One of the earliest ideas about how to determine longitude based on the compass was conceived by a Neapolitan scholar, Giovanni Battista Della (John Baptista) Porta, and described in his encyclopedia Magia Naturalis (Natural Magick), published first in 1558 and later, in twenty volumes, in 1589. Porta appears to have examined some early observations of declination. In 1492, for example, Christopher Columbus’s navigators had been alarmed to find that on occasion their compasses deviated significantly from the Pole Star. Columbus had apparently explained the effect to his crew in terms of some previously undetected motion of the Pole Star, but Porta supposed that the compass needle had moved in step with Columbus’s progress across the Atlantic Ocean, from some ten degrees east of north in Europe to a similar angle west of north by the time he reached the Caribbean region.
Porta concluded that the declination of the compass needle provided a direct measure of longitude east or west of the Azores, where it pointed true north. He even suggested making a giant compass, about five feet in diameter, and dividing the degrees and minutes into seconds and thirds in order to resolve longitude more precisely. Porta’s idea was short-lived, but the notion of a meridian of zero declination through the Azores survived a little longer, thanks to Mercator and other sixteenth-century cartographers who still ran the prime meridian through these islands.
Later, and more significant in the quest to understand Earth’s magnetism, was the discovery of what is now called “inclination.” This is a simple concept. Anyone who has traveled widely with a compass will have discovered that a compass that is balanced at one location tilts up or down when used at other latitudes. This is particularly noticeable between the hemispheres. If a compass designed for use in the northern hemisphere is taken to the southern hemisphere, the north end of the needle seems to be pulled upwards and the south end pushed down. The opposite is true of a southern hemisphere compass used north of the equator: the north end of the needle is pulled down and the south end pushed up. For this reason, a compass needle is usually weighted or balanced to make it swing in a horizontal plane.
If a magnetized but unweighted needle is freely suspended about its center, the angle it takes above or below the horizontal was known historically as the “dip.” Today it is called as the “inclination.” By convention, inclination is deemed positive if the direction is below the horizontal (as at most locations in the northern hemisphere), and negative if above the horizontal (as at most locations in the southern hemisphere).
English hydographer Robert Norman’s demonstration of the inclination, or dip, of a magnetized needle that is balanced and neutrally buoyant in a goblet of water so it is free to align due to the turning effect of Earth’s magnetic field. Reproduced in William Gilbert’s 1600 work De Magnete.
Georg Hartmann, a vicar in Nuremberg, appears to have been close to discovering inclination in 1544 when, in a letter describing declination measurements, he noted a slight tilting of his unweighted, pivoted needle. However, the angle of nine degrees that he recorded was much too low for his location.
Credit for finally recognizing inclination therefore probably belongs to an English hydrographer named Robert Norman. In The Newe Attractiue, a pamphlet published in 1581, Norman described inclination as “a newe discouered secret and subtill propertie concernyng the Declinyng of the Needle.” The discovery was a breakthrough for mariners and scientists alike. The Newe Attractiue was reprinted at least four times (in 1585, 1596, 1614 and 1720), and later included in Rara Magnetica, a collection of works on geomagnetism published by Gustav Hellmann in Berlin in 1897.
Norman had gone back to the old idea of a floating compass, but his version was three-dimensional. He stuck a needle through a piece of cork and carefully adjusted the size of the cork until he just achieved neutral buoyancy in a goblet of water: the cork floated beneath the surface, neither rising to the surface nor sinking to the bottom. Norman found that the needle, once magnetized, pointed northwards (give or take a few degrees due to declination, which was very small in England at the time), but inclined downwards at about seventy degrees below the horizontal.
Norman also described the construction of the first “dip needle,” a magnetized needle pivoted at its center of mass so it swung in a vertical plane. When this needle was oriented north−south, it finally settled at an angle below the horizontal equal to the inclination.
By 1600 inclination had been measured at many locations around the globe and a pattern was emerging. At the equator the inclination was close to zero: the dip needle was almost horizontal. The inclination steadily increased in steepness with latitude, until the dip needle pointed vertically downwards at the north (magnetic) pole and vertically upwards at the south (magnetic) pole.
