Fig. 3 – William Whiston, by an unknown artist, c.1690
{The Master, Fellows and Scholars of Clare College, Cambridge}
Fig. 4 – The limits of viewing the flash from a mortar fired at Shooter’s Hill, near Greenwich, from William Whiston’s The Longitude Discovered (London, 1738) (detail)
{National Maritime Museum, Greenwich, London}
Fig. 5 – A terrella (or ‘little earth’), a spherical lodestone used to model the Earth’s magnetic field, c.1600
{National Maritime Museum, Greenwich, London}
Whiston and Ditton presented their scheme more fully in a pamphlet addressed to the newly appointed Commissioners of Longitude. They offered it as a practical idea that, without universally solving the problem, would make a material difference. It was ‘easy to be understood and practis’d by Ordinary seamen, without the Necessity of any puzzling Calculations in Astronomy’ but would ‘prevent the Loss of abundance of Ships and Lives of Men’. The signals could provide both latitude and longitude, might be used to give exceptional warnings of bad weather, and would have most success in the areas of greatest danger – that is, near coasts. Here they invoked the maritime tragedy that occurred off Scilly in 1707, claiming their scheme ‘would certainly have sav’d all Sir Cloudsly Shovel’s Fleet, had it been then put in Practice’.
To put the idea into effect, Whiston, who continued the project after Ditton’s untimely death in 1715, relied on the skills of London’s firework makers and gunners as he began trials on Hampstead Heath and Blackheath (Fig. 4). There was merit in the idea, and explosives were later occasionally used to measure distances in survey work, but there were serious practical problems. Not least was the difficulty of mooring vessels in deep water, despite a claim that anchors might be secured by reaching down to supposedly still layers of water far below the surface.
It was all too easy for the firework scheme to be ridiculed, particularly by Tory satirists, who connected it to Whiston’s fiery and suspect theology, and cast the whole concept of longitude rewards as a Whig folly. Nevertheless, projecting, publicizing and finding patronage for longitude schemes remained one of Whiston’s major activities and sources of income. He explored all the accepted avenues of research, including one not mentioned by Newton: the Earth’s magnetism.
Magnetic variation and inclination
The idea that patterns in the Earth’s magnetic field might be a means of fixing position at sea had a long history and continued to be investigated in the eighteenth century and even into the nineteenth. It is interesting that Newton did not mention it in his evidence to the parliamentary committee, especially since he was joined there by Edmond Halley, an experienced astronomer, mathematician and navigator who had investigated these phenomena himself. As with Whiston’s signals, this method was about finding position relative to known locations rather than finding longitude itself. Presumably Newton and Halley therefore considered it discounted as a universal solution.
Most of the incoming proposals to the Spanish and Dutch longitude reward schemes were based on patterns in the Earth’s magnetic field, and many would be put to the British Commissioners of Longitude. There were two patterns that were investigated, with the hope that they were regular enough to be mapped and used. One was magnetic variation (also known as magnetic declination), which is the angular difference between magnetic north, shown on the compass, and true north, determined by the Sun or stars. A positive variation shows that magnetic north is east of true north, a negative one that it is to the west. The other was magnetic inclination, or magnetic dip, which is measured by the compass needle’s vertical rather than horizontal deviation. This is caused by the needle aligning itself with the Earth’s curving lines of magnetic force.
These were phenomena that had long been observed and investigated: variation had to be understood by navigators to correct steering directions, if not for position finding. In trying to make sense of the patterns of observational data, natural philosophers attempted to describe the Earth as, or containing, a giant magnet. One of the most famous of these accounts was De Magnete, published by William Gilbert, a London physician, in 1600. He undertook much of his research with spherical lodestones: known as terrellae, meaning ‘little earths’, these magnetic rocks were used to model patterns of geomagnetism (Fig. 5).
Magnetic inclination was also explored as a means of finding latitude, which, given that the Earth’s lines of magnetic force run north–south, had some plausibility. However, experiments had shown that the variations were too irregular and the observations too difficult to make at sea. In any case, astronomical observations were becoming much more effective for determining latitude. Thus, while schemes relating to magnetic inclination for latitude or longitude did not disappear, and even occasionally recurred in navigational textbooks, it was magnetic variation that had more impact. It involved significantly easier on-board observations and had a more plausible theoretical underpinning. This too was challenged, however, when it was demonstrated that the patterns change over time as well as place.
Fig. 6 – An amplitude compass, used for measuring magnetic variation from the apparent bearing of the Sun’s rising or setting; made by Ferreira, Lisbon, 1780
{National Maritime Museum, Greenwich, London}
Fig. 7 – Edmond Halley, by Thomas Murray, c.1690
{The Royal Society}
Nevertheless, navigation by magnetic variation was actually achieved, albeit in restricted locations or on familiar routes. The necessary tools were an amplitude compass (Fig. 6), to measure variation from observations of the sun and a chart recording previously observed lines of equal variation, against which to plot the ship’s position. This could be effective in specific circumstances, where the charting was detailed, and the lines ran nearly north–south and were reasonably close together. Some Portuguese navigators, for example, and those on Dutch East Indiamen, put the method into practice at various times during the seventeenth and eighteenth centuries, many apparently satisfied with the results.
Research into magnetism was a serious interest at the Royal Society, with demonstrations by their curator of experiments, Robert Hooke (1635–1703), who developed his own magnetic theory. Between 1668 and 1716, annual predictions by Henry Bond, a teacher of mathematics and navigation, were published in the Society’s journal, with the aim of encouraging magnetic observations against which his theory might be tested. Bond’s claims were investigated by a Royal Commission in 1674 and, although there was some doubt, he was paid £50 and given licence to publish his book The Longitude Found. On the basis that the six Commissioners were all Fellows, the book claimed the Royal Society’s approval, to which its President, Viscount Brouncker, objected strongly.
The Society’s interest nevertheless continued and was instrumental in persuading the government and the Navy to fund and equip a scientific voyage that would, among other things, chart magnetic variation as widely as possible. Edmond Halley (Fig. 7) was, very unusually for a civilian, given command of a specially built