Papers of Nevil Maskelyne : Instrument for taking angles at sea

Papers of Nevil Maskelyne

John Wood in Kriens near Lucerne in Switzerland sent this handwritten 'Description of an instrument for taking Angles at Sea' to the Board of Longitude via Henry Parker at the Admiralty in London on 1 March 1792. A pencil notation on the <a href='' onclick='store.loadPage(32);return false;'> title page (RGO 4/219:back cover)</a> records that it was received by Parker on 8 June. He then passed it to the Astronomer Royal Nevil Maskelyne for his opinion, with a <a href='/view/MS-RGO-00004-00187/'> cover letter (RGO 4/187:16)</a> marked 7 June (dating disparity unexplained), which is now in volume RGO 4/87. Maskelyne used the letter as scratch paper, which could partially be viewed as a reflection of his and thus the Board's unimpressed opinion of Wood's invention.Wood <a href='' onclick='store.loadPage(1);return false;'> begged (RGO 4/219:2)</a> the Commissioners' forgiveness for any inaccuracies and potential misapplication of technical terms in his treatise, attributing them to his youth and to his belonging to an unspecified profession which was unrelated to such activities. (He may have been the author John Wood who published books during the late eighteenth and early nineteenth centuries including A General View of the History of Switzerland; With a Particular Account of the Origin and Accomplishment of the Late Swiss Revolution.) He <a href='' onclick='store.loadPage(4);return false;'> proposed (RGO 4/219:2v)</a> that his instrument would be of great use not only to finding the longitude by the lunar-distance method but also to general navigation and to measuring angles on land. The projector said that in comparison, the sextant was too flawed for this purpose and that any similar instrument would have to be built to too large a scale in order to overcome such flaws.Wood <a href='' onclick='store.loadPage(1);return false;'> continued (RGO 4/219:3)</a> that his instrument was constructed on the same principles as the Hadley's Quadrant but in a different form, and that its scale could take any angle to a second of degree. Whereas Hadley's Quadrant had an index carrying a speculum along a graduated arch, his had an 'endless screw' moving an index and speculum over a divided circle. The instrument's body consisted of <a href='' onclick='store.loadPage(6);return false;'> two brass plates (RGO 4/219:3v)</a> affixed at right angles to each other. One plate was a four-inch circular scale, divided into sixty parts which were each divided into three parts, over which the endless screw index moved. The other was a rectangular plate which held the telescope, the one horizon glass, and a two-inch brass wheel with 180 teeth and a large speculum fixed to its centre. The plate also supported a hollow cylinder around which the endless screw travelled, moving an index along a subdivided quadrant on the opposite circular plate. The index affixed to the wheel revealed the degrees of an observed angle, while the index affixed to the screw showed minutes and seconds.The remainder of Wood's treatise after the physical descriptions of the instrument explain how to make observations with it, how to overcome obstacles to its accurate use, and how to compensate for the irregular motion of the screw index. It includes a <a href='' onclick='store.loadPage(16);return false;'> depiction (RGO 4/219:8v)</a> of the parts and movement of the instrument's screw cylinder, and a large fold-out <a href='' onclick='store.loadPage(1);return false;'> engraving (RGO 4/219:14)</a> of the instruments and its component parts and scale and of relevant angle measures. There is no signature for the engraver of the latter. As the taking of angles and altitudes was so vital to astronomy and often to general navigation, other projectors also proposed angle instruments for finding longitude at sea. Examples include J. Vice (<a href='/view/MS-RGO-00014-00045/'> (RGO 14/45:337-338a)</a>) and Samuel Dunn (<a href='/view/MS-RGO-00014-00005/174'> (RGO 14/5:169-170)</a>).Alexi Baker History and Philosophy of Science University of Cambridge