<p style='text-align: justify;'>This torn and dirtied manuscript within a marbled book represents the observations and calculations carried out by the astronomer Nevil Maskelyne on his voyage to the island of St. Helena from 21st January to 6th April 1761. He had been sent there by the Royal Society to observe the transit of Venus, as part of an international team of observers. The, sadly few, observations from this expedition were published in 1765 as <i>Astronomical Observations made at the Island of St Helena</i> .</p><p style='text-align: justify;'>This journey also introduced Maskelyne to practical navigation and gave him the opportunity to test Tobias Mayer's lunar tables and John Hadley's quadrant for measuring longitude at sea. This experience would make him an invaluable contributor to the Board of Longitude's debates and trials in 1763-1765 of the lunar distance method in comparison to John Harrison's chronometer. Indeed, Maskelyne's opinion of lunar distances derived from this voyage to St. Helena opened the <a href='/view/MS-RGO-00014-00005/82'> discussion of this method (RGO 14/5:78)</a> by the Commissioners at their meeting on 9th February 1765, just before Maskelyne was made Astronomer Royal and therefore became a Commissioner himself.</p><p style='text-align: justify;'>This Journal contains Maskelyne's calculations and observations from his journey, and shows his usual care, meticulous mathematics and attention to detail. We might compare it with his similarly detailed notes elsewhere in the archive, when <a href='/view/MS-RGO-00004-00196'> working out how to simplify the Nautical Almanac (RGO 4/196)</a>, or <a href='/view/MS-RGO-00004-00311'> conducting trials on Harrison's watch (RGO 4/311)</a>. Later pages include careful working out, <a href='' onclick='store.loadPage(58);return false;'> with diagrams (RGO 4/150:28v-34r)</a>, of problems on the elliptical orbit of heavenly bodies, and of the scholium. The majority of the booklet is made up of a mixture of rough and neat observations from the journey, opening with <a href='' onclick='store.loadPage();return false;'> tightly-packed calculations and notes to himself (RGO 4/150:1-5r)</a>, and then <a href='' onclick='store.loadPage(12);return false;'> neatly-set out tables (RGO 4/150:5v-9r)</a> comparing longitude by his observations with the dead reckoning. He includes the journals kept by the Captain Charles Haggis and first mate William Papworth, and carefully records the weather at the time of each observation. He discusses omissions in Mayer's tables and the accuracy of observing with Hadley's quadrant. He specifically mentions using 'Mr Waddington's Quadrant' which refers to the quadrant improvements made by his second observer Robert Waddington and published by him in 1763 in <i>A Practical Method for finding the Longitude and Latitude of a Ship at Sea</i> .</p><p style='text-align: justify;'>Katy Barrett<br />History and Philosophy of Science<br />University of Cambridge<br /></p>
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