Edmond Halley and the Magnetic Field of the Earth

Edmond Halley and the Magnetic Field of the EarthAuthor(s): Alan CookSource: Notes and Records of the Royal Society of London, Vol. 55, No. 3 (Sep., 2001), pp. 473-490Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/531953 .

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Notes Rec. R. Soc. Lond. 55 (3), 473-90 (2001)

EDMOND HALLEY AND THE MAGNETIC FIELD OF THE EARTH*

by

SIR ALAN COOK, F.R.S.

Selwyn College, Cambridge CB3 9DQ, UK ([email protected])

SUMMARY

Edmond Halley was interested in magnetism from his schooldays to the end of his life, and of all his notable contributions to natural knowledge, his survey of the magnetic field over the Atlantic Ocean surely stands out both for its achievement at the time and for its lasting value. His identification of the westerly drift and his speculations about its origin, his description of aurorae and his connection of them with the

geomagnetic field, all demonstrate his physical insight, the grounds for which have

only quite recently become apparent. How profound were Halley's studies of

geomagnetism has become much more evident in the 57 years that have passed since

Sidney Chapman's Halley lecture on the same topic; the advances in geophysics that have taken place in the intervening years are my justification for returning to the

subject.

INTRODUCTION

It is a particular honour to deliver this lecture in commemoration of a most

distinguished member of the University and of the Queen's College within it, a natural philosopher who was second only to Newton in his day. Fifty seven years ago, in wartime, Sidney Chapman gave his Halley lecture on the same subject, and Edward Bullard took up the same topic in 1950'; I do not apologize for returning to it. We have learned much since those lectures about Halley's activities; in particular, the Hakluyt Society has published the logs, edited by Norman Thrower, of his adventurous Atlantic cruises.2 The study of geomagnetism has changed profoundly. Space research is applied to problems that Halley studied. Only since Chapman's lecture have the

physical principles behind some of Halley's most penetrating ideas been established. Those are my grounds for going back to Sidney Chapman's subject.

Halley made remarkable advances in the study of the Earth's field. To see how remarkable they were we should look at what was known when, as a schoolboy, he

began to take an interest in magnetism; then we should look at how much or how little

changed until the years after Chapman's lecture; that is my plan.

*The 91st Edmond Halley lecture before the University of Oxford, 16 May 2000.

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) 2001 The Royal Society



Alan Cook

EDMOND HALLEY

Edmond Halley was a younger contemporary of Isaac Newton, born in 1656, the son of a wealthy London citizen. He went to St Paul's School in London just before the Great Fire destroyed it in 1666 and he returned for about two years after it was rebuilt. He came up to Oxford, to the Queen's College, but left in 1676 without taking a degree, so that he might spend a year on the island of St Helena to determine the positions of stars invisible from northern observatories, and to observe a transit of Mercury across the Sun. He took with him telescopic sights with a micrometer eyepiece, and had a good clock. Those were the most up-to-date instruments, only just available, and his catalogue of the southern stars was the first made with the new instruments, not just of the southern hemisphere but anywhere. He observed the transit successfully. He was only 20. It was a great achievement. The Royal Society elected him a Fellow on his return; he remained active in it until his death 64 years later. Oxford University conferred the M.A degree on him by Royal mandate.

Halley visited the venerable astronomer Johannes Hevelius in Danzig in 1679. In 1681 he went to Paris with Robert Nelson, a schoolfriend and later also F.R.S., and there with G.D. Cassini he observed the great comet that had first appeared in November 1680. He went on to Rome with Nelson, returned to England in January 1682, and married the daughter of a prominent London lawyer. His father was found dead, probably murdered, in 1684. He became Clerk of The Royal Society in 1686.

Halley prompted Newton to start writing the Philosophiae Naturalis Principia Mathematica, he saw it through the press and published it at his own risk. He calculated the orbits of some 23 comets and predicted that 'his' comet of 1682 would return after about 76 years, as it did before and has done ever since. He was a practical seaman, often at sea in the 20 or so years up to 1703. He surveyed the Thames approaches and waters around the south-east of England, and was a pioneer of diving for salvage. At the end of 1698 he set sail as a captain in the Royal Navy to observe the magnetic declination over the Atlantic Ocean. Two Atlantic cruises and a survey of the tides in the English Channel brought him to 1701, and then in 1703, early in the War of Spanish Succession, he surveyed and fortified harbours for the English Government on the Imperial coast near Trieste. In 1704, he was elected the Savilian Professor of Geometry. In Oxford he edited the Conics of Apollonius of Perga, an edition that has not since been surpassed. He became the second Astronomer Royal in 1720, in succession to John Flamsteed, and observed the Moon assiduously for more than 18 years. He died aged 86 in 1742, three years after his wife.3

