Halley’s Comet, as it is known today, appeared late in 1758, brightened and passed closest to the sun in March 1759. It has appeared on cue ever since. Once Halley realized that the comet’s orbit might be slightly modified by the gravitational pulls of the planets, he correlated it with sightings in 1066 (famous for its coincidence with the Battle of Hastings), 1145, 1301 and 1456.
Halley was still only twenty-seven, and despite his diversion into comet research had not lost interest in geomagnetism. He had, in fact, been busy searching for a rational way to explain declination and its secular variation—in his words “to reconcile the observations by some general rule.” He was not deterred by the pessimism of the prominent French mathematician René Descartes who, like Gilbert, apparently attributed declination to iron mines and lodestone deposits, randomly distributed about the Earth without any meaningful pattern.
Halley was looking for a solution with some degree of global symmetry, and “after a great many close thoughts” he came up with the idea of not two but four magnetic poles, two in the Arctic and two in Antarctica. He carefully estimated the locations of his poles. The European North Pole was “near the Meridian of the Lands end of England and not above seven degrees from the Pole Arctick.” The American North Pole was “in a meridian passing about the middle of California, and about fifteen degrees from the North Pole of the World.” The American South Pole was, he estimated, “about sixteen degrees” from “the South Pole of the World” and “some twenty degrees to the westward of Magellan’s Straights,” and the Asian South Pole “a little less than twenty degrees distant” from the south geographic pole and “in a Meridian which passes through Hollandia Nova and the Island Celebes.” These corresponded approximately to 83° N, 354° E; 75° N, 241° E; 74° S, 270° E; and 70° S, 120° E, respectively.
Halley’s reasoning was based on the assumption that a compass needle would be influenced most by the pole that lay closest to it; for example, the declination in Europe and the North Sea area would be dominated by the attraction of the European pole. He also supposed that secular variation was the result of the gradual drift of the poles. This drift, he deduced, affected all four poles to varying degrees, but was generally westwards.
However, Halley was not going to overstate his case. He concluded his 1682 paper by listing three obstacles to the further development of this concept. First, he said, although in the “frigid” polar regions it was easy to see which pole was closest and therefore dominated the declination, this was not the case in the “torrid” latitudes nearer the equator. Here, declination was likely to result from the combined effect of more than one pole, and to calculate declination accurately you needed to know how the magnetic attraction decreased with distance from each pole.
Halley was well aware that the gravitational force between two bodies depends inversely on the square of the distance between them: for example, if you double the distance between them, the force is reduced to a quarter of its original strength. In fact, both he and Robert Hooke had independently discovered this some time before learning that Newton had beaten them to it. Halley subsequently cajoled Newton into writing up this and much more of his work on the movements of the planets, and in 1687 personally saw through the publication of Newton’s great work, Philosophiae Naturalis Principia Mathematica.
Halley realized that progress in geomagnetism depended on a comparable mathematical understanding of magnetic force. He tried some experiments of his own, but without success. He then apparently urged Newton to investigate, but Newton does not seem to have been much inspired by magnetism. There are only a few very brief mentions of it in Principia, including a reference to “some crude experiments” that suggested magnetic force might decrease not with the inverse square of distance, as is the case with gravity, but more probably with the inverse cube of distance (in which case doubling the separation would decrease the force by a factor of eight).
The second barrier to further development of his concept, Halley noted, was that even the sparse and meager observations of secular variation then available painted too complex a picture to be explained by simple westward drift of the poles: maybe their latitudes also changed. It would, he concluded, take hundreds of years to accumulate sufficient observations “to establish a compleat doctrine of the magnetical system,” and until such time the motions of the magnetic poles would remain “secrets as yet utterly unknown to Mankind.” Whether this was visionary foresight, or a ploy to gain support for his future endeavors to chart declination over large areas of the globe, is anyone’s guess.
Halley continued to wonder at the origin of Earth’s magnetism, and in 1692 he came up with a novel explanation of his four poles. His idea may have been inspired by Gilbert’s comparison of the Earth with a lodestone terrella, but it also bore uncanny similarities with modern concepts of the Earth’s interior, as well as some features that nowadays seem quite absurd. In essence, Halley proposed that the Earth was composed of a solid inner sphere and a spherical outer shell, with some sort of fluid filling the space between them. Both the inner sphere and the outer shell were magnetized, and each carried two poles: the inner sphere carried the European North Pole and the American South Pole, while the shell carried the American North Pole and the Asian South Pole. Neither pair of poles was truly antipodal—that is, diametrically opposite each other—but this does not seem to have worried Halley.
To achieve secular variation, Halley allowed both the inner sphere and the outer shell to rotate about the geographic axis but at different rates: because of the viscous resistance of the intervening fluid, the sphere lagged further and further behind the shell. He estimated that it would take 700 years for the inner sphere to fall one whole rotation behind the shell, and so complete one cycle of secular variation.
Halley concluded with a curious discussion of the possible purposes of his shell and inner sphere in the overall scheme of Creation. He speculated that there might be several more shells, nested inside one another, corresponding in size to the planets Mercury, Mars and Venus and each carrying a pair of poles. He probably envisaged this as a means of accommodating yet-to-be-discovered complexities in the geomagnetic field, but he discussed at length the likelihood of life on the inner shells.
