<p style='text-align: justify;'>This is a rare surviving drawing by John Harrison of one of his famous timekeepers, developed for the purpose of keeping time accurately on a moving ship. From the passage of the <a href='/view/MS-RGO-00014-00001/19'> Longitude Act (RGO 14/1:10r)</a> in 1714, accurate time-keeping had been a recognised, but ill-trusted, potential method for finding longitude at sea. By keeping the accurate time of the place of departure of a ship, a time-keeper would allow the difference from the local time to be found, which was then easily converted into degrees of longitude. Although not the preferred method of most Commissioners of Longitude, who thought methods using celestial observations more likely to succeed, the time-keeper method was the first to be supported financially by the Board, beginning with John Harrison's proposal of his first time-keeper (known as <a target='_blank' class='externalLink' href='http://collections.nmm.ac.uk/collections/objects/79139.html'>H1</a>) in the 1730s.</p><p style='text-align: justify;'>Trialled in 1736 on a voyage to Lisbon, H1 showed marked promise in keeping accurate time, but also some desirable improvements. By the 1740s, Harrison had made a second machine, <a target='_blank' class='externalLink' href='http://collections.nmm.ac.uk/collections/objects/79140.html'>H2</a>, and received a number of payments in support of his work from the Board. The machine shown in this drawing, known as <a target='_blank' class='externalLink' href='http://collections.nmm.ac.uk/collections/objects/79141.html'>H3</a>, was the focus of his work from 1741 to 1760, when it was itself superseded by the very different idea of a watch (<a target='_blank' class='externalLink' href='http://collections.nmm.ac.uk/collections/objects/79142.html'>H4</a>), which would eventually win Harrison part of the 'great reward.' H3 was originally intended as the focus of the trial voyage to Jamaica in 1761-2, but replaced at the last moment by H4.</p><p style='text-align: justify;'>This drawing shows the rear of the clock, with two of the key improvements in clockwork that allowed Harrison to make his machines maintain accurate time on a moving vessel, and which were new to H3. A key aim was to achieve isochronal behaviour, whereby the vibration of the balances was maintained at a consistent frequency. The isochroniser physically impeded the longer arcs of the balance and so tried to even out the vibrations. The caged roller bearing visible to bottom left was one of Harrison's major contributions to modern mechanics as it allowed almost frictionless rotation of parts of the clock.</p><p style='text-align: justify;'>The light and elegant nature of Harrison's drawing was a departure in visual representations of machinery, that would become of central importance in his disputes with the Board of Longitude in the 1760s over the process of disclosure necessary for him to win the longitude prize. The Board required accurate drawings of H4, which were eventually published in 1767 as <i>The Principles of Mr. Harrison's Time-Keeper</i> with ten plates of beautiful, intricate diagrams of parts of the watch. In so doing, the Commissioners not only created to-scale diagrams from which other makers could copy the watch, but also publicised a new method of technical drawing; a method which this drawing from the 1740s suggests had been second nature to Harrison from the beginning of his work on time-keepers.</p><p style='text-align: justify;'>Katy Barrett<br />History and Philosophy of Science<br />University of Cambridge<br /></p>
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