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Solving the Problem of Longitude

 

March 25, 1704 - an urgent petition reached Parliament from "Certain Captains of Her Majesty's Ships, Merchants of London, and Commanders of Merchant-men" who wanted something done about the problems of Longitude in navigation. Up to now it was either guess (dead-reckoning) or carry long, heavy telescopes and awkward pendulum clocks, which couldn't be used aboard ship anyway, to track a tiny Jovian moon after the manner of Cassini, the elder. Neither method was proving satisfactory and there ware numerous ship-wrecks owing to incorrect guestimates of position.

Since "The Discovery of the Longitude is of such Consequence to Great Britain for the safety of the Navy and Merchant Ships is well as for the improvement of Trade that for want thereof many Ships have been retarded in their voyages, and many lost..." Parliament, in 1774, voted to offer a reward (£10,000 for any method capable of determining a ship's longitude within one degree; £15,000, within 40 minutes, and £20,000 within one half a degree) "for such person or persons as shall discover the Longitude." The Board of Longitude, composed of scientists and admirals was permanently set up to examine proposals and check results of accuracy tests.

John Harrison was born at Foulby in Yorkshire, the eldest son of a carpenter. He was given a watch when he was six to amuse him while in bed with smallpox. He spent hours listening to it and studying its moving parts.

Harrison learned his father's trade, made extra money surveying land, read lectures on mechanics and physics, made and repaired clocks. He was a man of many skills and used these to improve on the way clocks were built. For example, he developed the "gridiron pendulum", consisting of alternating brass and steel rods assembled so that the different expansion and contraction rates cancelled each other out and there was no loss or gain of time due to temperature. Another example of his inventive genius was the "grasshopper" escapement - a control device for the step-by-step release of a clock's driving power. Being almost frictionless, it required no oiling.

In 1728 Harrison packed up full scale models of his inventions and drawings for a proposed marine clock and headed for London seeking financial assistance. He was sent to George Graham, the country's foremost horologist. He must have been impressed with Harrison for Graham personally loaned him money and told him to build a model of his marine clock.

Harrison spent seven years building No. 1 - a large device weighing 72 pounds. Since pendulums had proved unreliable at sea, he contrived a system of two large brass balances connected by wires. Since the balances were opposed to each other the effect of the roll of the ship on one would be counteracted by the other. After due examination by the Board, it was deemed worthy of trial by sea. This was successful and Harrison was sent home with a subsidy from the Board of Longitude to build No. 2. This was completed while England was at war with Spain, and rather than risk having the timepiece fall into enemy hands, it was never tried at sea.

No. 3 took Harrison seventeen years to build. He must have been thinking of many improvements because he started No. 4 without submitting No. 3 for trial. His first three clocks were all of the same design -heavy box-like instruments. No. 4 was a thing of beauty - a large pocket watch, twelve centimetres in diameter, with a jewelled mechanism that was the product of "fifty years of self denial, unremitting toil, and ceaseless concentration. I think I may make bold to say," wrote Harrison, "that there is neither any other Mechanism or Mathematical thing in the World that is more beautiful or curious in texture than this my watch or timekeeper for the Longitude."

The next step was to try it out. In November 1761, Harrison, by now sixty-eight years old, entrusted his precious watch to his son and co-worker, William. The Board of, Longitude placed the clock aboard the Deptford, bound for Jamaica. It was in a case with four locks - William Harrison, Governor Lyttleton of Jamaica, Captain Dudley Digges, and his lieutenant all had keys and had to be present when the case was unlocked and the clock was wound. No cheating here!

The first leg of the voyage from Spithead to Madeira generated unusual interest in the timepiece since the ship's beer had spoiled and the crew didn't want to cross the Atlantic with only water to drink. Nine days out the Deptford's longitude by dead-reckoning was 13 degrees 50 minutes West, but by Harrison's calculations 15 degrees 19 minutes West - a difference of 160 kilometres. Digges, sceptical but committed to testing the timepiece, kept to Harrison's course and sure enough they sighted land right where it should be. If they had altered course to go by dead-reckoning they would have missed Madeira altogether.

The landing at Jamaica was equally successful. Digges followed Harrison's calculations and arrived there three days before another ship that had left port ten days before the Deptford. No. 4 was taken ashore and checked against Jamaica's longitude as determined by astronomical calculations. Making a predetermined allowance for two and two-thirds seconds a day for error it was found to be five seconds slow - an error of 1.25 minutes in longitude, more than close enough for the great prize.

However the Board of Longitude decided it was just a fluke that the watch performed so well. They gave Harrison £2,500 and said that further sea trials would be necessary. So in March 1764 William set sail for the Barbados aboard the Tartar. Once again the timepiece performed magnificently an error of only 34.4 seconds over seven weeks or on the round trip less than a tenth of a second a day!

Early in 1765 the Board of Longitude agreed that "the said timekeeper had kept its time with sufficient correctness, without losing its longitude in the voyage from Portsmouth to Barbados beyond the nearest limit required by the Act 12th of Queen Anne, but even considerably within the same."

The Board decided that Harrison would have to turn all four clocks over to them for testing and reproduction, then he would receive £7,500 or half the reward. The other half would be available only on completion and testing of two more timepieces.

Harrison felt "extremely ill used by the gentlemen who I might have expected better treatment from" and decided to enlist the aid of King George III. He asked for and was granted an audience with the King who became extremely annoyed with the Board. King George tested No. 5 himself at the palace and when it had lost only four and a half seconds in ten days he told Harrison to petition Parliament for the full prize. So in 1773, Harrison finally received his reward.

In 1776 James Cook took a copy of No. 4 with him to the Pacific. He proved it extremely useful many times over in charting and mapping the many places he visited on his second voyage.

Two centuries after John Harrison's death in 1776 Neil A. Armstrong paid tribute to him, saying that this, the earliest of chronometers, enabled man to explore Earth with precision and then to build navigational systems for voyages to the Moon.

BIBLIOGRAPHY

Armstrong, Richard: "The Discoverers", Time Life Science Library, New York, 1966. Wilford, John Noble: "The Mapmakers", chapter 9, New York, 1981.

Ruth Burkholder

Originally published in Cook's Log, page 222, volume 6, number 4 (1983).

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