The 1730 edition of Sir Edmund Halley's seminal map of the World, the map to include isogonals and the first edition of Halley's map to include wind directions.
Edmund Halley's great wall map of the World is one of the most important world maps of the 18th Century. The map is the first world map to show isogonals, or lines showing equal magnetic variation in the oceans, a feature considered of prime importance for determining longitude.
Edmund Halley (1656-1742), follows Flamstead as England's second Astronomer Royal, "the greatest of European astronomers, and next to Newton among the finest scientific Englishmen of his time." Halley is best remembered for the comet that bears his name. His knowledge of celestial phenomena caused him to be sent to the island of St. Helena to make the first scientific determinations of the positions of the stars in the Southern Hemisphere. Upon his return to England in 1678, Halley was elected a fellow of the Royal Society. In 1703, Halley was appointed Professor of Geometry at Oxford and, in 1721, he was made Astronomer Royal at Greenwich.
Halley's most celebrated work, his synopsis of the movement of comets, was published in 1706, and it was here that he correctly predicted its reappearance at seventy-six year intervals of the comet by which he is best remembered. In 1698, in the hope that Halley might be able to solve the longitude problem, he was made a Captain of the Royal Navy and was sent to sea aboard the Paramour at the command of William III. His primary mission was to chart the variation of the Earth's magnetic field in the Western and Eastern hemispheres. He returned in 1700 and this map is the product of his measurements.
The lines on this chart, called isogonals, depict the declinations of the compass. This is the first map on which they appear. As Rodney Shirley notes, thus the isoline, or line of equal [magnetic] value was invented. For more than a century Halley's magnetic lines were a familiar feature of the world chart. It was later learned that the variations fluctuate with time and cannot be used to find longitude at sea.
The Ottens brothers, Reiner and Joshua, operated a successful printing partnership in the mid-eighteenth century (fl. 1726-1765). They began the venture in 1726, publishing maps and other prints as “R & I Ottens.” They specialized in the reprinting of others’ work, especially Guillaume De L’Isle. In 1750, Reiner died; his soon, also Reiner, took his place, but the firm began listing their works as “Joshua & Reiner Ottens.” The firm lasted until Joshua’s death in 1765. Joshua’s widow, Johanna de Lindt, sold their remaining stock of plates in 1784.