<p style='text-align: justify;'>This volume of manuscript notes comprises the tables of observations taken while John Harrison's four chronometers, H1 [<a target='_blank' class='externalLink' href='http://collections.rmg.co.uk/collections/objects/79139.html'><img title="Link to RMG" alt='RMG icon' class='nmm_icon' src='/images/general/nmm_small.png'/></a>], H2 [<a target='_blank' class='externalLink' href='http://collections.rmg.co.uk/collections/objects/79140.html'><img title="Link to RMG" alt='RMG icon' class='nmm_icon' src='/images/general/nmm_small.png'/></a>], H3 [<a target='_blank' class='externalLink' href='http://collections.rmg.co.uk/collections/objects/79141.html'><img title="Link to RMG" alt='RMG icon' class='nmm_icon' src='/images/general/nmm_small.png'/></a>] and H4 [<a target='_blank' class='externalLink' href='http://collections.rmg.co.uk/collections/objects/79142.html'><img title="Link to RMG" alt='RMG icon' class='nmm_icon' src='/images/general/nmm_small.png'/></a>] were under trial at the Royal Greenwich Observatory between 1766-1767. They compare the rate of going of each chronometer with the astronomical regulator or transit clock of the Royal Observatory, and with regular observations of the sun's transit over the meridian. The writing varies between the hand of the Astronomer Royal Nevil Maskelyne, who was put in charge of these trials by the Board of Longitude, and the hands of his assistants Joseph Dymond and William Bayly. Sometimes writing by Dymond or Bayly is signed, or has clearly been dictated, by Maskelyne.</p><p style='text-align: justify;'>The majority of the volume focuses on the trials of Harrison's watch H4, which was under serious consideration to win the second half of the 'great reward' promised to him on certain conditions by the <a href='/view/MS-RGO-00014-00001/57'> 1765 Act (RGO 14/1:29r)</a>. The tables of observations, and Maskelyne's introductory comments on these formed part of the Account of the Going of Mr. John Harrison's Watch published by Maskelyne at the order of the Board in 1767. The <a href='' onclick='store.loadPage(4);return false;'> volume opens (RGO 4/311: 1v)</a> with notes by Maskelyne on how the trials were conducted and the precautions taken to ensure that they were fair and accurately replicated. These notes were printed word for word in the <i>Account</i>. Versions of this introduction in both Maskelyne's hand and Dymond's hand with corrections by Maskelyne appear in this volume. It is interesting to see how Maskelyne's corrections altered the text for the published version; <a href='' onclick='store.loadPage(8);return false;'> some (RGO 4/311: 3v-5)</a>, for instance, mark certain columns of the tables as 'not to be printed.' </p><p style='text-align: justify;'>It becomes clear how carefully Maskelyne sought to justify his conduct of the trials and the results for the accuracy of Harrison's clocks which emerged from them, especially as these would lead him to comment in the <i>Account</i> that 'Mr Harrison's watch cannot be depended upon to keep the longitude … within half a degree for more than a fortnight.' His introduction lays out carefully how each stage of the watch's transfer from Red Lion Square to Greenwich was conducted in the presence of John Ibbetson, Secretary to the Admiralty, Captain Thomas Baillie of the Royal Hospital Greenwich, and the watchmaker Larcum Kendall; how the watch was wound and set by Kendal in a sealed box constructed according to the <a href='/view/MS-RGO-00014-00005/125'> resolutions (RGO 14/5:121)</a> of the Board of Longitude; and how the watch was wound and compared each day in the presence of a member of the same Royal Hospital, these observers being the sole holders of the keys to the second lock on the box, the keys to the first being kept at the Royal Observatory. This volume includes the <a href='' onclick='store.loadPage(117);return false;'> certificates (RGO 4/311: 57r-68r)</a> for each day signed by Maskelyne, Bayly or Dymond and a representative from the Royal Hospital. The wording of these certificates is reproduced in the <i>Account</i>. Likewise, each observation is signed by Maskelyne, Dymond or Bayly with a note on what position the watch was left in, which are also carefully published.</p><p style='text-align: justify;'>The volume also includes trials of H1, H2, H3 and the copy of H4, each compared with the transit clock. <a href='' onclick='store.loadPage(83);return false;'> Tables (RGO 4/311: 41r)</a> also compare the relative going of H2 and H3.</p><p style='text-align: justify;'>Katy Barrett<br />History and Philosophy of Science<br />University of Cambridge<br /></p>
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