That is a very brief life of Halley, who notably advanced natural knowledge in very many ways, known in his own day as a distinguished mathematician, a great astronomer and respected as a servant of the State. He was renowned throughout Europe, and seen then and since as second only to Newton. He was very different from Newton. Newton, the country boy, never left England, Halley, a Londoner, travelled widely in Europe and sailed around the Atlantic. Newton was solitary, Halley sociable. Halley was a good classical scholar, and Leibniz admired the Latin ode with which he prefaced Newton's Principia. Halley was no mathematician of the originality and intensity of Newton, but

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his interests were very wide and he had a gift for making sense from large collections of data. He had a deep interest in navigation and was intensely preoccupied with its problems in the days before there were chronometers that would keep time at sea. His astronomical tables, published posthumously, were long used throughout Europe. He studied the motion of the Moon in the hope that it might be used as a clock to find longitude at sea, and he studied the magnetic field in the hope that it too might be a guide to mariners.

THE EARTH AS A MAGNET IN 1660

Halley began early as an astronomer. As a schoolboy, he knew the London astronomers Thomas Streete and Robert Hooke, and probably Sir Jonas Moore, the Surveyor General of the Ordnance who was Flamsteed's patron. He found errors in the tables of Jupiter and Saturn and discovered some 20 stars not in the existing catalogues. While an undergraduate, he was in the party with Wren and Hooke that chose the site of the new Royal Observatory at Greenwich. Later he assisted Flamsteed there on occasion. From his first observation in London as a schoolboy in 1672, he ever thereafter, as opportunity offered, observed the magnetic declination, and sometimes also the dip, wherever he might be-in Oxford as an undergraduate, during his year on St Helena, in Danzig, Paris and Rome, and in many other places. He noticed the magnetization of a vertical iron rod induced by the Earth's field. He experimented on the force between lodestones. His two cruises in the north and south Atlantic, with the chart of the declination that he derived from them, were his greatest contribution to the study of the Earth's field.

Much was known about the magnetic field of the Earth when Halley was at school. Mariners from Amalfi had used the magnetic compass for navigation about 1300, and about the same time a military engineer, Peter Peregrinus, studied the direction of the magnetic field around a spherical lodestone or terella; he realized that a magnet aligned itself more or less in the meridian. Early in the sixteenth century sailors found that the declination varied from place to place, and John Cabot, in 1522, suggested that it might be possible to find longitude from declination, or variation as it was then usually called. Values of variation were recorded on printed charts. In 1581 Robert Norman in London made the first dip needle.

Norman's measurement of dip enabled William Gilbert in the De Magnete of 1600 to show that the magnetic field of the Earth was like that of a magnetized sphere, although he knew that there were deviations from a simple model.4 Guillaume de Nautonier in France had rather similar ideas and, like Gilbert, studied the attraction of tiny compass needles at the surface of a spherical lodestone. In his book Mecometrie de Leymand of 1602-4, de Nautonier independently proposed that the Earth was a great magnet, but with the poles displaced from the geographic poles, and he drew a world map, constructed purely geometrically, with magnetic poles, magnetic meridians and the magnetic equator, all in addition to the geographic coordinates.5 Gilbert, on the other hand, placed the magnetic poles at the geographic poles and attributed variations to, for example, the attractions of continents.

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Gilbert was physician to Elizabeth I and died, as she did, in 1603. He was certainly concerned with the use of his geomagnetic model for navigation, but within a far wider concept of a magnetic cosmos in which magnetic phenomena were arguments for Copernican as against Ptolemaic cosmology. He considered that all magnetic bodies rotated (P.M.S. Blackett 350 years later suggested that all rotating bodies were magnetized) and that the magnetic field had a fixed orientation. He argued that the Earth, having a magnetic field, was rotating, and so aligned himself with those who thought that the Earth was rotating and moving around the Sun, as against Tycho Brahe, who thought that the Earth was fixed and the Sun, with the other planets, went round it. Both rejected the Ptolemaic cosmology but followed Copericus in different ways. Galileo thought highly of Gilbert, as did Kepler, who adopted his ideas and considered that the planets were moved round in their orbits by magnetic forces coupling them to the Sun.6

In about 1630, Henry Gellibrand, a professor of Gresham's College in London, found that the variation was changing over time. Many people, many with only dilettante interests like Charles II, observed it. Robert Hooke suggested that the magnetic poles were displaced from the magnetic poles and rotated around them. The forces between magnets were also being studied. The Roman polymath, the Jesuit Athanasius Kircher, was the author of two books on magnetism, Arte magnesia of 1631, and Magnes, sive de arte magnetica, opus tripartium, of 1643. In the first, he explained how the balance might be used to find the forces between magnets, an idea he attributed to Cardinal Nicola Cusano. He thought the forces might be used to produce a perpetual motion machine.7 Halley proposed experiments to discover how forces between magnets depended on their separation, but quite differently from Kircher, and he probably did not know of Kircher's suggestion. In Magnes, Kircher included extensive tables of variations at sites worldwide. Jesuits in particular had made many observations in the course of missionary journeys to the Far East and South America.8 Again, Halley may not have known of that collection. Although four of Kircher's books were among those sold on Halley's death, neither of his magnetic books was among them. Kircher died in Rome in 1680, about a year before Halley was there. Halley certainly knew that Kircher had worked on magnetism, but probably not in any detail.9

Halley did know of the views of Descartes,'° who knew that the magnetic poles were displaced from the geographic poles and that the variation changed from place to place and with time. He thought that it was due to the attraction of masses of iron at the base of mountains, and that it changed when the iron moved. His aim in the Principia Philosophiae was to show that all physical phenomena were the consequences of mechanical motions of corpuscles in contact, and he purported to show that the properties of magnets and of the magnetic field of the Earth did so follow.