Three centuries had now passed since Petrus Peregrinus had written his Epistola, making the case for experimentation in science and reviving interest in magnetism. Europe had emerged from the Dark Ages and the social and scientific climate had changed dramatically. The Renaissance had revived a passion for creativity and learning in both arts and sciences. As well as the ancient universities of Bologna, Oxford and Paris, centers of study now flourished across Europe at places such as Cambridge, Salamanca, Lisbon, Modena and Padua.
Given the slowness of travel and communication, it was remarkable how much interaction was taking place between academics and scholars scattered across the continent. Several great minds had been at work changing the way the universe was perceived—or trying their hardest to do so. But unfortunately for many, and disastrously for some, the Reformation in religious thought and tolerance had lagged some way behind the cultural and social renaissance. In earlier days, the geocentric model of the solar system had suited the young Christian church, from which Roman Catholicism emerged to dominate medieval Europe. The perceived circular orbits of the moon, sun, planets and stars about a central Earth were regarded as evidence of divine Creation: questioning such perfection was deemed as heresy, to be stamped out at all cost.
The problem was that the orbits of the planets had long been known to deviate from perfect circles: most notable was the so-called “retrograde” or occasional eastward motion of Mars across the sky. The suggestion that such observations might be explained better if all the planets, including Earth, orbited the sun can be traced back to the fifth-century BC Greek mathematician and astronomer Pythagoras and his followers. The idea of a sun-centered system had resurfaced several times down the following centuries, but had always failed to gain general approval. The overwhelmingly accepted solution to the problem of irregular planetary motions had been to modify the geocentric model with Ptolemy’s idea of epicycles: extra loops superimposed on perfectly circular planetary orbits about Earth.
The heliocentric theory was finally revived again by the Polish astronomer Nicolaus Copernicus in the early sixteenth century. Copernicus, born in 1473, was an official of the Catholic Church and did not dare publish his theory for fear of excommunication, or worse. It was only thanks to his friends that his manuscript De Revolutionibus Orbium Coelestium (On the Revolutions of the Celestial Spheres) was eventually printed in 1543: it was presented to him on his deathbed, when he was arguably beyond reprisal. Even so, his friends had inserted a disclaimer stating that, whereas the sun-centered model provided an excellent and superior means of making accurate predictions of planetary positions, it was no more than a hypothetical model and did not represent the universe in reality.
The Italian mathematician and astronomer Galileo Galilei, born twenty-one years after Copernicus’s death, was rather more outspoken and so fared less well with church authorities. Throughout his life he was issued with warnings to teach the classical theory, but he resisted and was eventually brought before the Holy Office of the Inquisition in 1633, found guilty of heresy, and ended his life in imprisonment, albeit in his home.
By the beginning of the seventeenth century, however, the Reformation was sweeping northwestern Europe, and with many countries becoming largely reformed there was a more clement environment
for the publication and dissemination of new ideas. In 1600 William Gilbert, a fifty-six-year-old physician with a passionate interest in the natural sciences, was fortunate to find himself in the relative safety of a moderately reformed Elizabethan England, with access to the works of his predecessors and contemporaries, publishers eager to circulate new ideas, and an audience hungry for new scientific discoveries. The time was right for a milestone work, and Gilbert, having conducted many years of research on magnetism and electricity, was ideally placed to write it.
Magnus Magnes
Magnus magnes ipseest globus terrestris.
—WILLIAM GILBERT, 1600
William Gilbert was born in Colchester, England, the eldest of eleven children. Historians are divided as to whether he was born in 1540 or 1544, but agree that he entered St. John’s College of Cambridge University in 1558 when he would have been either fourteen or eighteen. It was not unusual for talented children of well-to-do parents to begin university studies young, so this does not help with ascertaining his birth date. In any case, he gained a Bachelor of Arts in 1560, a Master of Arts in 1564, and finally became a Doctor of Medicine in 1569.