The Canadian geophysicist Michael Evans has described Halley’s model as a sort of global apartment block. Halley, he suggests, may have been having a hard time with the church, and may have even missed out on a chair at Oxford on suspicion that he believed Earth to be eternal, rather than divinely created. This curious discourse may have been his defence. Whatever the case, in 1703 Halley eventually became Savilian professor of geometry at Oxford University, a position he would hold until his death in 1742, and in 1720 he succeeded John Flamsteed to become the second British Astronomer Royal.
Edmond Halley’s final concept of Earth’s internal structure as a central sphere the size of the planet Mercury, and concentric shells equal in size to Mars and Venus. This was an elaboration of his earlier model, in which an outer shell enclosed a single inner sphere, with each carrying a pair of magnetic poles.
Before this, though, Halley scored another first for science when in 1698, in what has been claimed to be the first specially commissioned nautical geophysical expedition, he set out to measure and chart the magnetic declination (or “variation” as it was still called) over the Atlantic Ocean. Halley had been adamant that the route to understanding geomagnetic declination and secular variation, and possibly to determining longitude at sea, lay in collecting and documenting as much information as possible. He had made his case to the king, William III, and to the British Admiralty and been awarded a captain’s commission.
On October 20, 1698 he set sail aboard the Paramour to, as the commission described it, “improve the knowledge of the longitude and the variations of the compasse” and to explore the south Atlantic further than previous mariners “till you discover the coast of Terra Incognita.” The idea of this huge southern continent had existed since the anci
ent Greeks first speculated on the shape of the Earth and the distribution of its land masses. Most atlases and globes showed a Terra Incognita or Terra Australis, although the voyages of Ferdinand Magellan and Abel Tasman had already cast doubt on its extent.
Halley’s first voyage was short-lived. Leaks in the ship and disputes with his second in command forced him to return from the West Indies after just a few months at sea. Problems fixed and lessons learned, Halley set sail again in 1699 and, making declination measurements all the way, he eventually reached latitude 52°40’S on February 1, 1700. The closest thing he saw to a Terra Incognita were three huge icebergs that floated past on their way north, prompting him to name this part of the South Atlantic “The Icey Sea.” The myth of a vast southern continent would finally be laid to rest when Captain James Cook circumnavigated New Zealand and Australia in the 1770s, proving the extent of the southern oceans.
In all, Halley collected over 200 measurements of declination, covering the length and breadth of the Atlantic. His next important task was to report to his sponsors. What was the best way to present his measurements? Although Halley’s main interest was in Earth’s magnetism, the justification for his work was to improve navigation, and his results would be used mainly by mariners and sailors. He needed to create a chart that would be readily understood and easy to use on the high seas.
His answer was to invent the contour line. Although he had made measurements only at discrete, disconnected locations along the route of the Paramour, Halley drew lines that joined up, in the simplest and smoothest way possible, places with equal declination values. These lines were drawn at intervals of one degree of declination, producing a series of equally spaced “Halleyan lines,” which later came to be known as contours. Today we are familiar with the use of contours of equal height on topographic maps, and weather maps showing isobars or contours of equal atmospheric pressure, but in 1701 Halley’s curved lines of equal declination were a revolutionary idea.
As was customary at the time, Halley’s chart was adorned with elaborate text borders, compass roses and pictures. His route was marked out with tiny icons of the Paramour, while in The Icey Sea he depicted and described:
… two sorts of Animalls of a Middle Species between a Bird and a Fish, having necks like Swans and swimming with their whole Bodies always underwater only putting up their long necks for Air.
Was Halley seeing penguins for the first time, or some obscure species never seen before or since?
In 1702 Edmond Halley produced a declination chart for the whole world using observations made by other mariners in other oceans. Surprisingly, though, he had never measured inclination. Although the accuracy of the dip needle was certainly inferior to that of the mariner’s compass, the combination of declination and inclination measurements might have offered an opportunity for determining absolute location at sea and a solution to the longitude problem.
The first inclination chart, which covered the southeast of England, seems to have been made in 1721 by an Anglican priest called William Whiston. In addition to his religious calling,
The earliest chart of magnetic declination, or “variation,” over the Atlantic Ocean, published by Edmond Halley in 1701 from over 200 measurements made between 1697 and 1701. This was the first use of Halleyan lines, later known as contours. These lines of equal declination were at the time considered revolutionary.
Whiston was a keen mathematician and astronomer. At about the time Halley finally gained his Oxford chair, Whiston became Lucasian professor of mathematics at Cambridge, a position that had earlier been held by Newton, and would 200 years later be held by Stephen Hawking. However, he seems to have been too outspoken for the times, and in 1710 he lost the chair for apparently suggesting that Noah’s flood might have had natural causes. Even in the liberal society of William III’s England, academics had not achieved unbridled freedom of speech.
