2. The Astronomer's Role at Greenwich

Nevil Maskelyne's role as Astronomer Royal, which he held from February 1765, was to produce numbers that were to be printed and distributed, in the form of the Greenwich Observations and accompanying Requisite Tables, and the Nautical Almanac. These printed numbers had a wide audience, ranging from the King's library, to which it was ordered on 21 April 1774, at a Council of the Royal Society, that a copy of the first printed Observations be presented,Footnote ¹ to sea captains in both the merchant and Royal Navies, for whom the Nautical Almanac was (or became) affordable and desirable. To carry out this role, Maskelyne resided permanently and visibly at Greenwich.

These aspects of Maskelyne's role as the British Astronomer Royal were novel in scale and format. The Nautical Almanac was Maskelyne's creation, and the observations made previously by his predecessors John Flamsteed, Edmond Halley, James Bradley, and Nathaniel Bliss were of varied extent and reliability and were never published so painlessly and regularly, if at all, as they were under Maskelyne. In a communication to the Royal Society on 9 June 1763, the young Maskelyne, not yet Astronomer Royal, was less than complimentary with regard to the three previous Astronomers Royal's belief that their observations were their personal property.Footnote ² “Mr Flamsteed the first Astronomer Royal's tenaciousness in withholding his observations, not only from the public but even from his learned friends, a long while deprived Newton of the means of establishing his theory of the moon,” Maskelyne told the Society, and “with respect to Dr Halley's Observations, made with superior instruments to those of Mr Flamsteed we have been even more unfortunate; being entirely deprived of them, his executors having seized and to this very day detaining them as their own property.” Furthermore, according to Maskelyne writing circa 1800, his immediate predecessor Bliss was for the two years from his appointment in 1762 to his death in 1764 “incapable from his age and infirmities from taking any active part in making the observations” (Howse Reference Howse1989, 214–224), and it appears that Bliss was additionally, often absent from Greenwich, living in Oxford, for extended periods (Maunder Reference Maunder1900, 82).

Bradley had died in July 1762, with his twenty years of observations unpublished. Maskelyne asked the Society at the meeting in June 1763:

This is exactly what did happen, as was noted at the Royal Society on 13 September 1764, when it became apparent that Bradley's complete observations had been removed by Bradley's executors, who had arrived at the Observatory to demand them from Bliss's widow, Bliss himself having died on 2 September (Howse Reference Howse1989, 106). Thirteen folio volumes of unpublished observations by Bradley had been removed.Footnote ³

A letter written by Bliss eighteen months previously indicates how the issues of an absent, non-resident Astronomer Royal related to the security of manuscript observations and their potential future publication. Bliss wrote from Oxford to Charles Mason, former assistant to Bradley, giving permission for Mason to visit Greenwich in order to remove some personal belongings:Footnote ⁴

Bliss went on to ask Mason to pass on a message to Charles Green, the current assistant to the Astronomer Royal:

“Mr Peach” was Bradley's brother-in-law Samuel Peach, who was responsible for the removal of all Bradley's manuscript observations the following year. It is not clear exactly which “books” Bliss refers to in his letter to Mason, but, taken together, the two elements of this letter indicate a porous Greenwich Observatory where the Astronomer Royal was absent and access to the most precious (paper) materials there, the observations, was easily obtained.

In June 1763 the young Maskelyne had recommended institutional reform at Greenwich; in November 1764 his suggestions were implemented in nine regulations drawn up by the Royal Society in its role as Visitors to the Observatory. There was a dual emphasis on the regular publication of observations, and on the permanent residence of the Astronomer Royal at Greenwich. The first regulation noted that

The Astronomer Royal's movements were thus clearly restricted and controlled by the Visitors, who demanded that he reside at the Observatory “constantly.” The seventh regulation demanded that the manuscript observations never be removed from the Observatory “upon any pretence whatsoever”; and the eighth demanded that fair copies “of the annual observations made by the Astronomer Royal and his assistant shall be transmitted within six months after every year shall be elapsed to the Council of the Royal Society.”Footnote ⁶