Gilbert then spent several years traveling around Europe, during which time he seems to have developed an interest in magnetism.
William Gilbert (c. 1544–1603), “electrician,” physician to Queen Elizabeth I, and author of the epic work De Magnete.
On his return to London in 1573 he began practicing medicine and was elected a Fellow of the Royal College of Physicians, a society in which he would subsequently hold several offices, eventually becoming president in 1600. This was a prodigious year for him in several other ways as well: he was called to court and appointed physician to Queen Elizabeth I, and he published a monumental book on Earth’s magnetism. It would become his most famous piece of work.
Until his move to court, Gilbert had hosted regular meetings at his home, and here small groups of amateur scientists would discuss the science and philosophy of the day and the investigations they were undertaking. Such groups were forerunners of the Royal Society, which would be formally founded later in the seventeenth century with the explicit aim of promoting and fostering the exchange of ideas between scientists. Early on, Gilbert’s non-professional scientific interests had leaned towards chemistry, but he had soon switched to electricity and magnetism, and during his years as a physician he devoted his spare time and considerable personal funds to the experimental investigation of these subjects.
De Magnete, Magnetisque Corporibus, et de Magno Magnete Tellure (On the Magnet, Magnetic Bodies, and the Great Magnet of the Earth) is a formidable work. In a style reminiscent of the Bible, it is divided into six “books” and 115 “chapters.” Fortunately for modern readers, some chapters are very short and their titles are descriptive, in some cases almost abstracts in their own right.
De Magnete fulfilled a number of distinct purposes. It summarized and evaluated existing knowledge and philosophy of magnetism, described Gilbert’s numerous experiments, and was a treasure trove of his thoughts and conclusions, doubtless enriched through presentation and discussion at his house meetings. It has been described as the first textbook on the subject, but it is a good deal more. As late as 1822 a doctor, John Robson, would optimistically proclaim: “It contains almost everything that we know about magnetism.”
De Magnete was written in Latin, and almost before the ink had dried there were cries for an English translation. However, even in English—for example, the 1893 translation by P. Fleury Mottelay—it is far from easy to digest. Nevertheless, few textbooks survive over 200 years as the seminal work on any subject: De Magnete was something special. Gilbert’s work ranks alongside that of Galileo, and of Sir Isaac Newton later in the seventeenth century. Galileo himself received a copy of the book and was clearly impressed and influenced by it.
In Book I, Gilbert separates well-founded information about magnetism from speculation and myth. He endorses the importance of experimentation, and scorns metaphysical philosophy without practical verification as pointless:
… but they wasted oil and labor, because not being practical … having made no magnetical experiments, they constructed certain ratiocinations on a basis of mere opinions and old-womanishly dreamt things that were not.
Leaving aside what would today be regarded as blatant ageism and sexism, Gilbert’s message was unmistakable: from the seventeenth century on, scientific theory would be firmly grounded in experimentation. Although Peregrinus had carried out experiments three centuries years earlier, the practice did not take hold until Gilbert expounded its importance. The message was to be reiterated even more forcefully by Francis Bacon a few years later, whereupon it gained the title Baconism.
Not all of Gilbert’s conclusions were taken on board immediately. Like Porta, for instance, he ridiculed the idea that a lodestone smeared with garlic would lose its “virtue,” yet throughout the next century British sailors who were found with garlic on their breath still risked a flogging for fear they would demagnetize their ships’ compasses.
In the final chapter of Book I, Gilbert presents his most famous conclusion: “Magnus magnes ipse est globus terrestris” (“The Earth itself is a great magnet”). The experimental justification for this, and much else, follows in the next four books, which set out to discuss magnetic movements, direction, declination (which Gilbert calls “variation”) and inclination (which he calls “dip” or sometimes, confusingly, “declination”).
