Instrument and photograph courtesy of Dale Beeks.
"Apparatus for Dip and Intensity and variation of the Magnetic needle have been given to me in the most handsome manner from the Colonial Office."30
At the beginning of Douglas's stint in London, he was received with remarkable acclaim for the relatively unschooled son of a village mason. Yet he soon chafed at the restrictions of what was at that time a very strict class society, and focused instead on expanding the scope his work. The writings of Alexander Mackenzie, George Vancouver, and Lewis and Clark, combined with the memories of cartographer David Thompson's crew, provided Douglas with a deep appreciation for the role that careful surveys had played in these explorer's accomplishments. To match them would require that he learn some basic skills, and in London, quite by chance, he found a willing teacher.
Edward Sabine, the younger brother of Douglas's boss at the Horticultural Society, had served as an officer in the Royal Navy during the War of 1812 before turning his attention to the study of physical geography and the properties of terrestrial magnetism. As astronomer on Captain John Ross's first Arctic expedition in 1819, Sabine developed a particular interest in the problems of finding magnetic north.
David Douglas shared lodgings with Sabine during his stay in London, and in 1828 began assisting him in his geomagnetic measurements. When the naturalist secured approval for a second collecting trip to the Northwest, he reached out to Sabine for some practical advice. "While preparing for his departure in the summer of 1829, I heard [Douglas] frequently express his regret that his limited education prevented his being able to render those services to the geographical and physical sciences," Sabine later wrote. "He spoke with particular regret of his inability to fix geographical positions."28
Recognizing Douglas's combination of boundless physical energy with "vigor of mind,"29 Sabine offered to serve as his friend's tutor during the three months that remained before his scheduled departure. At the Royal Observatory in Greenwich, the two fell into a regime that echoed Thomas Jefferson's concentrated training of Meriwether Lewis at Monticello. Based on his own practical experience, Sabine had a sense of how to feed his pupil "just so much knowledge of plane and spherical trigonometry, and of the nature and use of logarithms, as was essential for his practical purposes. . . . For eighteen hours a day he bent all the powers of his mind to overcome difficulties, for which his previous education and habits had so little prepared him."31
Dolland Compass and Case
Instrument and photograph courtesy of Dale Beeks.
Thomas Jefferson, David Thompson, and David Douglas all obtained their surveying instruments at the London shop of Dollond & Son
Because of Sabine's deep interest in the earth's magnetic properties, Douglas received a very individual mode of astronomical instruction. The captain demonstrated how iron bars suspended from a tripod allowed an observer to measure "the relative intensity of magnetic attraction in different parts of the earth's surface."32 He acquainted Douglas with the use of the dip circle (fig. 14), a compass supported on gimbals that pivots on a plane to reveal the angle the magnetic field makes with the vertical. He also taught Douglas how to hang a magnetically charged needle from a tripod, then watch as Earth's magnetism made that needle vibrate. In London, such a needle suspended from a tripod on asbestos thread twitched exactly one hundred times in three hundred seconds. By measuring slight differences in the frequency of those vibrations at other locations around the world, Sabine had begun to describe sweeping "isographic" curves that covered the earth in exactly the same way "that iron filings arrange themselves around a magnetized iron sphere."33
Sabine combined these experiments in magnetism with a thorough knowledge of standard nautical measurements, including the use of accurate temperature and dew point readings to determine elevation, as well as the intricate trigonometry necessary to calculate latitude and longitude. In addition to the mariner's sextant (fig. 16), Sabine taught Douglas to use a repeating reflecting circle—a larger, heavier instrument that contained two reflecting mirrors and a full circle rather than sixty degrees of arc. The teacher soon felt confident that with a few weeks of steady instruction, combined with the better part of a year to practice on the outbound ship, his pupil would be fully competent "to undertake a variety of determinations which might render his mission important to other branches of science besides those of natural history."34
With a Pacific Northwest boundary settlement between British Canada and the United States pending, the Colonial Office in London took an interest in Douglas's newly acquired skills, and supplied him with a stipend to acquire both surveying manuals and a full set of secondhand but serviceable equipment. Edward Sabine, with a more sophisticated set of tools in mind, took Douglas to the shop of Dollond and Sons, the family that had supplied instruments to both Thomas Jefferson and David Thompson. Douglas's final list included not only both a quality sextant and repeating reflecting circle, but also a Dollond compass, dip needles, and dip circle; a tripod, magnetic bars, and the special asbestos thread to hang them on; two state-of-the-art chronometers; plus a thermometer, barometer, hydrometer, and hygrometer. The entire package weighed enough that in the field Douglas required an extra man to carry his equipment, and the competent use of such instruments would prove a daunting task for a novice. But then, his teacher had very lofty goals in mind.
Alexander von Humboldt had recorded anomalies in the earth's magnetic fields during his South American expedition of 1799-1804. Soon after his return to Germany, he had published a paper that, drawing on the work of astronomer Edmond Halley, introduced the notion of a map that described these fluctuations with parallel "isolines." Over the next few years, Humboldt developed a plan to employ scientists worldwide to collect the data necessary to chart them on a global map. By 1829, Edward Sabine was coordinating Humboldt's project from the Royal Observatory in Greenwich. He assigned David Douglas to cover a significant portion of North America, and expected the naturalist to make a very real contribution to the undertaking.
28. Edward Sabine, "Notes on the geographical observations made by David Douglas on the Pacific Coast of North America." MS-0623 (1837), 2. Archives of British Columbia, Victoria.
29. Ibid., 6.
30. Douglas to William Jackson Hooker, October 27, 1829. Papers. Lindley Library, Royal Horticultural Society, London.
31. Sabine, "Notes," 6.
32. Ibid., 4.
33. Edward Sabine, "Observations on the Magnetism of the Earth . . . ," American Journal of Science and Art, vol. 17 (1830): 146.
34. Ibid., 147.