Ten years later, the success of the implementation of these regulations was witnessed in the publication of the first sets of the Greenwich Observations, as discussed at a Royal Society Council on 13 June 1774.Footnote ⁷ Just as specified in the 1764 regulations, these were observations made by a permanent resident of Greenwich, which were then made public. Greenwich had become a place where numbers were produced and printed, promptly, for the benefit of the nation. Maskelyne was not present at this particular Council, because he was in transit heading north, intending to spend four months camping on a boggy Scottish hillside. Given that this took him away from the production of the Observations and Nautical Almanac, a production he nicely described as his “numerous and interesting . . . literary engagements” (Maskelyne Reference Maskelyne1775b, 504) and also took him away from his residence at Greenwich, we must ask what serious business it was that would need his attendance. It was, it should be pointed out, to no other than the King, George III, that Maskelyne had to petition in order to be granted “permission for absenting myself so long from my duty at the Royal Observatory” (ibid., 505). In the following section we shall see how much personal historical involvement Maskelyne had with the instrument at the heart of the Scottish operations, the zenith sector telescope and its troublesome plumb line, and hence why it was critical for him to attend.

3. Maskelyne and the Zenith Sector

Maskelyne had first highlighted a problem with the plumb lines of his zenith sector in a demonstration to the Royal Society in September 1762, at the British Museum, and at greater length in an incendiary paper published in Philosophical Transactions. This paper was based on his experience on St. Helena from April 1761 to February 1762, which was by all accounts wretched. He described St. Helena as a place where “the almost continual cloudiness of the skies . . . renders it a very inconvenient place for the making of astronomical observations” (Maskelyne Reference Maskelyne1764, 349). Maskelyne had been sent to St. Helena in order to observe the transit of Venus of 6 June 1761, to test his new “lunar distance” method of determining longitude at sea, and to attempt “by the help of a ten foot sector, provided me at the expense of the Royal Society, either to prove or disprove the existence of a sensible annual parallax of the star Sirius . . . by observations of the zenith distance of the star” (ibid., 350). This observation of an annual parallax would, if successful, “be the fullest and directest proof of the Copernican system” (Maskelyne Reference Maskelyne1760, 889), as it would convincingly demonstrate that the earth really moved. Based on estimates by the French astronomer the Abbé de la Caille, Maskelyne expected to observe an annual parallax of Sirius of around nine seconds of arc.

Having failed to observe the transit of Venus in June 1761 due to cloudy weather, Maskelyne's attempts to measure the parallax of Sirius were also a failure. He found “a strange irregularity in the observed zenith distances of the stars, amounting to 10, 20, and sometimes even 30 seconds.” The problem, he eventually found, lay with the suspension of the plumb line. The “fine silver wire” plumb line, when hung and then re-hung from the “pin” from which it was suspended, Maskelyne found, “would apply itself to a different part of the limb of the sector, commonly by 10, and frequently by 20 seconds” (Maskelyne Reference Maskelyne1764, 350). This was a fault so large that the annual parallax of Sirius could not be observed, and Maskelyne was unable to fix this problem on the island, “for want of workmen or tools.” He concluded that, although a failure, he had at least highlighted this instrumental error that had not been remarked upon previously:

The implication for any previous work was clear: the irregular suspension of the plumb line was “a fault, to which most of the sectors, made before mine, have probably been liable” (ibid., 351). This included the zenith observations taken of Sirius at the Cape of Good Hope by the Abbé de la Caille in 1751 and 1752, which were the stated inspiration for Maskelyne's initial proposal to attempt to determine the star's parallax in a 1760 presentation to the Royal Society (Maskelyne Reference Maskelyne1760). It also included all previous geodetic work to determine the length of degrees of latitude, such as that undertaken by Maupertuis in Lapland in the 1730s with the sector manufactured by George Graham in order to resolve the issue of the shape of the earth (Maskelyne Reference Maskelyne1764, 352). This instrument, then, which had been central to several international cosmographical schemes in the preceding decades, could not be fully trusted.

