Mackenzie is also credited with the invention of the ‘station pointer’ – an invaluable instrument that enables the coastal navigator quickly to fix his position by taking horizontal sextant angles between three or more fixed points.
Not until the 1790s did the newly established Ordnance Survey follow Mackenzie’s example and begin mapping Britain by triangulation. In 1797 the intricate network of triangles was extended from Land’s End to the Scilly Isles and, to general consternation, it emerged that the position of the islands shown on contemporary charts was out by the astonishingly wide margin of 20 nautical miles.9 Cold comfort for poor Shovell.fn5
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For all the progress that was being made in mapping the land, accurate position-fixing at sea still remained an impossible dream in the early eighteenth century. In fact it was no closer to reality than it had been 150 years earlier when the Spanish, conscious of the vital commercial importance of their overseas colonies and the difficulties surrounding accurate and therefore safe navigation, began to seek a shipboard solution to the longitude problem. In 1567 King Philip II offered the first cash prize to anyone who could crack it, and in 1598 his successor, Philip III, raised the stakes: the winner would receive a one-off payment of 6,000 ducats together with an annual pension of 2,000 ducats. So important was the goal that this princely annuity was promised even to the heirs of the eventual winner.10 Other prizes were later announced by the Dutch and Venetian Republics, by France and, eventually, by Britain. Under the terms of the British Longitude Act of 1714, a sum of up to £20,000 was offered as ‘a due and sufficient Encouragement to any such Person or Persons as shall discover a proper Method of Finding the said Longitude’. This would now be worth several million pounds.
The Longitude Act, however, imposed high standards of accuracy: to win the maximum amount the successful method had to be capable of determining longitude within a margin of error not exceeding half a degree of a great circle (equivalent to 30 nautical miles). Half the maximum prize would be payable when the Commissioners of the new Board of Longitude were satisfied that the proposed method extended to ‘the Security of Ships within Eighty Geographical Miles from the Shores, which are Places of the greatest Danger’, while the balance would be paid ‘when a ship … shall actually Sail over the Ocean, from Great Britain to any such Port in the West-Indies, as those Commissioners … shall Choose or Nominate for the Experiment, without Losing their Longitude beyond the Limits before mentioned’. Moreover, the reward would be paid only ‘as soon as such method for the Discovery of the said Longitude shall have been Tried and found Practicable and Useful at Sea, within any of the degrees aforesaid’. The words ‘Practicable’ and ‘Useful’ were to give rise to bitter disputes. Lesser rewards were available for proposals that the Commissioners judged of ‘considerable Use to the Publick’.11
Though pendulum clocks coupled with the new ephemeris tables permitted land-based observers to determine their longitude accurately, nobody had yet managed to devise a time-keeper that could be relied on at sea. Existing spring-driven clocks and watches were hopelessly erratic, and despite valiant attempts it proved impossible to make pendulum clocks work reliably on board ship. Strenuous efforts were therefore made to find methods of determining longitude that did not rely on astronomical observations and which could therefore be employed without the need to know the time. Mapping the geographical variations in the direction of the earth’s magnetic field seemed to offer some hope, but in the end this line of enquiry proved abortive and the heavens became the exclusive focus of scientific attention among those seeking to solve the longitude problem. If sea-going clocks were not to be relied on, then perhaps the sailor could find the time from observations of the sun, moon and stars. The challenge was to identify a frequently occurring astronomical event the precise time of which could be both accurately predicted and easily observed on board ship, anywhere in the world. Published tables of the predicted times of such events would in principle enable the navigator to find the time at a given reference meridian (such as Greenwich or Paris) wherever he happened to be – providing the skies were clear. Comparison with the local time – derived from astronomical observations – would then reveal the observer’s longitude.
Various methods of achieving this goal had already been suggested. For example, in 1616 Galileo opened discussions with Spanish officials about the possibility of using observations of the appearance and disappearance of the moons of Jupiter (the four largest of which he had recently discovered) as a means of determining the time at the reference meridian. In return for a large fee for travelling to Spain to demonstrate his method to King Philip III, an annual royalty both for himself and his heirs, as well as appointment to the chivalric Order of Santiago, he proposed to draw up the necessary tables and update them annually; he even invented a telescopic device to be worn on the head that was supposed to permit making the necessary observations at sea.12 But the Spanish lost interest and in 1635 an ageing Galileo turned to the Dutch, this time with improved tables and a mechanical device for representing the motions of the Jovian satellites that he called the ‘Jovilab’.
The Dutch States General responded enthusiastically and even appointed an astronomer to act as a technical go-between, but Galileo – who was by now going blind – was unable to generate the orbital parameters of the moons on which sufficiently precise predictions could be based.13 In any case, a fairly powerful telescope was required to observe the moons of Jupiter and such an instrument could not be held steadily enough on board ship. And there was another problem lurking in the background: without an accurate shipboard time-keeper, how exactly was the navigator supposed to compare local time (derived most easily from sun sights) with the time obtained from the tiny Jovian moons – visible only after the sun had set? Galileo claimed he knew how to make a sufficiently accurate pendulum clock but he had not succeeded in doing so by the time he died, and anyway it would have been of no use at sea.14
Jupiter’s moons were, however, very useful to land-based observers equipped with pendulum clocks – once the necessary tables had been produced at the Paris Observatory. In the 1680s a French expedition established the longitude of the Cape Verde Islands, Guadeloupe and Martinique using this technique,15 and Picard and La Hire also employed it when making their map of France. Eclipses of the sun were among the other possibilities, but they were too infrequent to be of much use, and it was not until the invention of the sextant that it was possible to observe them with sufficient accuracy on board ship. Spanish navigators and astronomers had experimented with the technique in the sixteenth century, but the results, even at land-based observatories, were of no value.fn6
So it is not surprising that when the Longitude Act was passed in 1714, few observers expected that anyone would soon succeed in claiming the big money. Many bizarre and frankly ludicrous proposals were put forward, and in consequence the quest became something of a standing joke. William Hogarth included a cheerful lunatic searching for a solution to the longitude problem in the background of the scene from the madhouse in the Rake’s Progress of 1735.
Such scepticism was misplaced. After a struggle lasting hundreds of years, two radically different solutions to the problem of finding the time on board ship emerged almost simultaneously in the 1750s – one mechanical, the other astronomical. Both, however, relied on accurate angular measurements made with a quadrant or, better still, a sextant. As we shall see, one method was based on a new kind of clock, while the other depended on the first accurate tables of the motions of the moon. In practice, however, the two techniques were to be mutually dependent for many years to come.
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The extraordinary story of the development by John Harrison (1693–1776) of the first accurate shipboard time-keeper – and his long struggle for official recognition of his feat – is by now well known. In 1759, after more than thirty years of experimentation, he produced a highly innovative ‘watch’ (known as ‘H4’). It was not regulated by a pendulum and exploited the ingenious principles of compensation he had developed in earlier experimental devices; it was also a great deal smaller and more practical. H4 easily passed the second (and possibly also the first)16 of two rigorous