National Maritime Museum Manuscripts : 'Theory and Practice of finding the Longitude at Sea or Land'
Mackay, Andrew
National Maritime Museum Manuscripts
<p style='text-align: justify;'>This is a manuscript copy of a work by Dr Andrew Mackay [<a target='_blank' class='externalLink' href='http://collections.rmg.co.uk/collections/objects/107278.html'><img title="Link to RMG" alt='RMG icon' class='nmm_icon' src='/images/general/nmm_small.png'/></a>] which would be published in two editions in 1793 and 1801. It might almost be a handbook on the longitude problem, as it discusses in turn each of the major methods for finding longitude, for some gives a history of their development, presents example problems and calculations, and remarks on their efficacy. For some, Mackay gives the example of establishing the longitude of the Observatory at Aberdeen, of which he was keeper. He discusses on the <a href='' onclick='store.loadPage(13);return false;'> first page (MKY/6:5)</a> how he had sent the work to Nevil Maskelyne [<a target='_blank' class='externalLink' href='http://collections.rmg.co.uk/collections/objects/379043.html'><img title="Link to RMG" alt='RMG icon' class='nmm_icon' src='/images/general/nmm_small.png'/></a>], Astronomer Royal, in 1787 and had received a supportive letter assuring him that, ‘I shall be ready to present it to the board at their next meeting, as I think it ingenious, and that you might reap the credit you deserve.’ Correspondence between Mackay and the Commissioners is included in <a href='/view/MS-RGO-00014-00033/506'> Volume 33 (RGO 14/33:518r)</a> of the Board of Longitude archive. Mackay was extended the thanks of both the British and French boards of longitude for the work. </p><p style='text-align: justify;'>The volume opens with Mackay's own suggestions for improving the lunar distance method [<a target='_blank' class='externalLink' href='http://collections.rmg.co.uk/collections/objects/554426.html'><img title="Link to RMG" alt='RMG icon' class='nmm_icon' src='/images/general/nmm_small.png'/></a>]. He considers how to make observations for longitude, latitude and time without needing existing data, and discusses the easiest methods of calculation. He does this both for the stars included in the <i>Nautical Almanac</i> and for those that are not. He goes on to consider other methods, discussing using eclipses of the moon, sun and Jupiter's satellites. He notes that all are reasonably accurate, notably the last, but not particularly practicable at sea. For Jupiter's satellites, he gives the history of work by Galileo [<a target='_blank' class='externalLink' href='http://collections.rmg.co.uk/collections/objects/14175.html'><img title="Link to RMG" alt='RMG icon' class='nmm_icon' src='/images/general/nmm_small.png'/></a>], Cassini [<a target='_blank' class='externalLink' href='http://collections.rmg.co.uk/collections/objects/40096.html'><img title="Link to RMG" alt='RMG icon' class='nmm_icon' src='/images/general/nmm_small.png'/></a>], and Wargentin as well as Christopher Irwin's marine chair trialled by Maskelyne for the Board in 1764. He considers 'variation charts' [<a target='_blank' class='externalLink' href='http://collections.rmg.co.uk/collections/objects/540213.html'><img title="Link to RMG" alt='RMG icon' class='nmm_icon' src='/images/general/nmm_small.png'/></a>] showing lines of magnetic variation, outlining the history of work to create accurate charts and a reliable theory by Bond, Blackborow, Halley [<a target='_blank' class='externalLink' href='http://collections.rmg.co.uk/collections/objects/14208.html'><img title="Link to RMG" alt='RMG icon' class='nmm_icon' src='/images/general/nmm_small.png'/></a>], Mountaine and Dodson, and Samuel Dunn [<a target='_blank' class='externalLink' href='http://collections.rmg.co.uk/collections.html#!csearch;authority=agent-167415;makerReference=agent-167415'><img title="Link to RMG" alt='RMG icon' class='nmm_icon' src='/images/general/nmm_small.png'/></a>]. He discusses a new azimuth compass by Kenneth M Culloch (whose own letter to the Commissioners is in <a href='/view/MS-RGO-00014-00029/65'> Volume 29 (RGO 14/29:34r)</a> of the Board of Longitude archives), and quotes extracts from Captain Cook’s [<a target='_blank' class='externalLink' href='http://collections.rmg.co.uk/collections/objects/14102.html'><img title="Link to RMG" alt='RMG icon' class='nmm_icon' src='/images/general/nmm_small.png'/></a>] last voyage discussing why variation was not a reliable method</p><p style='text-align: justify;'> Chronometers, Mackay concludes, are the method '<a href='' onclick='store.loadPage(267);return false;'> the best, and most easy to be understood, by the generality of seamen (MKY/6:259)</a>.’ Interestingly, this is the only place at which he specifically mentions work as inspired by the <a href='/view/MS-RGO-00014-00001/19'> 1714 longitude act (RGO 14/1:10r)</a>. He gives a brief history of work by Huygens [<a target='_blank' class='externalLink' href='http://collections.rmg.co.uk/collections/objects/40093.html'><img title="Link to RMG" alt='RMG icon' class='nmm_icon' src='/images/general/nmm_small.png'/></a>], Frisius, Metius, Hooke, Sully [<a target='_blank' class='externalLink' href='http://collections.rmg.co.uk/collections/objects/238369.html'><img title="Link to RMG" alt='RMG icon' class='nmm_icon' src='/images/general/nmm_small.png'/></a>], le Roy [<a target='_blank' class='externalLink' href='http://collections.rmg.co.uk/collections.html#!csearch;authority=agent-13026;makerReference=agent-13026'><img title="Link to RMG" alt='RMG icon' class='nmm_icon' src='/images/general/nmm_small.png'/></a>] and Berthoud [<a target='_blank' class='externalLink' href='http://collections.rmg.co.uk/collections/objects/79684.html'><img title="Link to RMG" alt='RMG icon' class='nmm_icon' src='/images/general/nmm_small.png'/></a>]. He discusses the trials and processes of disclosure of <a target='_blank' class='externalLink' href='http://collections.nmm.ac.uk/collections/objects/79142.html'>Harrison's watch</a>, and mentions him as having won the £20,000. He then deals at some length with improvements made to chronometers by John Arnold (see <a target='_blank' class='externalLink' href='http://www.britishmuseum.org/research/search_the_collection_database/search_object_details.aspx?objectid=55293&partid=1'>portrait</a> at the British Museum), quoting extracts from Arnold's 1780 publication and discussing his own trials of an Arnold belonging to the Duke of Gordon. Finally he quotes Cook at length again on the merits of chronometers.</p><p style='text-align: justify;'>The manuscript is mostly beautifully and neatly written suggesting, perhaps, a presentation copy or one for the printer, but in general only the recto of each page is used and there are blank pages left between sections. This suggests that space was left to add further examples, comments, or even entire methods. The section on using the moon's transit over the meridian appears to be a later addition, as are discussions of Huygens' work on chronometer's and Halley's work on magnetic variation.</p><p style='text-align: justify;'>Katy Barrett<br />History and Philosophy of Science<br />University of Cambridge<br /></p>