When people realized that the variation changed from place to place and time to time, they found it more difficult to account for celestial phenomena by magnetic forces. Magnetism gradually dropped out of the argument between Copericus and Ptolemy. It was still, however, being invoked almost up to the time when Newton began to compose the Principia in 1684. Early in November of 1680, a comet

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appeared, moving towards the Sun. It became one of the most spectacular ever seen and was observed across Europe. Just before Christmas it reappeared moving away from the Sun, and Halley and Nelson saw its brilliant extensive tail as they travelled from Calais to Paris. Kepler's ideas of magnetic forces are evident in correspondence between Newton and Flamsteed about the comet, in which Halley took part. Was the comet that had approached the Sun the same as that moving away from it? Flamsteed thought it was, Newton thought not. In the end, he agreed they might be one and the same, provided the comet was always attracted by the Sun and went behind it, not in front. Flamsteed had suggested that the comet was first attracted, then repelled, by a magnetic field of the Sun, but Newton replied that the Sun, being hot, could not be magnetized." Just a few years later Newton discussed the motion of the comet in great detail in the Principia. He showed that it followed a parabolic path under the gravitational attraction of the Sun. Gravitation replaced magnetism in celestial mechanics, although Leibniz did continue to argue for it as an alternative. Studies of magnetism then concentrated upon the nature and effects of magnetized bodies and on empirical studies of the Earth's field and its possible use in navigation. That was the background against which Halley pursued his magnetic studies, and through his own work he reinforced the emphasis on geomagnetism. His studies were quite distinct from those of his predecessors and foreshadowed the modem world.

THE WESTERLY DRIFT

Halley and his contemporaries well knew that the Earth's field was close to, but yet deviated from, that of a uniformly magnetized sphere; that for instance, the variations at London and Paris had, over a lifetime, changed sign and gone through zero. Some understood that the major deviant features appeared to move westerly around the geographic poles. Along with observations made on land, mariners commonly observed the variation at sea, and people such as John Flamsteed, with interests in navigation, collected observations from sea captains. When in 1683, Halley examined the nature and origin of the westerly drift he had many observations to hand. He had collaborated with Flamsteed in the early years of the Royal Observatory and so knew of a substantial set that Peter Perkins had amassed. Perkins, a friend of Flamsteed, was the mathematical master at Christ's Hospital School in London, and a Fellow of The Royal Society. When Perkins died in 1680, Halley bought some of his papers from his widow, and so may have acquired part or all of his collection of magnetic observations. Perhaps he used some of them in his first paper on the westerly drift in 1683. Perkins had spoken to The Royal Society about representing the Earth's field by a set of six magnetic poles, and Flamsteed later accused Halley of stealing Perkins's ideas about the origin of the westerly drift. Although Perkins's remarks were entered in the Journal Book of the Society, he published no paper, and Halley's ideas were apparently not like his. Halley, however, was probably in Perkins's debt for a collection of observations, which can no longer be found.

Halley assembled representative observations of the variation from 1587 to 1680, covering the geographical range from New Zealand (170 °E) to Baffin Bay (80 °W).

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They came from mariner's reports, as well presumably as from Perkins, but Halley regretted that he did not have access to Dutch or Spanish observations in the Pacific. If he knew of Kircher's lists, which included material from the Pacific, he made no use of them and did not refer to them. He showed that the variation kept its sign over areas of continental extent and over many decades. He was able to define the westerly drift better than before and to show that it was not everywhere the same. He argued that because the anomalies were so extensive they could not arise from masses of iron near the surface, in the way that Descartes had suggested, but had much deeper sources. He did recognize that there were some anomalies of smaller extent that must arise from sources nearer the surface. He represented the Earth's field by two pairs of dipoles in relative motion, but pointed out that such a simple model could not show the detailed structure of the actual field. In fact, two dipoles are equivalent to a single one and so cannot represent the deviations from a dipole field-higher multipoles must be included, as Halley seems to have appreciated. Halley did know of Kircher's representation of the field as lines of action drawn from the poles, and dismissed it.12

Halley returned to the westerly drift in 1692 and refined his description.'3 He kept to his model of two pairs of poles, but placed one in an outer shell of the Earth and the other in a core (Figure 1). He supposed that the core and the shell had separate directions of magnetization and that they could rotate relative one to the other. He realized two things about the westerly drift. First, he saw, before the development of potential theory, that local deviations of the field must have sources closer to the surface than widespread ones. Second, he saw that moving deviations must arise from relative motions of parts of the Earth, and that prompted his suggestion that the Earth had what we now call a mantle and a core, although that was perhaps more of a guess.

