I will write about how Halley, an astronomer and mathematician, came to have command of a Royal Navy ship in a future post, but I thought I’d begin by explaining the purpose of his voyage.
Halley was interested in navigation throughout his life, and especially the search for a method of finding longitude at sea and the variation of the magnetic compass (magnetic declination*). Longitude is your position east or west of a reference meridian and it was very difficult to find at sea, with accurate, practical methods only becoming available in the later eighteenth century through the time-keepers of John Harrison and the lunar-distance tables of Nevil Maskelyne. The difficulty of finding longitude applied not just to the position of a ship at sea, but also to the port to which it was heading. As the Astronomer Royal, John Flamsteed, remarked, ‘Tis in Vain to talke of the Use of findeing the Longitude at Sea, except you know the true Longitude and Latitude of the Port for which you are designd’. In other words, knowing the longitude of your ship was of little use, if the place you were sailing towards was inaccurately located on your chart, and so improving the accuracy of maps was another concern of the period.
Halley was also interested in the related problem of compass variation and he published two important papers on the subject in 1683 and 1692. In the first he explained that the ‘deflection of the Magnetical Needle from the true Meridian’ was of great importance in navigation and that the ‘neglect thereof, does little less than render useless one of the noblest Inventions mankind ever yet attained to’, since if a navigator failed to adjust his course for the local variation, he would sail in the wrong direction. Halley was critical of work that had been done on the subject to date, complaining that no-one had yet sought to establish a general rule of the variation, an endeavour that was characteristic of Halley. He therefore sought to provide such a rule in his paper and proposed that both magnetic declination and secular variation** could be explained if the ‘whole Globe of the Earth is one great Magnet, having Four Magnetical poles’.
Halley refined this theory in his 1692 paper by suggesting that the Earth comprised an external shell and an inner nucleus, and that each possessed two magnetic poles and rotated at slightly different speeds, thereby accounting for the observed features of magnetic variation. (He also suggested there might be more than one inner globe and that these might be habitable, but we’ll draw a veil over that one!) Halley called on mariners to ‘use their utmost Diligence to make, or procure to be made, Observations of these Variations in all parts of the World’ in order to advance the study of the declination, and it was perhaps this that gave him the idea of undertaking the task for himself, as it was shortly after the publication of this paper that his voyage was first proposed.
I will discuss that proposal and the events that followed in a later post, but for now we’ll just look at the instructions he received from the Admiralty on 15 October 1698:
Halley’s Instructions (© National Archives (£), ADM 2/25, pp 155-156)
Whereas his Ma[jes]ty. has been pleased to lend his Pink the Paramour for Your proceeding with her on an Expedition, to improve the knowledge of the Longitude and variations of the Compasse, which Shipp is now compleatly Man’d, Stored and Victualled at his Mats. Charge for the said Expedition; You are therefore hereby required and directed, forthwith to proceed with her according to the following Instructions.
You are to make the best of Your way to the Southward of the Equator, and there to observe on the East Coast of South America, and the West Coast of Affrica, the variations of the Compasse, with all the accuracy You can, as also the true Scituation both in Longitude and Latitude of the Ports where You arrive.
You are likewise to make the like observations at as many of the Islands in the Seas, between the aforesaid Coasts as You can (without too much deviation) bring into Your course: and if the Season of the Yeare permit, You are to stand soe farr into the South, till You discover the Coast of the Terra Incognita, supposed to lye between Magelan’s Streights and the Cape of Good hope, which Coast You are carefully to lay downe in its true position.
In Your returne home You are to visit the English West India Plantations, or as many of them as conveniently You may, and in them to make such observations as may contribute to lay them downe truely in their Geographicall Scituation. And in all the Course of Your Voyage, You must be carefull to omit no opportunity of Noteing the variation of the Compasse, of which You are to keep a Register in Your Journall.
You are for the better lengthning out Your Provisions to put the Men under Your Comand when You come out of the Channel, to Six to four Mens Allowance, assureing them that they shall be punctually pay’d for the same at the End of the Voyage.
You are dureing the Term of this Voyage, to be very carefull in conforming Your selfe to what is directed by the Generall Printed Instructions annex’d to Your Comission, with regard as well to his Mats. honor, as to the Government of the Shipp under Your Comand, and when You returne to England, You are to call in at Plymouth and finding no Orders there to the contrary, to make the best of Your way to the Downes, and remaine there till further Order. Giving Us an Accot. of Your arrival.
So Halley had three tasks on his voyage:
- to measure the variations of the compass, in order to identify a general theory that would be of use to navigators
- to ascertain the longitude and latitude of the places he visits, in order to improve maps and charts
- to seek the coast of the Terra Incognita in the southern Atlantic
Was Halley successful in his mission? Well, you can follow the progress of his expedition via his Twitter account @HalleysLog and he’ll tell you himself!
This post was first published on 22 October 2012 and revised on 22 October 2016.
* magnetic declination (or variation) is the angular difference between magnetic and geographic north
** secular variation is the variation of the variation over time
 The Correspondence of John Flamsteed, ed. by Forbes, Murdin, and Willmoth, 3 vols (Bristol, 1995), Vol. 2, p. 641.
 Halley, ‘A Theory of the Variation of the Magnetical COMPASS’, Philosophical Transactions, vol. 13 (1683), p. 208.
 Ibid, pp. 215-6.
 Halley, ‘An Account of the cause of the Change of the Variation of the Magnetical Needle’, Philosophical Transactions, vol. 16 (1686-92), pp. 567-8.