The instrument maker John Bird devised a new method for suspending plumb lines of zenith sectors almost immediately after Maskelyne demonstrated the fault at the British Museum. This new method of suspension, with the plumb line hanging from a moveable v-shaped notch, rather than from a pin, was incorporated into the new 6-foot zenith sector “contrived” by Bird for Charles Mason and Jeremiah Dixon to measure the boundary between Pennsylvania and Maryland (ibid., 351). On 28 March 1765, Maskelyne, newly appointed as Astronomer Royal, wrote to the Royal Society with a detailed “account of the repairs which appear to me to be necessary to be made to the instruments at the Royal Observatory.” Maskelyne informed the Society in its capacity as Visitors that he had consulted with Bird, “the artist who made the principal instruments,” regarding what the Observatory required. Both the astronomer and the artist agreed that many instruments needed to be mended, altered or replaced, in particular the Observatory's 12 ½ foot sector:

This sector, distinct from the 10-foot one found faulty at St. Helena, had been in use at the Greenwich Observatory since 1749, having been originally made for James Bradley by George Graham in 1727 (Howse Reference Howse1975, 60–64). The suggested alterations to the 12 ½ foot zenith sector's plumb line were eventually made by Bird in June 1768 (Maskelyne Reference Maskelyne1776, “Observed distances of the fixed stars and the planets from the zenith,” 38, June 12th 1768). At this point in the summer of 1768, Maskelyne was preparing a paper which he delivered to the Royal Society the following November, introducing and interpreting Mason and Dixon's “Observations for determining the length of a degree of latitude in the provinces of Maryland and Pennsylvania, in North America,” observations which were made with the previously mentioned altered 6-foot zenith sector.Footnote ⁹ Their work on measuring a degree of latitude was formally presented to the Royal Society on 24 November 1768 (Mason and Dixon Reference Mason and Dixon1768), prefaced by an introduction by Maskelyne in which he praised their sector:

Following Bird's alterations to the 12 ½ foot sector, Maskelyne used it for settling collimation errors of the two mural quadrants, a technique he described in 1776 in the first printed Greenwich Observations:

The mural quadrants, crucial to the observational regime at Greenwich dedicated to “the rectifying the tables of the motion of the heavens, and the places of the fixed stars,” could not be dismounted and reversed to check for collimation errors because of their size and weight. Hence the more portable zenith sector was moved around the mural quadrants in order to check and correct their accuracy. This technique was used by Maskelyne at Greenwich throughout his tenure as Astronomer Royal.

To summarize, during the period from 1761 to 1768, Maskelyne on an island in the South Atlantic highlighted a previously unnoticed fault with the suspension of a plumb line on the 10-foot zenith sector, and proposed with the instrument maker John Bird in London a method to fix this serious fault which had implications for the value of a large number of historical astronomical and geodetical projects. In the American colonies the new 6-foot sector was successfully trialled, and back at Greenwich, the American successes were used in order to adjust the existing zenith 12 ½ foot sector, itself critical for the correction of errors in the other Observatory instruments, and hence critical for the Greenwich Observations in general. This shows how important the zenith sector and its correctly functioning plumb line was for the Greenwich Observatory, and furthermore how personally involved Maskelyne was in its correct functioning.

4. Preparations for Scotland

Maskelyne reassured the Royal Society in 1768 that because the American plain that Mason and Dixon had surveyed was “as level as if it had been laid out by art, . . . there is no room for suspicion that the plumb line of the sector could be deflected materially from its proper position by the attraction of any mountain” (Maskelyne Reference Maskelyne1768a, 271, 273). The results, collated with previous degree measurements in Peru, Lapland, and elsewhere proved inconclusive, however, and Maskelyne eventually conceded that, as had been pointed out to him by Henry Cavendish, even in this flat terrain, the attraction of distant mountains, and the “anti-attraction” of valleys and oceans, may have had an influence. For all the labor, expense, and danger that it took to measure a degree of latitude, cutting through the thick American forest “the straightest and most regular, as well as extensive vistas that, perhaps, ever were made” (ibid., 270), Mason and Dixon's degree of latitude was in error by as much as 640 English feet. Not only that, but other degrees measured in other parts of the world were incorrect too:

So, just as the problem of the suspension of the plumb line of sectors was shown to have been solved, a new “palpable” problem with the plumb line was revealed, the effects of the attraction of mountains, an old problem, of course, but one which Maskelyne had, until Henry Cavendish pointed out the error, thought would be avoided in a “level” terrain.

