Sir Edmund Halley's Foundational Map of the World – Early, Influential Use of Isogonic Lines
Edmund Halley's chart of the world is one of the most important world maps of the eighteenth century. The map is the first surviving map to show isogonic lines, or lines connecting points of equal magnetic variation in the oceans, a feature then considered of prime importance for determining longitude.
The world map is presented in a long-strip format of a Mercator projection, which Halley called a nautical projection. Eastern Asia and Australia are repeated, so as to better show the flow of the isogonic lines. These lines radiate over the Atlantic and Indian Oceans, but not the Pacific. A note in the Pacific explains his innovation and why the lines stop as they approach the world’s largest ocean:
The Curve lines which are drawn over the Seas in this Chart, shew the Variation of the Compass in all the known Seas, the double lines divide the tracts of East and West Variation & under them the Compass stands true without Varying. In any other place, the degrees of Variation is [sic.] seen by the number on the Line that passes over that place. I durst not presume to describe the like Curves in the South Seas wanting account thereof. (quoted from the English edition of Halley’s chart, published by Emanuel Bowen, 1722).
While the isogonic lines are certainly a distinctive feature of this map, Halley also strove to include all the latest and most comprehensive geographic information, so that it would be of utmost use to the navigator. In the South Pacific is a half-hemisphere map of the North Pole. A helpful note explains, “That nothing might be wanting in this Chart we have added this Polar, partly to shew the Inclination of the Meridians toward the Pole, partly to avoid the too great contraction of our Scale” (ibid).
Halley also includes several details that we now know are cartographic myths. California is shown as an island (see below). A tiny island near what is today Argentina is labeled as Pepys Island. Pepys was a contemporary of Halley’s and had served as President of the Royal Society. This island was named in the 1680s by a buccaneer and lingered on maps for a century before naval explorers like Byron and Cook confirmed that it did not exist.
Another chimera is repeated twice in Northeast Asian waters. Mainland Asia fades out, giving way to the unfinished coastlines of Terre de Iesso and Terre de la Compagnie. Iesso, or Yesso, is a name for Hokkaido and its size was usually exaggerated on maps of this period. Compagnie’s Land, and its neighbor, Staten Land, were mis-discoveries found while searching for another mythic island, Gamaland (see below).
There are decorative details tucked into the blank and inland spaces of the map. For example, a poem, in Latin, graces the North Pacific in a temple-style cartouche and lauds the unknown inventor of the maritime compass. A similar framed cartouche carries another Latin poem, this one praising the consolidation of maritime power by Queen Anne (a holdover from the English edition), in Russia. Muses carrying the instruments of astronomy, navigation, and cartography grace the title cartouche in North America.
While it was later learned that magnetic variation fluctuates with time and cannot be used to find longitude at sea, Halley’s charts and his use of isogonic lines were considered an indispensable contribution to the study of navigation by savants and sailors alike. As Samuel Pepys asked rhetorically, “Mr Hawley—May he not be said to have the most, if not to be the first Englishman (and possibly any other) that had so much, or (it may be) any competent degree (meeting in them) of the science and practice (both) of navigation?” (as quoted in Thrower, 15).
Halley’s voyage and his charts
Halley was a prolific publisher and his output included several important maps and charts, including his untitled world map of 1686 showing the trade winds. During the 1680s, Halley became increasingly concerned with the implications of magnetic variation for navigation. Not one to just read about a phenomenon, Halley sought command of a Royal Navy vessel, the Paramore, and took it on three voyages between 1698 and 1701. The first two, to the South Atlantic, were to study geomagnetism. The third, in the English Channel, focused on tidal phenomena. While his journals were not published during his lifetime, his charts from those voyages were.
The first chart he published was a chart of the Atlantic with isogonic lines, “A New and Correct Chart Shewing the Variations of the Compass in the Western & Southern Oceans as observed in ye year 1700 by his Ma/ties Command.” Dedicated to William III, who died on March 8, 1702, the chart is usually dated to 1701, as Halley returned from his second voyage on September 18, 1700. It was printed by Richard Mount and Thomas Page of Tower Hilll and engraved by John Harris. He presented the chart to the Royal Society on June 4, 1701.
The chart of the world’s oceans which Halley created, of which the present chart is an example, would prove more influential and grow to greater prominence, appearing in several states, foreign editions, and even updated revisions. It was the first printed world map to employ isogonic lines.
The chart first appeared in 1702, datable by the dedication cartouche in Africa that references “Prince George of Denmark, Lord High Admiral.” The Prince Consort was elevated to that position on April 17, 1702.
The world chart contains fewer decorative details than the Atlantic chart, as well as a few additions. One of these is the Falkland Islands, which had been named as such by John Strong in 1690. Halley mentions Strong’s journal in his own and this chart would help to fix that name to the archipelago.
