National Maritime Museum Manuscripts : Directions for Walker's Meridional and Azimuth Compass

Platt, John

National Maritime Museum Manuscripts

<p style='text-align: justify;'>A set of instructions for a magnetic compass [<a target='_blank' class='externalLink' href='http://collections.rmg.co.uk/collections/objects/42475.html'><img title="Link to RMG" alt='RMG icon' class='nmm_icon' src='/images/general/nmm_small.png'/></a>] designed by Ralph Walker (1749-1824) [<a target='_blank' class='externalLink' href='http://collections.rmg.co.uk/collections/objects/107212.html'><img title="Link to RMG" alt='RMG icon' class='nmm_icon' src='/images/general/nmm_small.png'/></a>]. The compass was intended to solve a number of navigational problems, including finding a ship's longitude. The instructions are from a group of papers belinging to John Platt (fl. 1780-1832), a seaman who served on a number of ships including HMS Centurion and HMS Alligator, both of which operated around the West Indies in the 1790s.</p><p style='text-align: justify;'>Ralph Walker, the son of a Scottish farmer, went to sea in 1768. After becoming master of various vessels trading to the West Indies, the Baltic and America, he settled as a planter in Jamaica in 1783 with his Irish wife, Jane. While in Jamaica he built a prototype compass, mostly out of wood, that so impressed the Governor of Jamaica that he procured Walker a passage to London on HMS Providence under Captain William Bligh in 1793, so that he could present his invention to the Admiralty and the Board of Longitude.</p><p style='text-align: justify;'>Although initial reactions were favourable, Nevil Maskelyne considered that the compass was neither innovative nor useful. Reporting to the Board on <a href='/view/MS-RGO-00014-00006/370'> 6 December 1794 (RGO 14/6:2:224)</a>, Maskelyne criticised the compass and the theory behind it, noting that the idea of finding longitude from magnetic variation had nothing to offer. Nevertheless, the Board met with Walker several times between 1793 and 1796, as well as sending his compass for trials. Although the compass was not ultimately considered a viable longitude solution, it was seen as an improvement on other compasses then available. This seems to be why the Board finally granted Walker a reward of £200. The <a href='/view/MS-ADM-A-02869'> order for payment by the (ADM/A/2869)</a> was issued by the Commissioners of Longitude in June 1795.</p><p style='text-align: justify;'>The instructions describe firstly the different parts of the compass, then how to use it to observe the apparent or local time and the magnetic variation, and finally two methods for measuring the sun's altitude and azimuth. To find longitude, a mariner would use the sundial attachment to determine true north. Comparing this with magnetic north from the compass needle gave the magnetic variation. This could, in theory, be used to discover the longitude, by finding where supposed 'magnetic meridians' intersected with the observed latitude.</p><p style='text-align: justify;'>Richard Dunn<br />Royal Museums Greenwich<br /></p>