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>

Page: cover

'Theory and Practice of finding the Longitude at Sea or Land' (MKY/6)

This is a manuscript copy of a work by Dr Andrew Mackay [] 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 first page (MKY/6:5) how he had sent the work to Nevil Maskelyne [], 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 Volume 33 (RGO 14/33:518r) of the Board of Longitude archive. Mackay was extended the thanks of both the British and French boards of longitude for the work.

The volume opens with Mackay's own suggestions for improving the lunar distance method []. 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 Nautical Almanac 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 [], Cassini [], and Wargentin as well as Christopher Irwin's marine chair trialled by Maskelyne for the Board in 1764. He considers 'variation charts' [] showing lines of magnetic variation, outlining the history of work to create accurate charts and a reliable theory by Bond, Blackborow, Halley [], Mountaine and Dodson, and Samuel Dunn []. He discusses a new azimuth compass by Kenneth M Culloch (whose own letter to the Commissioners is in Volume 29 (RGO 14/29:34r) of the Board of Longitude archives), and quotes extracts from Captain Cook’s [] last voyage discussing why variation was not a reliable method

Chronometers, Mackay concludes, are the method ' the best, and most easy to be understood, by the generality of seamen (MKY/6:259).’ Interestingly, this is the only place at which he specifically mentions work as inspired by the 1714 longitude act (RGO 14/1:10r). He gives a brief history of work by Huygens [], Frisius, Metius, Hooke, Sully [], le Roy [] and Berthoud []. He discusses the trials and processes of disclosure of Harrison's watch, and mentions him as having won the £20,000. He then deals at some length with improvements made to chronometers by John Arnold (see portrait 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.

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.

Katy Barrett
History and Philosophy of Science
University of Cambridge

Information about this document

Physical Location: National Maritime Museum
Classmark: MKY/6
Author(s): Mackay, Andrew
Date of Creation: 1793 (circa)
Associated Name(s): Mackay, Andrew, 1758-1809
Associated Place(s): Aberdeen; Aberdeen
Extent: 1 volume
Material: Paper
Author(s) of the Record: Dr Emma Saunders Robert Steiner , Dr Emma Saunders , Robert Steiner , Katy Barrett (summary text)

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Information about this document

Physical Location: National Maritime Museum
Classmark: MKY/6
Author(s): Mackay, Andrew
Date of Creation: 1793 (circa)
Associated Name(s): Mackay, Andrew, 1758-1809
Associated Place(s): Aberdeen; Aberdeen
Extent: 1 volume
Material: Paper
Author(s) of the Record: Dr Emma Saunders Robert Steiner , Dr Emma Saunders , Robert Steiner , Katy Barrett (summary text)

Section shown in images 13 to 48

Classmark: MKY/6: 5-40
Title: New method of finding the longitude and latitude of a ship at sea
Date of Creation: 1793 (circa)
Extent: 18 ff.
Material: Paper

Section shown in images 49 to 62

Classmark: MKY/6: 41-54
Title: Of finding the longitude by the observed distance between the Moon and Sun or a fixed star, the apparent time, and its latitude and longitude by account
Date of Creation: 1793 (circa)
Extent: 7 ff.
Material: Paper

Section shown in images 63 to 86

Classmark: MKY/6: 55-78
Title: Of finding the longitude by comparing the Moon with a star not used in the Nautical Almanac
Date of Creation: 1793 (circa)
Extent: 12 ff.
Material: Paper

Section shown in images 87 to 98

Classmark: MKY/6: 79-90
Title: Of finding the longitude by observing the distance between the Moon and a planet
Date of Creation: 1793 (circa)
Extent: 6 ff.
Material: Paper

Section shown in images 99 to 108

Classmark: MKY/6: 91-100
Title: Of finding the longitude by the meridian altitude of the Moon
Date of Creation: 1793 (circa)
Extent: 5 ff.
Material: Paper

Section shown in images 109 to 122

Classmark: MKY/6: 101-114
Title: Of finding the longitude by the observed altitude of either limb of the Moon, the apparent time, and its latitude and longitude by account
Date of Creation: 1793 (circa)
Extent: 7 ff.
Material: Paper

Section shown in images 123 to 136

Classmark: MKY/6: 115-128
Title: Of finding the longitude by the transit of the Moon over the meridian
Date of Creation: 1793 (circa)
Extent: 9 ff.
Material: Paper

Section shown in images 137 to 158

Classmark: MKY/6: 129-150
Title: Of finding the longitude by eclipses of the Moon
Date of Creation: 1793 (circa)
Extent: 11 ff.
Material: Paper

Section shown in images 159 to 240

Classmark: MKY/6: 151-232
Title: Of finding the longitude by occultations, solar eclipses, etc.
Date of Creation: 1793 (circa)
Extent: 41 ff.
Material: Paper

Section shown in images 241 to 266

Classmark: MKY/6: 233-258
Title: Of finding the longitude by the eclipses of Jupiter's satellites
Date of Creation: 1793 (circa)
Associated Name(s): Galilei, Galileo, 1564-1642; Cassini, Giovanni Domenico, 1625-1712; Wargentin, Pehr, 1717-1783; Irwin, Christopher (fl 1762-1765) petitioner to the Board of Longitude; Maskelyne, Nevil, 1732-1811
Extent: 13 ff.
Material: Paper

Section shown in images 267 to 313

Classmark: MKY/6: 259-303
Title: Of finding the longitude by a chronometer or timekeeper
Date of Creation: 1793 (circa)
Associated Name(s): Harrison, John, 1693-1776; Arnold, John (c 1736-1799) watch and chronometer maker
Extent: 23 ff.
Material: Paper