Interestingly, although the world chart proved popular and was reissued, translated, and updated (see Montaine and Dodson’s chart (1744) which extends the isogonic lines to the Pacific), the chart became scarce in all iterations over time. Indeed, by 1870, Halley’s charts were so scarce that Sir George Airy, Astronomer Royal, exasperatedly exclaimed that, despite having seen references to “Halley’s Magnetic Chart,” he had “not ascertained that any writer had ever seen it…As I was desirous of making myself acquainted with a document so important in the history of magnetic science, I made enquiries in nearly every Academy in Europe, but could not find anywhere a copy of this Chart.” He eventually did find an example of the world chart nearby, at the British Library; he seems not to have known of the existence of the Atlantic chart.
The present bilingual edition (French and Dutch) was issued by Reiner and Joshua Ottens in Amsterdam. It is widely considered to be the most visually attractive edition of the map. It is also rarely seen on the market, as are Halley’s charts in general.
The history of geomagnetism
Humans have known for centuries that there was a magnetic property to metals, and to navigation. Polarity and orientation were first recorded in China in the sixth century; they also recorded the first known compasses, a needle floating in a bowl of water, in the twelfth century. This technique for finding direction was also developed in Europe in the Medieval period. The first maps to include an inkling of declination are markings on German sundials from the mid-fifteenth century. Soon mapmakers adopted the convention, including declination markings on their wind roses.
Until the sixteenth century, the seat of magnetic attraction was thought to be housed in the heavens. In the early modern period, more and more data was gathered. This data was collected by navigators at sea and consisted of magnetic inclination, declination, and deviation. The former, also known as magnetic dip or the dip angle, is the angle made with the horizontal by the Earth’s magnetic fields. Magnetic declination, or variation, is the angle on a horizontal plane between magnetic and true north. Magnetic deviation is the effect of local magnetic fields on compass error. Directional data led savants to conclude that these phenomena varied considerably based on one’s location. They found there was terrestrial polar attraction, creating waves, or lines, of magnetic variation across the globe.
More information also shifted understanding of the source of magnetic variety. As more and more ships took to the open seas, they contributed new data sets. Many found magnetic declination to be zero near the Azores, suggesting that it was a natural prime meridian from which to measure longitude. A tilted dipold was thought to lie 180°E of the Azores, affected by the great magnetic mountain that supposedly lay in the Arctic—it appears famously on Mercator’s map of the North Pole. While this idea was mistaken, as were other hypotheses of two and up to six dipolds, the ideas of an Earth-bound source for magnetism, and of terrestrial locations for the magnetic poles, were not.
The first map to show isogonic lines—lines connecting points of equal declination—was a manuscript chart by the Jesuit Christoforo Borro; made in the 1620s, it is now lost. The seventeenth century was an important period in the theorization of geomagnetism; William Gilbert and others contributed to the ideas of global crustal heterogeneity, rather than a single Arctic magnetic pole. Observations conducted over time at a single point also showed that there was a temporal element to magnetic readings. Precisely why these changes occurred was what drove Edmond Halley to conduct the first naval surveys of magnetic declination in the 1690s.
In the eighteenth century, isogonic lines such as those employed by Halley on his influential charts in the early 1700s would become a useful tool for those eager to crack the secrets of geomagnetism. They appeared on maps by Frezier (1717), Van Musschenbroeck (1729, 1744), van Ewyk (1752), Mountaine and Dodson (1744, 1756), Dunn (1775), Lambert (1777), and Le Monnier (1778). The first map to include isoclinics, or lines of equal dip, was made by Johann Karl Wilcke in 1768. Towards the end of the century, John Churchman, a surveyor, published a magnetic atlas that employed both isogonics and isoclinics. They called on a huge amount of data gathered on shore by national observatories and local natural philosophers, as well as at sea by naval officers and trade company employees. All of this information led to the abandonment of the idea of even multiple fixed poles and gave way to an understanding of shifting magnetism based on disjointed dipoles that were dynamic, tilted, and nonantipodal.
From the 1830s, astronomers and physicists became the primary gatherers of data. They measured the full magnetic vector; that is, they recorded both the direction and intensity of magnetism. These surveys allowed them to map the field as a whole, a process that accelerated in the mid-twentieth century when scientists were able to carry out various magnetohydrodynamic simulations.
The etymology of the idiom Yesso (Eso, Yeco, Jesso, Yedso) is most likely the Japanese Ezo-chi; a term used for the lands north of the island of Honshu. During the Edō period (1600-1886), it came to represent the ‘foreigners’ on the Kuril and Sakhalin islands. As European traders came into contact with the Japanese in the seventeenth century, the term was transferred onto European maps, where it was often associated with the island of Hokkaido. It varies on maps from a small island to a near-continent sized mass that stretches from Asia to Alaska.