Those are remarkable precursors of what we now see as principles ofgeomagnetism, although Halley lacked the knowledge of physics and of the interior of the Earth to put flesh upon them. We now realize that the mantle is not magnetized and that both the main field and the westerly drift are generated by motions in the conducting core. Halley foresaw a possible question about his ideas. Would there be people in the space between the core and the mantle? Although in many ways Halley seems almost of our own time, he was also of his, and in his time people seriously contemplated the possibility, indeed the likelihood, of other worlds in other places and at other times. Perhaps, there too, Halley is not so far from us, when the search for extraterrestrial life is seriously pursued.

Halley's perception of the westerly drift depended on an outstanding characteristic of his most significant studies-he collected observations of natural phenomena as widely as possible and presented them in ways that displayed their outstanding features. He used similar methods in his studies of the trade winds and later of aurorae, collecting material over a wide geographical range and making maps of them to reveal their structure. So he related the trade winds (incorrectly) to the heating effect of the Sun, and the aurorae (correctly) to the magnetic field. As with his other studies, he collected evidence of past occurrences, of the history of the phenomena. He had a keen sense that terrestrial and celestial phenomena changed and had histories.

Halley emphasized the value of pictures against words to bring out worldwide relations and structures (Figure 2). In his paper of 1686 on the trade winds, he wrote:

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Figure 1. Halley with a sketch of his shell model of the Earth. In the Royal Society (Michael Dahl).

Figure 2. Chart of trade winds (Phil. Trans. R. Soc. Lond.) 16, 153-168 (1686).

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To help the conception of the reader in a matter of so much difficulty, I believe it is necessary to adjoin a Schema showing at one view all the various Tracts and Courses ofthose Winds, whereby 'tis possible the thing may be better understood, than by any verbal description whatsoever.14

So Halley did very effectively in geomagnetism and even though he did not publish a map of the westerly drift, it is clear he had one in mind in his account of the drift. Visual representations have the further value that as they bring out the structure of phenomena, so they also stimulate the imagination to perceive their causes. Halley used images to great effect in his other geomagnetic studies.

THE ATLANTIC CRUISES

In 1693, The Royal Society strongly supported Halley and Benjamin Middleton when they proposed to the Government that they should undertake a voyage around the world to improve geographical knowledge, to observe the magnetic variation worldwide and to try out different ways of discovering the longitude. They had royal support, and a small ship, the pink Paramore, was built at Deptford (Figure 3). Halley received a commission as captain in the Royal Navy, but for various reasons, the visit of the Tsar Peter the Great and war with France among them, he could not set sail until October 1698. His voyages were then restricted to the Atlantic Ocean.

Figure 3. A pink in the distance setting sail. (reproduced by permission from a painting in the National Maritime Museum).

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The declination, or variation, is the angle between the horizontal direction of the field and the local meridian, as defined by the direction of the Sun at noon. Halley had an azimuth compass for that observation, but whenever possible he did not observe the Sun at noon but at sunrise and sunset as more convenient horizontal directions. It is relatively straightforward to obtain the latitude at the time of observation from the elevation of the Sun at noon, but finding the other coordinate, the longitude, was much more difficult 60 years before James Cook could rely on the Kendal watch for Greenwich time. Sometimes Halley was fortunate to observe a lunar eclipse, but they were infrequent. When on land, he observed the eclipses of the satellites of Jupiter, but had to use long telescopes which were inconvenient on land and impossible at sea. Thus many of his longitudes at sea depended on dead reckoning, and some were grossly in error because of the effects of unknown currents.

Many difficulties beset Halley on his first cruise. He could not leave the Thames until late in the season, and was further delayed by bad weather, so that by the time he had gone south to the equator and across to South America, it was too late to continue in the South Atlantic. So he returned north through the West Indies, to arrive back in England at the end of June 1699. He had difficult relations with his officers, so much so that his lieutenant, a correspondent of Flamsteed, faced a court martial on their return. Halley evidently kept the confidence of the Admiralty. He sailed on a second cruise on 16 September 1699, this time with good officers. After crossing as before to South America (Figure 4), they continued south until, beyond the Polar Front, they entered the ice fields of Antarctica in longitude 35 °W and about 50°S, some 200 nautical miles from South Georgia. They were just north of the Weddell Sea and of the track of Shackleton's epic boat voyage to South Georgia. They went further into the ice than anyone had gone before, or would go again until James Cook was in the same region almost 70 years later. In great danger from icebergs and fog, they turned north. Apart from a very severe storm off South Africa, their return to England was mostly uneventful. They arrived back early in September 1700. Halley was very proud that in his two voyages he had lost only one of his crew, a boy washed overboard. Less than two months later, at The Royal Society, he presented his chart of the variation displayed as isogonic lines over the Atlantic (Figure 5). For many years it hung in the rooms of the Society but has long since disappeared.15

