Description
The publicity given by Dava Sobel's book 'Longitude', together with the BBC and Channel 4 'Longitude' films, has increased the awareness of the public to the work of John 'Longitude' Harrison. In particular, Harrison has become widely known for his efforts to win the £20,000 prize offered by the British Government in 1714 to anyone capable of devising a method for the determination of longitude at sea, accurate to within 30 nautical miles. Longitude is the measure of how far east or west something is from the Prime Meridian (a line that passes from the North to South pole though the Royal Observatory at Greenwich) and its accurate determination was sought in order to improve the navigation of
British ships.

Harrison's solution to the problem of determining longitude required the accurate keeping of time, and to this end he built three large sea clocks (or "chronometers") now known of as H1, H2 and H3, all designed to keep very accurate time even whilst being tossed about on a ship at sea. Harrison's final successful design, H4, was in the form of a watch about 5 inches (140mm) in diameter, and although this ultimate design incorporated much he had learned through producing H3, it was quite different from the previous three larger clocks.

This is a full-size working replica of the original H3 clock, now held in the National Maritime Museum, Greenwich.

John Harrison was born in 1693 near Wakefield, moving with his family to Barrow just south of the Humber when four years old. Little is known of his early education, except that he learned woodworking skills from his father and became very interested in music. He was a precocious child who worked hard to widen his knowledge, helped by a friendly visiting clergyman. How he came to develop his interest in clockmaking is unknown, but by the age of 20 Harrison had already completed a longcase clock with the motion work made largely of oak.

After working alone on a number of clocks (three of which have survived) Harrison was later joined by his brother James and together they produced a turret clock and two regulators, still incorporating wood: In this early work Harrison demonstrated that he was an impressive thinker as well as craftsman, solving a number of problems, most notably how to avoid the need for oil in clock mechanisms. His solution was to use the oily hard wood lignum vitae with brass as bearing material. These same materials are used in this replica.

Following initial attempts to keep time at sea using H1 and H2, Harrison spent 10 years trying to get H3 to perform satisfactorily, partly financed by grants from the Government's Board of Longitude. However, Harrison came to realise by 1755 that H3 had certain basic faults, so began quietly working on his next chronometer before eventually abandoning H3 to produce the successful H4.

Apart from it being Harrison's third attempt to win the £20,000 Board of Longitude prize, H3's other claim to frame is that it incorporated a number of Harrison's outstanding inventions. One was the caged roller bearing (still used today) and another even more significant innovation was the temperature sensitive bimetallic strip. Examples of the bimetallic strip are still widely used industrially and incorporated in almost every domestic appliance. Another significant innovation used in H3 was the grasshopper escapement, a device that involved no sliding and derived its name from the manner in which the pallets jump when released. other notable firsts in H3 include the use of a coarse ratchet wheel with two pawls set half the tooth pitch apart (giving the effect of a fine ratchet wheel but with strong teeth) and the introduction of maintaining gear, which provided drive to the clock whilst it was being wound up.
03/02/2023
Created by: Guey-Mei hsu on 03/02/2023