At a Council of the Royal Society in May 1771, Maskelyne proposed a new experiment “to be tried with respect of the attraction of hills.” “Some alterations would be necessary in the sector of the Society” for this experiment: the 10-foot sector, found faulty at St. Helena, needed to be altered in a manner similar to the other sectors so that its plumb line could be suspended with sufficient exactness such that a deflection from the vertical could be measured.Footnote ¹⁰ The following July, in response to a paper by Maskelyne, a “Committee on Attractions” was formed, “for considering some experiments which are proposed to be made for measuring the attraction of hills and mountains.”Footnote ¹¹ Several potential mountains in the north of England were proposed by Maskelyne. Based on rough estimations, he believed that these hills would have an attraction that would cause a deflection from the vertical of between four seconds and twenty seconds of arc. These quantities were “not too small to be measured and demonstrated by an accurate zenith sector, such as that belonging to the Royal Society . . . if the fault in the suspension of the plumb-line, which I there [St. Helena] discovered, was corrected,” he emphasized (Maskelyne Reference Maskelyne1775a, 499). Hence Maskelyne made it explicit that attraction could only be measured with a sector with the new plumb-line suspension. The zenith sector, no longer faulty thanks to the endeavors of Maskelyne and Bird, was now capable of measuring deflections of an order of seconds of arc. This would be demonstrated on a mountainside, where the extreme gravitational conditions would reveal how “exact” the instrument had become.

Charles Mason was employed in June 1773 to undertake a tour in order to further investigate suitable mountains, and a payment was made to the instrument maker Jeremiah Sisson to make improvements as directed by Maskelyne.Footnote ¹² The following January, the Committee on Attractions presented its findings, noting that “a mountain in Perthshire called Schehalian or Maiden Pap seems for its situation size and figure to be a proper place for the intended experiments.”Footnote ¹³ Mason was offered the job of undertaking the experiments, but he refused. Reuben Burrow, himself, like Mason, a former assistant at the Greenwich Observatory, was therefore appointed with the task of observing zenith distances on the Scottish mountainside. Burrow did not appear to be quite so well trusted as Mason, however, for on 5 May 1774 it was determined that Burrow would be demoted to assistant:

Maskelyne immediately gave his “ready compliance” to the request, and refused any payment. He later noted firstly that his presence was essential “to ensure the success of so important and delicate an experiment” and secondly that it was essential for him to “have his Majesty's permission for absenting myself so long from my duty at the Royal Observatory” (Maskelyne Reference Maskelyne1775b, 504–505).

5. Public Demonstrations

Pringle explained well the technique employed to determine the attraction of the mountain:

Schiehallion, a “remarkable hill,” was chosen because it was of “sufficient height, tolerably detached . . . and considerably larger from East to West than from North to South.” It was a steep and narrow mountain aligned roughly East to West, so that its mass would cause a maximum deflection of the plumb line in the meridian (Maskelyne Reference Maskelyne1775b, 502–503).

Maskelyne assembled a collection of well traveled instruments. In addition to having its plumb line suspension modified, a new wooden stand was provided for the 10-foot sector that had been to St. Helena. For timing observations, he employed the regulator clock by John Shelton that had traveled to St. Helena with Maskelyne, America with Mason and Dixon, and Tahiti with James Cook (in 1769, to observe the transit of Venus).Footnote ¹⁵ For the triangulation between the two stations on either side of the mountain, Maskelyne used an “excellent” 9-inch theodolite by Jesse Ramsden in order to discover the “true difference of their latitudes.” For measuring the baselines that would serve as the origins of the triangulation he had “two fir poles of 20 feet each,” and for “adjusting them” he had a “brass standard of five feet.” This was the same standard made by Bird that Mason and Dixon had used in America (Maskelyne Reference Maskelyne1775a, 500–501).

