<P> Harrison built his first longcase clock in 1713, at the age of 20 . The mechanism was made entirely of wood . Three of Harrison's early wooden clocks have survived: the first (1713) is in the Worshipful Company of Clockmakers' collection previously in the Guildhall in London, and since 2015 on display in the Science Museum . The second (1715) is also in the Science Museum in London; and the third (1717) is at Nostell Priory in Yorkshire, the face bearing the inscription "John Harrison Barrow". The Nostell example, in the billiards room of this stately home, has a Victorian outer case, which has small glass windows on each side of the movement so that the wooden workings may be inspected . </P> <P> In the early 1720s, Harrison was commissioned to make a new turret clock at Brocklesby Park, North Lincolnshire . The clock still works, and like his previous clocks has a wooden movement of oak and lignum vitae . Unlike his early clocks, it incorporates some original features to improve timekeeping, for example the grasshopper escapement . Between 1725 and 1728, John and his brother James, also a skilled joiner, made at least three precision longcase clocks, again with the movements and longcase made of oak and lignum vitae . The grid - iron pendulum was developed during this period . These precision clocks are thought by some to have been the most accurate clocks in the world at the time . Number 1, now in a private collection, belonged to the Time Museum, USA, until the museum closed in 2000 and its collection was dispersed at auction in 2004 . Number 2 is in the Leeds City Museum . It forms the core of a permanent display dedicated to John Harrison's achievements, "John Harrison: The Clockmaker Who Changed the World" and had its official opening on 23 January 2014, the first longitude - related event marking the tercentenary of the Longitude Act . Number 3 is in the Worshipful Company of Clockmakers' collection . </P> <P> Harrison was a man of many skills and he used these to systematically improve the performance of the pendulum clock . He invented the gridiron pendulum, consisting of alternating brass and iron rods assembled so that the thermal expansions and contractions essentially cancel each other out . 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 . Developed from the anchor escapement, it was almost frictionless, requiring no lubrication because the pallets were made from wood . This was an important advantage at a time when lubricants and their degradation were little understood . </P> <P> In his earlier work on sea clocks, Harrison was continually assisted, both financially and in many other ways, by George Graham, the watchmaker and instrument maker . Harrison was introduced to Graham by the Astronomer Royal Edmond Halley, who championed Harrison and his work . This support was important to Harrison, as he was supposed to have found it difficult to communicate his ideas in a coherent manner . </P>

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