The final chapter of the first book (along with Book VI) is a rare early foray into geophysics, containing the first modern-style, reasoned discussion of the internal composition and structure of the Earth. Gilbert notes that the water-filled ocean basins, although apparently of great depth, are insignificant compared with the huge size of the Earth. He marvels at the irregular and enormously varied nature of the planet’s surface, criticizing the ancient Greek philosophers who lumped together the vast diversity of rocks, sediments and soils as just one element, “earth.” He wonders at the origin of these diverse surface rocks and sediments and discusses possible compositions of the interior, lamenting man’s inability to probe beyond the 500 fathoms of the deepest known mine. (One fathom is six feet, or 1.83 meters, so 500 fathoms is just over 900 meters. Even now the deepest hole drilled on land, the Kola Superdeep Borehole in the former USSR, reaches just over twelve kilometers into the crust, only one five-hundredth of Earth’s radius.)
Gilbert also notes that nearly all rocks seem to show some degree of attraction towards a lodestone:
… provided only they not be fouled by oozy and dank defilements like mud … or that a greasy slime oozes from them like marl … they are all attracted by the loadstone …
And he seems to wonder if magnetic attraction in Earth’s interior may be enough to bind and keep the planet in its spherical form.
This was a significant step forward in scientific thinking: it would be another eighty-seven years before Isaac Newton published his theory of a universal force of gravitation, a fundamental force of nature that acted between each and every particle of matter irrespective of composition, and that we now know accounts for both the binding together of planetary bodies and their orbital motions. In 1600, Galileo had yet to risk his neck describing the apparent imperfections of Creation, and the German astronomer Johannes Kepler had yet to show that planetary orbits were not circular but elliptical in shape. It was hardly surprising, then, that Gilbert and others invoked magnetism to explain geophysical and astronomical observations.
In Books II to V of De Magnete Gilbert describes his many experiments, including those he had conducted on a lodestone sphere, or terrella, which had led him to conclude that the Earth itself was a great magnet. In experiments remarkably similar to those of Peregrinus, he had plotted the orientations assumed by “bits of iron wire, one barleycorn in length” placed at different locations on the surface of the terrella. (A barleycorn is an old Anglo-Saxon measure equal to one-third of an inch, or
approximately 8.5 millimeters.)
Unlike Peregrinus, Gilbert knew all about the angle of inclination and how it increased with latitude, from zero near the equator to 90 degrees near the poles. He had, therefore, immediately recognized the pattern described by the barleycorn-length wires: the angles they made with the surface of the terrella duplicated exactly the inclinations of the dip needle at equivalent latitudes on Earth.
He reasoned that just as the magnetism of the terrella was due to the magnetic material of the lodestone, Earth’s magnetism must originate in the magnetism of the planet itself—not beyond the heavens, as had been argued, for example, in 1545 by a Spanish geographer, Martinus Cortes, who, Gilbert wrote, “would be content with no cause whatever in the universal world;” not at the celestial poles, as favored by Peregrinus; not at the Pole Star; and certainly not in polar mountains of lodestone. Instead, Gilbert said, it was spread through the whole substance of the Earth.
Sixteenth-century physician William Gilbert was deeply interested in electricity and magnetism, and believed strongly in experimentation, then a new scientific concept. Top: Gilbert’s demonstration of the orientations of short iron needles placed around a magnetized lodestone terrella, or “little earth.” The poles of the terrella are at opposite ends of the “axis,” where the needles stand at right angles to the surface. Bottom: Gilbert’s sketch of the inclination of a dip needle placed at various latitudes around the Earth. Earth’s north geographic pole is at left (point C) and south pole at right (point D). Notice the resemblance to the orientations of the iron needles around the lodestone terrella in the illustration above.
Gilbert’s concept of a uniformly magnetized planet, symmetrical about its axis of rotation, was a masterstroke. Nineteenth-century mathematicians would take the idea further, showing that the magnetic effect of a sphere of uniformly magnetized lodestone is identical to that of a bar magnet located at its center. Each has two poles, and so is commonly called a magnetic “dipole.” Today Gilbert’s concept is technically known as a geocentric (centered on the Earth) axial (along the axis) dipole.
North Pole, South Pole: The Epic Quest to Solve the Great Mystery of Earth's Magnetism Page 4