Halley presented his results as a chart of lines of equal declination over the whole Atlantic, the first time such a chart of any variable had ever been printed and published. Kircher had written in Magnes of a manuscript chart of isogonic lines drawn up by the Jesuit Cristoforo Borri about 1630. Borri seems to have used it to propose a scheme for the determination of longitude that he presented to the king of Portugal, but it was rejected. A Portuguese correspondent told Kircher of it, but Kircher evidently had not seen it himself. The chart may have been of the waters around India and the East Indies. Kircher followed his account of Borri's work with clear instructions for drawing up a worldwide map based on his tables of declinations. Places at which the declination was zero should be laid down on a map and a line drawn through them. The same was to be done with places at which the declination was +2 °, at which it was -2 , and so on; he does not say that he ever did so.16

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The copy ofMagnes in the University Library at Cambridge once belonged to Henri Justell, the Parisian correspondent of The Royal Society when Halley was staying in Paris, and whom he met there. Halley may have heard from Justell of Kircher's magnetic studies, or people he met in Rome may have told him of them, but Kircher's books on magnetism were then 40 years old and perhaps unobtainable. Kircher's museum was so famous that Halley might have visited it, even though Kircher was dead. There is at present a project under way, associated with the Pontificia Universita Gregoriana, the successor of the Collegio Romano, to reconstitute Kircher's museum, and it is possible this could reveal something of visitors to the museum at the end of Kircher's life. Since there were no copies of Kircher's magnetic books among those sold on Halley's death, it may be that Halley did not know of Borri's chart or of Kircher's prescription.

Another visitor to Rome, whom Halley just missed, was Count Luigi Marsigli, later F.R.S., who had made magnetic observations in Hungary and may have represented them on a chart. There are two manuscript charts from before 1700 in which the depths

Figure 4. Halley's tracks of his second voyage (after Thrower, 1981 The three voyages of Edmond Halley in the Paramore, 1698-1701, London, The Hakluyt Society.

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of water in Dutch rivers are represented by isobaths, but Halley would not have known of them.17 Thus when Halley was preparing his Atlantic chart in 1700, a number of people seem to have had the idea of representing the geographic variation of some quantity by isarhythmic lines, but had not carried it beyond a few private manuscripts of which Halley was probably unaware. Halley claimed that his was the first chart with isogonic lines to have been made and published:

What is here properly New is the Curve-Lines drawn over several Seas, to show the Degrees of the Variation of the Magneticall Needle or Sea Compass: which are design'd according to what I myself found in the Western and Southern Oceans in a Voyage I purposely made at the Publick Charge in the Year of our Lord 1700.

(Legend on the Atlantic chart)

So it was, and there seems no reason to doubt that it was entirely original. It was the first chart of any sort with isarhythmic lines to be engraved, printed and published;

Figure 5. The Atlantic chart (reproduced by permission of the President and Council of the Royal Astronomical Society).

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with it Halley became one of the founders of modem cartography and his chart the model for all manner of maps and charts ever since. His introduction of isogonic lines was his most notable contribution to geomagnetism and more generally to the representation of geographic data. His lines were known as Halleyan lines, and were soon used for other kinds of data, such as isotherms and isobaths.

Halley's chart of the Atlantic was constructed on quite different principles from the prescription of Kircher. With two exceptions, Halley observed around the margins of the basins of the north and south Atlantic, for he only crossed the oceans from east to west near the equator where Africa and South America are closest together, from west to east in the south on the edge of the Antarctic ice fields and in the north from Newfoundland to England. He would have had a number of reasons for avoiding the open oceans: the safety of a small, lone ship, ease of obtaining provisions and the problem of longitude at sea. As a result, Halley had no observations over most of the basins of the north and south Atlantic and could not join up points where the declination had specified measured values as Kircher proposed. Halley interpolated his isogonic lines across the open oceans from his observations around the margins. In the south Atlantic, where the lines run more or less north and south, their forms are reasonably well controlled by the observations in the south and near the equator; but in the north, where the lines swing round from roughly north-south in the east to east-west in the north, the forms are by no means so well controlled by observations. Rather they seem to indicate that Halley was drawing the simplest forms that would fit the observations.