Maskelyne arrived on the south side of the mountain on 30 June 1774 to find a canvas and wood observatory, instruments, and Reuben Burrow waiting for him. The sector was housed in a “large, parallelopiped tent, 15 ½ feet square and 17 feet high” (Maskelyne Reference Maskelyne1775b, 501); a “bothie, or temporary hut” was put up nearby for Maskelyne's “residence” (ibid., 508). He spent the next two weeks waiting for weather favorable enough for making observations to set the sector in the meridian. He spent the last ten days of July observing 34 different stars with the sector facing east, making 76 observations in total. He then reversed the sector so that it faced west, and spent the first two weeks of August observing 39 different stars, making 93 individual observations (ibid., 508–509). The boggy mountainside was infirm and unstable: he described how “on account of the gradual changes of the wooden frame” he “hardly ever observed [with the sector] without examining the bisection of the point at the centre of the instrument, by the plumbline.” He mentioned “these minute circumstances . . . to caution future observers”: “Whoever makes use of an instrument of this kind, supported on a wooden frame, will find the greatest attention necessary to attain the same degree of accuracy in his observations, as if his instrument were fixed to an immoveable wall” (ibid., 511). Hence Maskelyne was claiming that with “the greatest attention” he could make the unstable mountainside as stable as an observatory with its “immoveable walls.” He was showing how “exact” his zenith sector was: it could travel to a boggy Scottish mountain and still be set up and relied upon as if it were secured to a wall at Greenwich. Furthermore, if it could be used to detect tiny deflections from the vertical in such exacting circumstances, then it could certainly be relied upon in its use at Greenwich: the mountain trial helped show the value of the use of the sector back at Greenwich.

Due to bad weather, the observations at the south station had already taken longer than anticipated, and Maskelyne was required to petition the Council of the Royal Society as Visitors to the Greenwich Observatory for extended leave to continue his observations.Footnote ¹⁶ Having finished his sector observations at the south station on 15 August 1774, he “prepared . . . for removing the observatory and instruments” to the other side of the mountain. Transporting the instruments over the ridge of the mountain was “a work of great labour and difficulty, as everything was carried over the ridges of the hill on men's shoulders, and some of the packages were very weighty; it employed the labour of twelve men for a week, and was compleated on the 26th” (ibid., 516). Having set up the observatory and instruments at the north station, Maskelyne observed 32 separate stars, making a total of 68 observations, with the sector turned to the west, and then observed 37 stars, making a total of 100 observations with the sector turned east, between 4 September and 24 October (ibid. 517–518).

The apparent latitudinal separation of the north and south stations via the observed zenith distances of stars was calculated by Maskelyne to be 54.6 seconds (Maskelyne Reference Maskelyne1775b, 529). This was based on the observations at both stations of ten selected stars, with reductions made for precession, aberration, and nutation. The latitudinal separation according to the trigonometrical survey by Burrow using a theodolite, which will not be described here in any detail, was calculated to be 42.94 seconds. Hence Maskelyne concluded that “the difference 11”,6 is to be attributed to the sum of the two contrary attractions on the hill” (ibid., 531): the attraction of Schiehallion had been quantified, using an instrument that only very recently was capable of such precision.

Maskelyne gave two distinct reasons for the large number of observations he made at both stations on the mountainside. Firstly he explained how he had observed at each station with the sector in the meridian facing west, and then reversed to face east. The “doubling” of the number of observations caused by this reversing of the sector “may be thought by many to have multiplied observations unnecessarily,” but it was necessary in order to “obviate all danger of any alteration” in the line of collimation of the sector “in its removal from one side of the hill to another.” Observing east and then west would remove any collimation errors; Maskelyne believed that “the sector was carried from one station to the other without incident,” but thought it nevertheless “proper to guard against” any possible damage or alteration. Secondly, multiplying observations was necessary because “it might be expected, that some unsteadiness or warping of the wooden stand . . . might affect the accuracy of the observations” (ibid., 519). It is clear that it was the instrument that was on trial on the mountainside from a further remark on the multiplication of observations: “It was important to demonstrate the exactness of the instrument from the near agreement of a number of observations taken with it, as its excellence was not to be entirely presumed” (ibid., 519, emphasis added).

