<p style='text-align: justify;'>The log book of Nevil Maskelyne from his voyage to Barbados on behalf of the Board of Longitude from 1761 to 1764, shortly before he became Astronomer Royal. Maskelyne had been made an FRS of the Royal Society in 1758, and had already been sent by it to the island of St Helena in 1761 to observe the transit of Venus. When weather stymied the astronomer's efforts there, he had worked on the lunar-distance method of finding longitude at sea, which he would publish two years later in The British Mariner's Guide. In 1763, Maskelyne was appointed as the astronomical observer to travel to Barbados, rather than to <a href='/view/MS-RGO-00014-00005/32'> fever-ridden Jamaica (RGO 14/5:28)</a> as was originally planned, as part of the second sea trial of John Harrison's marine timekeeper 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>]. The Board directed him to observe eclipses of Jupiter's moons in order to find the longitude difference between England and the island, and to observe the equal altitudes of the Sun before and after noon in order to check the going rate of Harrison's watch. These would be simultaneously observed in England at Portsmouth by John Bradley, nephew of the the late Astronomer Royal James Bradley.</p><p style='text-align: justify;'>On the way to and from Barbados, Maskelyne was also to test the lunar-distance method of finding longitude of which he became a lifelong champion, the Irish projector Christopher Irwin's marine chair for purportedly steadier astronomical viewing at sea, and Tobias Mayer's latest lunar tables. He was appointed a naval Chaplain with a sizeable gratuity of £300 plus expenses and was accompanied on the voyage by Charles Green, assistant to the current Astronomer Royal Nathaniel Bliss. This volume is important because it sheds light on the details of, and Maskelyne's working processes and thoughts during, this important voyage. Firstly, the expedition <a href='/view/MS-RGO-00014-00005/45'> extinguished hopes (RGO 14/5:41)</a> for a reward for Irwin, a relatively well-known longitude projector of the period who had managed to drum up public and institutional interest in part through strategic use of the newspapers and periodicals, and who had received a grant of £500 from the Board on <a href='/view/MS-RGO-00014-00005/41'> 17 August 1762 (RGO 14/5:37)</a>. More importantly, it set the tenor for many of the ensuing activities and conflicts of the Board, with Maskelyne's lunar-distance method (aided by Mayer's tables) and John Harrison's longitude timekeepers both gathering steam and, as a result, John and his son William Harrison developing a lifelong enmity towards the astronomer.</p><p style='text-align: justify;'>The elder Harrison had in fact worked well with the Commissioners of the Longitude from at least 1737 until the early 1760s, as the Commissioners repeatedly gave him financial and logistical support with little question and limited requirements. This began to change as the Commissioners increasingly institutionalised as a sitting Board, and as the issues of the timekeepers' dependability and affordable replicability become more pressing. William Harrison arrived at Barbados in May 1763 and later wrote that he was told by a third party that Maskelyne was an active competitor for the longitude reward, which led Harrison to confront him about the implied impropriety - although he and his father did in fact <a href='/view/MS-RGO-00014-00005/30'> know (RGO 14/5:26)</a> of the astronomer's assigned role in the voyage and of his interest in lunar distances well before the journey took place. Maskelyne recorded experiencing some success with lunar distances on the journey to the island, and it is possible that his relating this to bystanders and to correspondents upon arrival encouraged William Harrison in thinking that he was actively competing against him and could not be trusted with any part of the testing of an alternative longitude scheme.</p><p style='text-align: justify;'>The confrontation between the two men reportedly distressed the astronomer greatly, and a compromise of sorts was reached whereby Maskelyne and Green observed on alternating days with the Captain and three other witnesses present. Harrison and Green left Barbados on 4 June 1764, while Maskelyne remained until 30 August to make more longitude observations and observations for the Moon's parallax in right ascension which he had begun in St Helena. On the return journey, he continued to have encouraging results with lunar-distance observations. Soon after returning to England, a successful campaign began for him to replace the late Nathaniel Bliss as Astronomer Royal. In late 1764, the Board <a href='/view/MS-RGO-00014-00005/66'> began hearing testimonies (RGO 14/5:62)</a> on the results of the voyage to Barbados and began directing what was to be done with the different types of observations made.</p><p style='text-align: justify;'>This log book is divided into three parts. The <a href='' onclick='store.loadPage(3);return false;'> first (RGO 4/321:1r-7r)</a>, which the astronomer entitled 'Celestial observations made on board his Majesty's ship the Princess Louisa, Admiral Tyrell by Nevil Maskelyne FRS', contains: <a href='' onclick='store.loadPage(4);return false;'> mathematical equations (RGO 4/321:1v-2v)</a> and methods for finding the inclination of the ecliptic to the horizon; the <a href='' onclick='store.loadPage(7);return false;'> means of and instruments for (RGO 4/321:3r)</a> making these observations; a log of the observations from <a href='' onclick='store.loadPage(7);return false;'>RGO 4/321:3r-7r</a> and then from <a href='' onclick='store.loadPage(78);return false;'>RGO 4/321:38v-40v</a>.</p><p style='text-align: justify;'>The <a href='' onclick='store.loadPage(16);return false;'> second part (RGO 4/321:7v-43r)</a>, entitled 'Journal of a voyage from England to Barbados in his Majesty's ship Princess Louisa, Captain Norwood, Admiral Tyrell by Nevil Maskelyne FRS Chaplain of the ship', contains: tables of the course set and of meteorological conditions; a <a href='' onclick='store.loadPage();return false;'> sketch (RGO 4/321:14)</a> of two unlabelled islands (presumably Barbados); <a href='' onclick='store.loadPage();return false;'> comparisons (RGO 4/321:15)</a> of observations in order to measure the error of the quadrant; <a href='' onclick='store.loadPage(33);return false;'> stellar observations (RGO 4/321:16r-17v)</a>; <a href='' onclick='store.loadPage(34);return false;'> meridian altitudes (RGO 4/321:16v)</a> of the Sun's latitude; and <a href='' onclick='store.loadPage(37);return false;'> computations (RGO 4/321:18r-20v)</a> for finding longitude by the lunar-distance method.</p><p style='text-align: justify;'>The <a href='' onclick='store.loadPage(88);return false;'> third section (RGO 4/321:43v-54v)</a> of the volume, entitled 'Observations for the longitude made on board the Princess Louisa Nov 2nd 1763 in the morning by Mr. Nevil Maskelyne', contains observations and computations for the longitude made on and before 2 November, a summary of and comments upon errors in and alterations to instruments, as well as <a href='' onclick='store.loadPage();return false;'> bearings (RGO 4/321:54)</a> for landmarks in Barbados. Some of the writings in this log are on loose sheets rather than on pages bound into the volume, especially in part two. This is reminiscent of many of Maskelyne's surviving records from later decades including in the RGO archive, which heavily feature loose sheets of calculations and notes, including on letters and the like which were recycled as scratch paper. These records also often reflect the imperfect conditions of observing at sea and in distant lands, as the astronomer frequently notes when attempts have gone partially or wholly wrong and when rougher estimates of values have been necessary.</p><p style='text-align: justify;'>Alexi Baker<br />History and Philosophy of Science<br />University of Cambridge<br /></p>
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