Tuesday, 2 December 2014

Dreamers and Schemers - the search for longitude

The final plate of Hogarth's A Rake's Progress from 1735 depicts a madhouse, in which the protagonist has finally found himself after squandering his riches and losing his mind. In the background of the scene is a detail I had not noticed before, until my attention was drawn to it at the excellent Ships Clocks and Stars exhibition currently running at the Maritime Museum. On the back wall of the scene, a deranged inmate is puzzling over solutions to the great challenge of the age; how to accurately determine longitude at sea.

A Rake's Progress - 8th plate 1735
Such is the way in which the search for longitude was viewed by satirists such as Hogarth; as a fools quest. The vast sums in prize money promised by the Board of Longitude, formed in 1714, were a veritable pot of gold that tempted dreamers and schemers to devise solutions to the problem. In the coffee houses of London schemes for solving the longitude challenge were a popular topic of conversation and the more outlandish proponents were fair game for lampooning.
No man was a more popular target for derision than William Whiston. Far from being a crackpot, Whiston succeeded to the Cambridge professorship of no less a luminary than Isaac Newton in 1702. Nevertheless he did hold unorthodox religious beliefs and was fascinated by miracles and prophecies. He caused alarm in 1736 by claiming that a comet was about to hit the earth. His other enduring fascination was with the problem of longitude and he was behind a number of proposals to the board.

Whiston's first proposal was as straightforward as it was impractical. Along with mathematician Humphrey Ditton, Whiston proposed using a system of anchored rocket ships around the globe. These ships, at positions of known longitude, would send up rockets at regular intervals. Ship's captains, observing the explosion of the rocket, would calculate their position based upon the time it took for the sound of the rocket to reach them. The admiralty, it need hardly be said, were unimpressed. Where were the funds and the thousands of men required to crew these ships to be found, they demanded. And how long would it be before they mutinied out of sheer boredom?

Halley's chart of magnetic variation

Unperturbed, Whiston next proposed using magnetism to find longitude. By producing charts showing the lines of magnetic variation across the globe, it would be possible for ship's captains to compare true north, established through solar observation, with the magnetic north determined by their compasses and then refer to the chart to find their longitude. Indeed these lines of magnetic variation had already been charted in the Atlantic by Edmund Halley during his command of HMS Paramour in 1698-99 on the first ever purely scientific naval mission. Whiston received approval from the board of longitude to produce a chart of magnetic variation for southern  England in 1719. Unfortunately for Whiston this idea too was ultimately doomed, for the variation in the earth's magnetic field would render any charts produced obsolete within a matter of years, requiring the entire painstaking process to begin again. The board eventually paid him £500 for his trouble.

In 1730 Whiston brought forth another proposal, this time based upon observing the regular motion of the moons of Jupiter. By noting the local time of an eclipse of one of the four moons as it passed behind or in front of the planet and comparing it to a table of the known times based on Greenwich, it would be possible to determine longitude. Once again there was nothing wrong with the theory, Galileo himself had suggested the use of Jupiter's moons as a celestial timepiece and it works perfectly well on land. The practicality however of observing the moons of Jupiter from the deck of a ship in anything other than perfect conditions, once again condemned the idea to the dustbin of longitude history. Whiston was not unaware of the difficulties and suggested using up to 7 telescopes in combination to ensure that the observer could keep Jupiter in their sights.

Whiston's unorthodox religious views eventually caused him to fall from establishment favour and he spent the rest of his life engaging in increasingly outlandish theological speculation. He died in 1752.

Whiston brandishes another cunning plan
Ultimately the solution to the longitude problem boiled down to two methods; lunar distance and marine chronometers. The lunar distance method, couched as it was in mathematics and astronomy, was more attractive to the gentleman scientists of the Royal Society, who struggled to overcome their disappointment that 'Harrison's blasted watch', as third astronomer royal James Bradley described it, could provide a complete solution.
In 1761 the Royal Society dispatched astronomer Neville Maskelyne to the island of St Helena to observe the transit of Venus. During the voyage Maskelyne was keen to try out the ideas of one Tobias Mayer. Mayer was a remarkable individual. Having come from poor and humble beginnings, he was entirely self taught and published his first mathematical work when he was just eighteen, subsequently obtaining work as a mapmaker. By the 1750's Mayer was the director of Gottingen observatory and had turned his mind to the issue of longitude. Mayer focussed on the lunar distance method, which allowed longitude to be determined by observing the angle between the moon and a chosen star, typically Regulus. By comparing the observed angle with tables compiled from Mayer's exacting observations, the mariner would be able to establish their precise local time and thereby their longitude. Mayer sent his tables to the Board of Longitude in 1755. They were assessed by Bradley, who had succeeded the great Edmund Halley in 1742, and were found to be most accurate. Bradley continued to build on Mayer's work, compiling further tables based on Greenwich. He was assisted in this work by Charles Mason, best known for his later partnership with Jeremiah Dixon in establishing the line that bears their names.
Mr Irwin's marine chair undergoes testing
So off sailed Maskelyne aboard the Prince Henry bound for St Helena, putting Mayer's methods to practical use and becoming in the process an advocate of the lunar distance method. Two years later he was dispatched to Barbados aboard HMS Princess Louisa. He took with him Harrison's final chronometer H4, the culmination of 30 years of work, the accuracy of which would be compared directly with the lunar distance method. Maskelyne also took with him Mr Christopher Irwin's marine chair for testing. This gimballed chair was designed to allow astronomical observations to be made more easily from the pitching and rolling deck of a ship. Maskelyne found the chair to be completely useless. On arrival, Maskelyne would determine the longitude of Bridgetown using Jupiter's satellites and see which of the rival methods came closest. Harrison's chronometer was found to be accurate to within 10 miles, whereas the lunar distance calculations were only within 30.
Maskelyne, who succeeded to the post of astronomer royal upon his return, has been somewhat vilified by posterity for his opposition to Harrison's clocks. Whilst Harrison is celebrated today as the man who solved the longitude problem, in reality the high cost of producing accurate marine chronometers ensured that both methods would continue to be employed for finding longitude at sea. Maskelyne oversaw the completion of the first Nautical Almanac containing lunar tables for determining longitude in 1766.
Mr Harrison poses proudly with his chronometers
Both Harrison and Mayer were ultimately awarded prizes by the Board of Longitude, although Harrison would require the intervention of the king to receive his £9,000 reward and Mayer's award of the same amount would be paid to his widow after his death aged just 39.
Ships, Clocks and Stars is on at the Royal Maritime Museum, Greenwich until 5th January. Go see it if you have the chance. The highlight is undoubtedly the sight of all four of Harrison's beautiful chronometers together in the same place but as I found out, there was more than one dog in the hunt.
The RMG longitude blog
You may also enjoy - The Hourglass Sea

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