Halley followed up his Atlantic chart with others. In 1703, he told Leibniz, whom he met in Hannover, that he was still hoping to make a magnetic survey of the Pacific Ocean18; but then he was elected to the Savilian Professorship of Geometry. Nonetheless he drew up a world isogonic chart in which he used logs of ships trading overseas to supplement his Atlantic survey. He had to leave the Pacific empty, for although Spanish and Dutch sailors had made many observations, there he could not obtain them. About 200 years later W. van Bemmelen in the Dutch East Indies, having access to the logs of Dutch ships, was able to compile worldwide isogonic charts at regular epochs that covered the Pacific as well as the rest of the world. In general, his chart for 1700 is close to Halley's where they overlap, many of the discrepancies being due to errors in the longitudes of observations. It may well be that in the Archive of the Indies in Seville, there are logs of Spanish ships that would extend our knowledge of the magnetic field over the Pacific in the sixteenth and seventeenth centuries.

Many people tried to represent Halley's chart by some mathematical formula, something he himself never ventured. Emmanuel Swedenborg, the Swedish engineer and mystic, devised an elaborate formula from the 'fluxion of a perpetually spiral vortex' to calculate the variation at any place. Anders Celsius could not reproduce his results for Uppsala and, unimpressed, wrote to Swedenborg:

I believe that Dr Halley has more than any other applied the deepest thought in establishing a theory of the declination of the magnet which the learned have been seeking to improve ever since; yet he does not venture to determine by geometry the situation of the magnetic poles upon the earth and to establish rules for computing the declination. Meanwhile however he has empirically constructed crooked lines representing the declinations of the magnet on

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the largest ocean of the world, and he has had the good fortune to see those lines, which were constructed on the basis of the observations taken during his voyages confirmed more and more by later experiments.'9

That contemporary appreciation of Halley's achievement stands after 250 years. In later years, extensive observations of the declination, and eventually of the dip

and total force, came to be made worldwide, for example by General Edward Sabine, Sec.R.S., among others, but the methods were essentially Halley's. The main change in practice came about some 50 years after Halley's cruises, as the result of the development of the Kendal watch for finding time, and thus longitude, at sea. Positions were then far more reliable and could be found far from land. Special surveys were organized, for example Sabine, over many years, assembled observations for the southern polar regions, a study in the mould of Halley. The structure of the westerly drift became much better established, yet little changed for almost 250 years. Then, after the Second World War, and after Chapman's lecture, electronic magnetometers came into use and were towed behind ships or aircraft and ultimately from spacecraft. Now there are surveys supported by public funds not just of the Earth but of the Moon and many of the planets. Halley would surely have appreciated the great extension of his Atlantic cruises and that it is done at the public charge.

AURORAE

Halley's third major contribution to geomagnetism came when he observed an intense display of aurorae over London in 1716.20 He described the luminous cloud, shot with coloured rays reaching almost to the zenith, that lasted for some hours until 3am. He compared the rays to glories around the head of God in some religious paintings and to the Star of the Order of the Garter. He made a list of many displays of aurorae from early in the sixteenth century onwards (he credited the French philosopher and astronomer Pierre Gassendi with the name aurora borealis). He collected reports of the phenomena from places far from London and was able to plot the forms of the auroral arcs. He considered that they could not arise from volcanic eruptions because they were too widespread but he showed that they corresponded to the forms of the field lines of the Earth's magnetic field in the polar regions, as displayed by iron filings around a magnetized sphere (Figure 6). He found that the rays were most intense around the magnetic, not the geographic, pole. He argued that they were controlled by the field and that they were produced by matter circulating around the field lines, a remarkably prescient suggestion. With his model of a core and mantle of the Earth, he proposed that the matter leaked out of hollow spaces in the Earth, perhaps between the core and the mantle, and that the aurorae were most intense around the poles because that was where the mantle was thinnest. While that is not so, Halley's analysis of the structure of the aurorae and their relation to the magnetic field remains striking and valid after 300 years. Johann Bernoulli appreciated Halley's account of the aurorae, but commented that he had not explained the colours, a perceptive criticism as it would turn out, although neither Bernoulli nor Halley could know why at the time.

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Figure 6. Magnetic field lines of the Earth (Phil. Trans. R. Soc. Lond. 29, 406-428 (1716).

Halley was unable to make the important connection between aurorae and the Sun. In his day, nothing was known of the daily variation of the geomagnetic field, nor of magnetic storms, for there were no instruments to detect them. There was then no reason to relate magnetic phenomena on the Earth to events on the Sun, although sunspots had been seen for more than a century. Towards the end of Halley's life George Graham, F.R.S., the great clock and instrument maker, devised a magnetometer with which daily variations of the Earth's field could be detected. Celsius, in Uppsala, obtained a magnetometer from Graham and found that some of the variations he observed occurred at the time of auroral display. In April 1741, Graham, in London, and Hiorter, in Uppsala, both saw a magnetic storm coincident with the same auroral display.

By the middle of the nineteenth century many observatories, some of them on British colonial stations worldwide, were recording the regular daily variations and magnetic storms, and a lunar effect had been detected. At first, only the declination was observed, but by 1870 there were records of dip and total force as well. Many of

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the programmes at colonial stations such as Toronto or St Helena were organized by General Sabine. Instruments for some observatories in the British Isles, such as Kew and Stonyhurst, were provided from the Government Grant for Science, administered by The Royal Society. Stonyhurst was a Jesuit college and recalls the source of Kircher's data from Jesuit missionaries in distant lands. So in due course, connections between magnetic storms and events on the Sun were established.

