Arnold, are to be seen at Dunsink Observatory Dublin.
Earnshaw developed a simple type of clockwork chronometer that is to all intents and purposes identical to those made until the middle of this century when quartz clocks became available. It could be mass produced comparatively easily and was within the financial reach of any ship owner. Eventually, after considerable wrangling, when Earnshaw’s health was nearly broken, he was awarded £3000 for his efforts. His appeal to Parliament for compensation was bitterly expressed and in a letter to Dr. J.A. Hamilton Director of Armagh Observatory he requests Hamilton’s testimony as to the accuracy of his first clock, Earnshaw No 1. As a result of this evidence, plus that of Maskelyne and others, Parliament finally acceded to Earnshaw’s request.
