The Expansion Of Great Britain Essay Research — страница 2

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joiner. Three of Harrison’s early wooden clocks have survived; the first (1713) is in London, at the Worshipful Company of Clockmakers’ Collection in Guildhall;. the second (1715), is in the Science Museum; the third (1717) is at Nostell Priory in Yorkshire. He married his first wife, Elizabeth, in 1718. She died just eight years later and he remarried within six months, to another Elizabeth. During the latter part of his early career, he worked with his younger brother James. Their first major project was a revolutionary turret clock for the stables at Brocklesby Park, seat of the Pelham family. The clock was revolutionary because it required no lubrication. 18th century clock oils were particularly poor and one of the major causes of failure in clocks of the period. Rather

than concentrating on improvements to the oil, Harrison designed a clock which didn’t need it. It was radical thinking of this sort that would be important later on, when he tackled the problem of designing a marine timekeeper. During the mid-1720’s, John and James designed a series of remarkable precision longcase clocks, to see how far they could push the capabilities of the design. By inventing a pendulum rod made of alternate wires of brass and steel, Harrison eliminated the problem of the pendulum’s effective length increasing in warmer weather, slowing the clock. As a result, Harrison’s regulators from this period achieved an accuracy of one second in a month, a performance far exceeding the best London clocks of the day. To solve the longitude problem, Harrison

would have to devise a portable clock which kept time to the same accuracy as these precision regulators… Constructed between 1730 and 1735, H2 is essentially, a portable version of Harrison’s precision wooden clocks. It is spring-driven and only runs for one day (the wooden clocks run for eight days). All of the moving parts are counterbalanced and controlled by springs so that, unlike a pendulum clock, H2 is independent of the direction of gravity. The linked balance mechanism also ensures that any change in motion which affects one of the balances is compensated for by the same effect on the other balance. Harrison’s linked balance mechanism negates the effects of motion of the clock. H2 was brought to London in 1735 and displayed to the scientific community. Harrison

was beseiged by requests from both scientists and socialites to see the timekeeper. In 1736, Harrison and his timekeeper travelled to Lisbon aboard the ship Centurion, as a test of the clock, and returned on the Orford. H2 performed well in the trial, keeping time accurately enough for Harrison to correct a misreading of the Orford’s longitude on the return voyage. However, Harrison did not ask for a second trial but, instead, requested financial assistance from the Board of Longitude to make a second marine timekeeper. Larger and heavier than H2, H3 is of fundamentally the same design as H2. Harrison began work on H3 in 1737 but in 1740 realised a deficiency in its design and requested more money from the Board to work on a third timekeeper. Harrison worked on his third

timekeeper from 1740 to 1759. After 19 years of labour, it refused to reach the accuracy required by the Board of Longitude. H4 incorporated two inventions of Harrison’s; a bimetallic strip, to compensate the balance spring for the effects of changes in temperature, and the caged roller bearing, the ultimate version of his anti-friction devices. Both of these inventions are used in a variety of machines nowadays. Despite these innovations, work on H4 seemed to lead nowhere and its ultimate role was to convince Harrison that the solution to the longitude problem lay in an entirely different design. In 1753, Harrison had commissioned John Jefferys, a London watchmaker, to make him a watch following Harrison’s own designs. The watch was intended for Harrison’s own personal use

– to help with his astronomical observing and clock testing. Noone in the 1750’s thought of the pocket watch as a serious timekeeper. However, Harrison discovered with his new watch that if certain improvements were made, it had the potential to be an excellent timekeeper. In 1755, as well as asking for continued support for the construction of H4, he asked the Board of Longitude for support H5 is completely different from the other three timekeepers. Just 13cm in diameter and weighing 1.45kg, it looks like a very large pocket watch. Harrison’s son William set sail for the West Indies, with H5, aboard the ship Deptford on 18 November 1761. They arrived in Jamaica on 19 January 1762, where the watch was found to be only 5.1 seconds slow! It was a remarkable achievement but