Aurorae were related to solar flares about the same time and Arthur Schuster, F.R.S., and others developed the idea of a conducting layer above the neutral atmosphere of the Earth. The discovery of the ionosphere in the 1930s apparently confirmed those ideas, but not until the middle of the last century, after Chapman's lecture, was the physics developed that supported Halley's ideas about aurorae and the westerly drift. David Bates, F.R.S., in Belfast, and Nicolet, in Belgium, studied the physicochemical reactions in the upper atmosphere driven by solar radiation, reactions that produced the ionized conducting layers and that also led to visible recombination radiation. So Bernoulli's question was answered: the colours arise because the auroral rays are formed by radiation from ionized atoms and molecules, and the rays are linked to magnetic field lines because the ions, being electrically charged, spiral around the field lines. In the past 50 years, the study of how charged particles circulate in the magnetic fields of the Earth and other planets has been a major preoccupation of space research.

HALLEY'S LEGACY

Halley saw that the westerly drift, involving as it does large-scale features of the field, could have a deep origin. It remained an enigma until Edward Bullard, F.R.S. and his colleagues studied it in 1950 and linked it to currents near the surface of the liquid conducting iron core of the Earth.21 Loops of current are sources of local dipole fields, so that in a gross way Bullard's currents have the same effect as Halley's moving poles. The differences lie in the respective pictures of the interiors of the Earth. Halley placed one of his pairs of poles in the mantle, whereas we now consider the mantle to be unmagnetized. Halley supposed that the mantle rotated relative to the core, whereas today we place the motions within the conducting fluid core. Halley thought that the different parts of the Earth were permanently magnetized, whereas we consider that the sources of the main field lie in magnetohydrodynamic interactions within the electrically conducting core. The concept of the fluid core had to wait upon the findings of seismology and is less than a century old. Magnetohydrodynamics is still younger. So long did it take for the physics and hydrodynamics to be developed that would show, only well after Chapman's lecture, that there were indeed substantial grounds for Halley's model of the westerly drift.

There remain difficult questions about the origin of the main field and the westerly drift. Chapman, no more than Halley, suspected reversals of the main field, but as soon as they were identified in palaeomagnetic records it was clear that the geomagnetic dynamo was unstable. Recently, more refined data have shown that there are instabilities with relatively short timescales. Is it possible that the westerly drift of

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eddies, with timescales of the order of a century or so, on the larger motions of the core, may be consequences of instabilities in the main dynamo system? What happens to the westerly drift when the main field reverses?

Until the last half-century the methods of geomagnetic survey, especially at sea, were little different from those that Halley employed. Sailors on non-magnetic ships found their positions by the astronomical methods that James Cook had used, and observed the variation with instruments not very different from Halley's. Then after the Second World War, electronic fluxgate magnetometers that had been developed for military purposes were towed at a distance behind ships and aircraft, and used both for worldwide survey and local prospecting. Data were acquired electronically and were analysed in early electronic computers to generate models composed of a large number of spherical harmonic terms in place of charts. Those analyses, dependent on potential theory unknown to Halley, go back to Gauss, but only through electronic methods has it become practical to perform them in the great detail of today. Magnetometers that operate on the shifts of frequency of atomic transitions in a magnetic field are a more recent development. The first spacecraft carried magnetometers far above the Earth. The magnetosphere and the effects of the solar wind were identified. Spacecraft travelling close to other planets have explored their magnetic fields; radio emissions from Jupiter have already shown that it has a particularly strong field.

CONCLUSION

Planets, the Sun and stars, white dwarfs and pulsars all have magnetic fields; magnetic fields permeate the tenuous clouds of gas that emit maser radiation, and participate in the condensation of clouds of gas into stars. In a curious way, our present perception of a magnetized cosmos goes back to the century before Halley, when Gilbert and Kepler and Galileo and others were speculating about magnetism controlling the motions of the stars and the Sun and the planets. They had little or no basis in observations for their speculations, and after Newton had demonstrated that it was gravity, not magnetism, that held the solar system together, magnetism as a cosmic force fell out of fashion. Now it is back. We recognize that there are regions of space, such as those permeated by clouds of ionized gas, where magnetic fields dominate the dynamics.

Halley grew up and developed as an astronomer at a time when many people were observing the declination in many places. Experiments on magnetic forces were being devised in laboratories, and techniques such as the use of iron filings to delineate field lines, and methods of measuring forces between magnets, were coming into use. The ideas of Gilbert and Kepler about the cosmic influence of magnetism were still seriously accepted. Halley had many current ideas and empirical results to draw on in his studies of geomagnetism; in each of his three major contributions, he established entirely new lines of enquiry. Such were his scientific insight and imagination that ideas and methods of geomagnetism changed little over more than 200 years from his

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death to Chapman's lecture, and that his ideas of the westerly drift and aurorae are not superseded but rather are now understood.