The toponym held interest for Europeans because the island was supposedly tied to mythic riches. Father Francis Xavier (1506-1552), an early Jesuit missionary to Japan and China, related stories that immense silver mines were to be found on a secluded Japanese island; these stories were echoed in Spanish reports. The rumors became so tenacious and tantalizing that Abraham Ortelius included an island of silver north of Japan on his 1589 map of the Pacific.
Yesso is often tied to two other mythical North Pacific lands, Gamaland and Compagnies Land. Juan de Gama, the grandson of Vasco de Gama, was a Portuguese navigator who was accused of illegal trading with the Spanish in the East Indies. Gama fled and sailed from Macau to Japan in the later sixteenth century. He then struck out east, across the Pacific, and supposedly saw lands in the North Pacific. These lands were initially shown as small islands on Portuguese charts, but ballooned into a continent-sized landmass in later representations.
Several voyagers sought out these chimerical islands, including the Dutchmen Matthijs Hendrickszoon Quast in 1639 and Maarten Gerritszoon Vries in 1643. Compagnies Land, often shown along with Staten Land, were islands sighted by Vries on his 1643 voyage. He named the islands for the Dutch States General (Staten Land) and for the Dutch East India Company (VOC) (Compagnies, or Company’s Land). In reality, he had re-discovered two of the Kuril Islands. However, other mapmakers latched onto Compagnies Land in particular, enlarging and merging it with Yesso and/or Gamaland.
In the mid-eighteenth century, Vitus Bering, a Danish explorer in Russian employ, and later James Cook would both check the area and find nothing. La Perouse also sought the huge islands, but found only the Kurils, putting to rest the myth of the continent-sized dream lands.
The popular misconception of California as an island can be found on European maps from the sixteenth through the eighteenth centuries. From its first portrayal on a printed map by Diego Gutiérrez, in 1562, California was shown as part of North America by mapmakers, including Gerardus Mercator and Abraham Ortelius. In the 1620s, however, it began to appear as an island in several sources.
The myth of California as an island was most likely the result of the travel account of Sebastian Vizcaino, who had been sent north up the shore of California in 1602. A Carmelite friar, Fray Antonio de la Ascensión, accompanied him. Ascension described the land as an island and around 1620 sketched maps to that effect. Normally, this information would have been reviewed and locked in the Spanish repository, the Casa de la Contratación. However, the manuscript maps were intercepted in the Atlantic by the Dutch, who took them to Amsterdam where they began to circulate. Ascensión also published descriptions of the insular geography in Juan Torquemada’s Monarquia Indiana (1613) (with the island details curtailed somewhat) and in his own Relación breve of ca. 1620.
The first known maps to show California as an island were on the title pages of Antonio de Herrera’s Descripción de las Indias Occidentales (1622) and Jacob le Maire's Spieghel Der Australische Navigatie (1622). Two early examples of larger maps are those by Abraham Goos (1624) and another by Henry Briggs, which was included in Samuel Purchas’ Hakluytus Posthumus or Purchas his Pilgrimes (1625). In addition to Briggs and Goos, prominent practitioners like Jan Jansson and Nicolas Sanson adopted the new island and the practice became commonplace. John Speed’s map (1626-7), based on Briggs’ work, is well known for being one of the first to depict an insular California.
The island of California became a fixture on mid- and late-seventeenth century maps. The island suggested possible links to the Northwest Passage, with rivers in the North American interior supposedly connecting to the sea between California and the mainland. Furthermore, Francis Drake had landed in northern California on his circumnavigation (1577-80) and an insular California suggested that Spanish power in the area could be questioned.
Not everyone was convinced, however. Father Eusebio Kino, after extensive travels in what is now California, Arizona, and northern Mexico concluded that the island was actually a peninsula and published a map refuting the claim (Paris, 1705). Another skeptic was Guillaume De L’Isle. In 1700, De L’Isle discussed “whether California is an Island or a part of the continent” with J. D. Cassini; the letter was published in 1715. After reviewing all the literature available to him in Paris, De L’Isle concluded that the evidence supporting an insular California was not trustworthy. He also cited more recent explorations by the Jesuits (including Kino) that disproved the island theory. Later, in his map of 1722 (Carte d’Amerique dressee pour l’usage du Roy), De L’Isle would abandon the island theory entirely.
Despite Kino’s and De L’Isle’s work, California as an island remained common on maps until the mid-eighteenth century. De L’Isle’s son-in-law, Philippe Buache, for example, remained an adherent of the island depiction for some time. Another believer was Herman Moll, who reported that California was unequivocally an island, for he had had sailors in his offices that claimed to have circumnavigated it. In the face of such skepticism, the King of Spain, Ferdinand VII, had to issue a decree in 1747 proclaiming California to be a peninsula connected to North America; the geographic chimera, no matter how appealing, was not to be suffered any longer, although a few final maps were printed with the lingering island.