It was suggested earlier that few needed to be convinced of the truth of Newtonian gravity in 1774. It is probable that few doubted the truth of the attraction of mountains either, but Maskelyne was the first to quantify it with some certainty, and in the process produced useful information as to the structure and density of the earth. It was, however, the expected result: the attraction was expected, and it was the zenith sector's ability to measure to an accuracy of seconds of arc that was on trial, rather than the theory of attractions. The instrument, after modifications to its plumb line based on Maskelyne's travels, was only very recently capable of such precision. The mountain, due to its extreme gravitational conditions, allowed the sector to be trialled. The maneuvering of the sector over the top of a boggy mountainside showed how it could be managed and used: if it could detect something so tiny as the attraction of mountains in such a terrain, then it was certainly capable of precision use in the Greenwich Observatory, where, as we have seen, it played an important role in correcting the observations made with the large mural quadrants. All of this required an audience: Schiehallion was a philosophical tourist site.

Schiehallion is situated in the very center of Scotland. Kinloch Rannoch, Maskelyne and Burrow's victualling station, was a settlement greatly expanded in the previous two decades as a result of the planned village movement, which, amongst other things, encouraged settlement by disbanded servicemen (Turnock Reference Turnock1982, 82–96). Bridges and roads, begun by General Wade in the 1720s in response to the Jacobite uprisings, and continued by his successor, Caulfield, helped, in tandem with the building of new forts, to “ring the Highlands.” The overland journey taken by Burrow, and a few weeks later by Maskelyne, from Perth to Schiehallion, followed directly a major military road, including the celebrated Tay Bridge at Aberfeldy built by Wade, which cut straight through the moor (Tabraham and Grove Reference Tabraham and Grove1995, 74). This was a controlled and accessible landscape, and a well surveyed landscape: William Roy's military survey of Scotland made in the late 1740s and 1750s lent a cartographical framework in which to situate the Schiehallion operations. Whilst it was, in Roy's own words, “rather to be considered as a magnificent military sketch, than a very accurate map of a country” (Roy Reference Roy1785, 386–387), and remained unpublished, as a resident in London since 1763 and a Fellow of the Royal Society since 1767, Roy's knowledge of the Highlands was utilized by the Committee of Attractions in their choosing Schiehallion. On the journey north in June 1774, Roy sailed with Maskelyne as far as Edinburgh, and was one of the many esteemed guests who made a pilgrimage to the hillside camp (Maskelyne Reference Maskelyne1775b, 525).

The site, then, was remote, but it was accessible. Maskelyne had a learned audience to witness his spectacular demonstration of mastery of instruments and conditions. He thanked a long list of dignitaries who visited him on the mountainside:

Maskelyne thus spent the summer of 1774 on the mountainside showing this long list of visitors how “exact” his zenith sector was. Maskelyne was demonstrating to political and scientific dignitaries how even on a boggy mountainside, an instrument critical to observations at the Greenwich Observatory could be mastered. The extreme nature of the gravitational conditions on the mountainside allowed for a trial of the precision of the recently improved instrument. The difficulty of the terrain, and its mastery, reinforced the lesson: if the sector could be “exact” here, it could be “exact” everywhere.

William Roy's involvement was more than merely suggesting the site. Roy had from 1771 been traversing Britain in order to make measurements of heights using barometers, a technique that Maskelyne himself had lectured the Royal Society on in January 1774 (Roy Reference Roy1777; Maskelyne Reference Maskelyne1774). Maskelyne's paper provided a method of “reducing” “to the English measure of length” De Luc's rules for measuring heights with barometers. Maskelyne's conversion utilized an earlier examination of the relation of French measures of length to English ones undertaken by John Bird, where Bird compared a brass “standard” he had manufactured with a standard provided from Lalande, the Parisian editor of the French almanac, Connaissance des Temps. Lalande's standard was adjusted to the standard used by La Condamine and Bouguer in Peru; Bird's standard had been earlier manufactured for Mason and Dixon to use in their determination of a degree of latitude, and shipped to them under the directions of Maskelyne in December 1765 (Maskleyne Reference Maskelyne1768b). This same brass standard was used in the survey of Schiehallion, along with, amongst other instruments, “2 De Luc's barometers, with staff and bagg.”Footnote ¹⁷ Using such a barometer, Roy measured the height of the south “station” on Schiehallion on 11 July 1774, and, “furnished” with Maskelyne and Burrow's data, estimated the height of the mountain itself (Roy Reference Roy1777, 725, 721–722). “I took my leave of Maskelyne on the 15th,” he wrote to a correspondent, probably James Lind, on 26th July, and further noted “I am glad to hear that the Sector is at last adjusted” (quoted in Close [1926] Reference Close and Harley1969, 7).

