The simplest solution to the problem of longitude was one of time difference. You can determine how far east or west you are of a given location if you know the difference between local time (determined by the sun) and time in that location. All you need is a clock that is set to Greenwich time. So far, so good. But, as Newton explained when testifying to Parliament in 1714, ‘by reason of the motion of a Ship, the variation of heat & cold, wet and dry, and the difference of Gravity in different Latitudes, such a Watch hath not yet been made’.
Before Harrison set to work on his ‘marine chronometers’ (a word that wouldn’t come into use until after his death), several very clever people had tried to make a clock which would be accurate enough and not affected by the movement and environment on board a ship at sea. All had failed, and given up their attempts to make "the impossible clock". An early attempt to counteract this problem of the motion of the ship included Henry Sully’s experimental marine chronometer known as ‘Sully C’ made in 1724.
The most accurate clocks at that time were long case pendulum clocks, which were impractical at sea thanks to the movement of the ship. One of Harrison’s brainwaves was to do away with the pendulum entirely. In H1, the pendulum is replaced by elegant balances and springs to accommodate the motion of the ship.
Metal components expand and contract with changes of temperature, leading to errors in time keeping. Harrison used metals of different kinds, to compensate for each other. Changes in temperature also affect the consistency of lubricating oils, so Harrison's clocks had a mechanism ,the grasshopper escapement, that was almost friction free, making oil unnecessary.
After Harrison demonstrated H1 to the Board of Longitude in 1735, a sea trial was ordered by the Admiralty. Harrison and H1 set sail for Lisbon aboard HMS Centurion. Although H1 did not quite make the grade, the Board were impressed enough to grant Harrison £500 to start work on a second clock. Always the perfectionist, in 1739 Harrison abandoned work on H2, but persuaded the board to grant him a further £500 to work on a third timekeeper. H3 is the most complex of Harrison's timekeepers, it was tested and adjusted time after time, with each adjustment adding a new level of complexity.
H3 would have made the grade, but Harrison believed he had been going in the wrong direction. John Jefferies, an apprentice of George Graham, made Harrison a pocket watch that incorporated many of the features that Harrison had put into his clocks. Harrison was so impressed by the accuracy of the watch, that in 1755 he asked the board for funding to work on a watch. In 1759, work on H3 ceased and the complete watch, H4 went on trial, to Jamaica and Barbados, accompanied by Harrison's son Ben.
Harrison's timekeepers are the work of a perfectionist. But horological masterpieces are expensive, and by the time Parliament awarded £8750 to John in 1773, he had already received grants of money from them totalling more than £13,000. What was needed now was an accurate timekeeper that was also affordable.
Larcom Kendall, an apprentice of George Graham, pointed this out to the board of longitude. A sextant and Lunar tables could be purchased for just £20, but a ships clock cost upwards of £200. Kendall’s replica of H4, K1, was used by Captain Cook on his second voyage, along with the lunar distance tables. Cook found K1 so reliable, that he called it his “trusty friend and never failing guide”.
By the mid 1800's chronometers, along with sextants and nautical almanacs, were widely available and charting of the oceans had been done on a grand scale. This supported Britain's territorial expansion, improved trade, communication and knowledge of the world.