In 1956, Edward Bullard wrote in Nature:

By reading Halley's two papers on the Earth's magnetic field one can learn more about the

origin of the field and its secular variation than will be found in all that was written in the succeeding 250 years.22

Bullard was referring then to the papers on the westerly drift, but his assessment holds even more forcefully today, and for all Halley's studies of the Earth as a great magnet.

Halley asked to be buried with his wife in the churchyard of St Margaret's Lee, just across Blackheath from the Royal Observatory. The gravestone that his daughters had carved with its memorial inscription is now in a wall of the Observatory. He does not lie with Newton in Westminster Abbey but a tablet to his memory was unveiled in the cloisters a few years ago. His greatest memorial is the observatory established by the Royal Scoiety at Halley Bay on Antarctica that links his name to the magnetic and auroral observations and to the cruise that went so close to the Antarctic continent. Well may those say who go there, 'Si monumentum requiris, circumspice'.

ACKNOWLEDGEMENTS

I am grateful to the University of Oxford for appointing me the 91st Halley lecturer, and to the Halley Professor, Professor F. Taylor, for his hospitality.

NOTES

1 S. Chapman, 'Edmond Halley and geomagnetism (Halley Lecture)', Nature 152, 231-147 (1943); Terr. Mag. 48, 131-144, 151 (1943). E.C. Bullard, 'The origin of the Earth's magnetic field (Halley Lecture)', The Observatory 70, 2 (1950). N.J.W. Thrower (ed.) The three voyages of Edmond Halley in the 'Paramour', 1698-1701, 2nd series, vol. 156, p. 157 (Hakluyt Society Publications, London, 1981).

3 A. Cook, Edmond Halley: charting the heavens and the seas (Oxford University Press, 1998).

4 W. Gilbert, De Magnete (London, 1600). 5 G. de Nautonier, Mecometrie de Leymand (1602-4). 6 S. Pumfrey, 'Magnetical philosophy and astronomy, 1600-1650'. In R. Taton and C. Wilson

(eds), The General History of Astronomy, vol. 2, pp. 45-53 (Cambridge University Press, 1989).

7 A. Kircher, Ars magnesia (Herbipoli, 1631). 8 A. Kircher, Athanasius, Magnes, sive de arte magnetica, opus tripartium (Rome, 1643). 9 Cook, op. cit., note 3, p. 484, note 62. 10 R. Descartes, Principia Philosophiae (Amsterdam, 1644). 11 See Flamsteed to Halley, 17 Feb 1680/1; Newton to Crompton for Flamsteed, 28 Feb.

1680/1; Flamsteed to Crompton for Newton, 7 March 1680/1. All in E.G. Forbes, L. Murdin and Frances Willmoth (eds), The correspondence of John Flamsteed, the first Astronomer Royal (loP Publishing, Bristol and Philadelphia, 1995).

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12 E. Halley, 'A theory of the variation of the magnetical compass', Phil. Trans. R. Soc. Lond.

13, 308-321 (1683). 13 E. Halley, 'An account of the cause of the change of the variation of the magnetick needle,

with an hypothesis of the structure of the internal parts of the Earth', Phil. Trans. R. Soc. Lond.

17,563-578 (1692). 14 E. Halley, 'An historical account of the trade winds and monsoons observable in the seas

between and near the tropicks, with an attempt to assign the cause of the said winds', Phil. Trans. R. Soc. Lond. 16, 153-168 (1686).

15 Cook, op. cit., note 3, p. 281. 16 Kircher, op. cit., note 8, pp. 502, 503. 17 Thrower, op. cit., note 1, p. 57. 18 Leibniz to J. Bernoulli, Hannover, 15 January 1704. In C.L. Gerhardt, Leibniz Mathematische

Schriften, vol. III/2 (Georg Olms, Hildesheim, 1962). 19 Celsius to Swedenborg, 23 June 1741. In R.I. Tafel, Documents concerning the life and

character of Emanuel Swedenborg, pp. 578-589 (The Swedenborg Society, London, 1875-77).

20 E. Halley, 'An account of the late surprising appearance of lights seen in the air, on the sixth of March last, with an attempt to explain the principal phenomena thereof, As it was laid before the Royal Society by Edmund Halley, J.V.D., Savilian Professor of Geometry, Oxon, and Reg. Soc. Secr', Phil. Trans. R. Soc. Lond. 29, 406-428 (1716).

21 E.C. Bullard, C. Freedman, H. Gellman and J. Nixon, 'The westward drift of the earth's

magnetic field', Phil. Trans. R. Soc. Lond. A 243, 563-578 (1950). 22 E.C. Bullard, 'Edmond Halley, the first geophysicist', Nature 175, 891-892 (1956).

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