Following the experiments on Schiehallion, the rough estimation of the relative densities of the earth and its surface was calculated with greater precision by Charles Hutton, combining Maskelyne's astronomical data with data taken from a detailed survey of the shape of Schiehallion undertaken by Maskelyne's assistant Reuben Burrow, who measured hundreds of vertical and horizontal angles between “stations” marked by wooden posts erected all over the mountain's surface. Hutton used this to model thin geometrical sections of mountain, “forming an imaginary group of vertical columns” or “pillars,” all gravitating. Informed by geological evidence, Hutton then calculated the shape and density of the mountain. In order to ease his calculations and their representation, he furthermore utilized a method of “connecting together by a faint line all the points which were of the same relative altitude,” helping to popularize the cartographic device now known as a contour line (Hutton Reference Hutton1778, 757). Hutton's complex calculations led him to conclude firstly that the mean density of the earth exceeded “that of the hill” by a ratio of 9 to 5, and secondly that “the mean density of the whole earth is about 4 ½ times the density of water” (ibid., 782–783).

Astronomy, hypsometry, surveying, geology and theories of attraction and the internal structure of the earth, then, were all connected, in a motley of terrestrial and celestial practices that Eric Forbes has labeled “mathematical cosmography” (Forbes Reference Forbes, Rousseau and Porter1980). The Greenwich Observatory had been tasked since 1675 with making observations for “the rectifying the tables of the motion of the heavens, and the places of the fixed stars, so as to find out the so much-desired longitude of places for the perfecting of the art of navigation” (Forbes Reference Forbes1975, 19). The shape of the earth and the question of the attraction of mountains were all critical to this navigation-inspired project of making detailed observations at Greenwich of the moon and stars to aid the determination of longitude at sea. Therefore considering the Astronomer Royal in 1774 as a cosmographer (and not just an astronomer) additionally helps to explain why Maskelyne, the Greenwich-based Astronomer Royal, spent four months away from his Observatory measuring attractions in Scotland.

However, this article has argued that a crucial reason for Maskelyne going to Scotland was to demonstrate the exactness of his sector. Why is this not emphasized in other accounts? John Pringle in 1775 claimed that Maskelyne's principle achievement was to end any uncertainty regarding Newtonian attraction, declaring that “all doubts about an universal attraction must at last be terminated, and every philosopher in that respect must now become a Newtonian” (Pringle Reference Pringle1775, 28). Edwin Danson presents a different view, where most savants are already convinced by the reality of attraction by the mid 1770s, but Maskelyne is the first to quantify it. For Danson, the Schiehallion experiment is but one point in a long series of complex investigations of the shape and structure of the earth, investigations which had great practical utility (Danson Reference Danson2006). This is an accurate picture, with the exception that it treats the “zenith sector” as an unproblematic entity. By following the biography of an instrument, however, we have seen a different story. There were less than half a dozen zenith sectors existing in the world in 1774 (Howse Reference Howse1986). Hence this was not a widely available instrument, but a very rare set of apparatus. Its value was not yet established; the Scottish mountain was used to trial the instrument, or to establish the value of the instrument. Once the value of an instrument has been established, it is referred to (by historical actors and historians alike) as if it had always existed, and always was a robust piece of equipment. Hence we learn, typically, that Maskelyne determined the attraction of mountains using “a zenith sector,” implying that there was such a natural kind before 1774. This deflects our attention away from the messy and laborious business of disciplining materials and making instruments work. Historians of instruments need to recover the pre-history of instruments: what an instrument looked like when “its excellence was not entirely to be presumed” (Maskelyne Reference Maskelyne1775b, 519), and what strategies were available in order to make it excellent and exact. In doing so, a better understanding can be achieved of how instruments, theory, and practice operated.