Like others before him, Whiston was tempted by the grail of solving the longitude problem. He had previously concocted several ingenious but unworkable schemes with his mathematician friend Humphrey Ditton; one involved regularly timed cannon blasts from a series of ships anchored at known locations throughout the oceans of the world. Now, on studying Halley’s declination charts, Whiston realized that simultaneous measurements of declination and inclination might provide the answer.
The idea had considerable merit, but it was still compromised by the difficulty at sea of making sufficiently accurate measurements, particularly of inclination, because of the motion of the ship and iron objects near the compass. It would not be until 1768, just five years before the longitude prize was finally awarded to John Harrison for his fifth improved marine chronometer, that the first world inclination chart was finally published by Johannes Wilcke, a Swedish physicist.
Like declination, the angle of inclination at any given location was also soon found to undergo steady changes from year to year. Gellibrand’s secular variation was slow, and could be detected only by collating and comparing compass and dip needle observations year after year. There was no indication yet as to where secular variation would lead in the long term. Between the times of Gilbert and Halley, the declination in London had shifted from 11° east to 7° west of north, a change of eighteen degrees. It was tempting to think the compass needle would soon turn again, and would eventually be found to slowly swing back and forth around true north, but there was little sign of this happening yet.
Then, just as geomagnetists and mariners alike began to settle into a routine of repeat observations, patiently amassing the data that might eventually lead to an interpretation of secular variation, another chance discovery occurred to upset their theories.
George Graham was a London-based maker of precision instruments. He made clocks, watches, quadrants and some of the most stable and sensitive compasses available, so in 1722 he was concerned to find that the needles of his best compasses were actually in constant, seemingly random motion. He went back and checked meticulously and found that:
… all the needles I made use of would not only vary in their direction on different days, but at different times on the same day …
Sometimes the needles fluctuated by more than half a degree in a day.
To be sure he was not imagining things, Graham set up three compasses. Each had a needle 31 centimeters long, filed to a fine edge at the end. These needles moved over a scale that was graduated at intervals of one-sixth of a degree, or ten minutes. Subdivisions of two minutes could be estimated with the use of a magnifying glass.
All three compasses behaved the same way: Graham had discovered the daily variation of Earth’s magnetism. This was quite different from the secular variation—at least ten times smaller in amplitude and something like 10,000 or 100,000 times more rapid. Further investigations showed that the average amplitude of these daily variations was greater in summer than in winter: in London it was about 13 minutes in summer, as opposed to seven in winter.
In Uppsala, Sweden, at a latitude of 60° N and 18 degrees further east than London, the astronomer Anders Celsius and his assistant, Olof Hiorter, had also noticed and become interested in the daily fluctuations of the compass needle. They coordinated their measurements with Graham, and between them discovered two features. First, the regular daily variation correlated with local time, beginning shortly after sunrise and quietening down with sunset. Secondly, on certain days the needle seemed to go crazy. On April 5, 1741, for example, Hiorter reported that “the needle at 2 pm began to be disturbed, so that at 5 pm it was 1°40’ to the west of its declination at 10 am,” while Graham recorded:
The alterations that day were greater than I have ever met with before … The observations on the other days were made with the same care, but they were much less and more regular.
These irregular disturbances were truly simultaneous at London and Uppsala, and were not a function of local time.
Following a hint Halley had given twenty years earlier, Hiorter and Celsius hit on
the explanation: at the times when the compass needle was most disturbed, the northern lights, or aurora borealis, were putting on their most spectacular show. The two scientists wrote:
A motion of the magnetic needle has been found that deserves the attention and wonder of everyone. Who could have thought that the northern lights … when they draw southwards … could within a few minutes cause considerable oscillations of the magnetic needle through whole degrees?
Gilbert’s explanation of declination may have been disproved, but his chief argument—that the major source of Earth’s magnetism lay within the Earth itself—had seemed incontrovertible. However, these new observations seemed to again point to a source outside the Earth: the daily movement of the compass needle was obviously related to local time, beginning at sunrise and ending at sunset, while the bigger disturbances that were simultaneous in London and Uppsala clearly correlated with auroral activity. Hiorter reasoned that the auroras must occur extremely high in the atmosphere. Here, then, were magnetic variations seemingly equally associated with processes in the heavens.
Was the source of Earth’s magnetism internal or external, or were there perhaps two sources? How could this apparent dilemma be reconciled?
Measuring the Force
Numbers may be said to rule the whole world of quantity … the four rules of arithmetic may be regarded as the complete equipment of the mathematician.
—JAMES CLERK MAXWELL
The ancient Greeks had recognized that while lodestone attracted only iron objects, the translucent, golden, fossil-resin amber, when rubbed with fur, would magically pick up light objects of many different materials. Curiously though, while lodestone, the compass and Earth’s magnetism had continued to intrigue both scientists and explorers, by the early seventeenth century the attractive properties of amber, or “elektron,” as the Greeks had called it, had been almost forgotten. Even though Gilbert’s portrait in the frontispiece of De Magnete was entitled “William Gilbert, M.D.—Electrician,” the book was essentially about magnetism: Gilbert had devoted just a single chapter to electric attraction, the amber effect.
North Pole, South Pole: The Epic Quest to Solve the Great Mystery of Earth's Magnetism Page 6