In his own words in his Personal Narrative of Travels…. Humboldt carried "A dipping needle of tweleve inches, constructed on the principles of Borda and Le Noir. ... An azimuth circle serves to find the plane of the magnetic meridian, either by correspondent dips, of by seeking the position in which the needle is vertical or observing the minimum of the dippings. The instrument is verified by observing on the east and west side, and changing the poles." (Alexander von Humboldt. Personal narrative of travels to the equinoctial regions of the New continent, during the years 1799-1804, by A. von Humboldt and A. Bonpland. translated from the French by Helen Maria Williams, 1814, p 36.) Helferich notes in Humboldt's Cosmos that Humboldt made 124 magnetic observations on his expedition.*
Selected on-line examples of early 18th and 19th-century artificial horizons that may be similar to Humboldt's at the Royal Museums, Greenwich, include:
Used to measure the horizontal angle of the Earth’s magnetic field (dip) as it varies with latitude and local mineral deposits etc.
The glazed case is mounted on a cast tripod mount with three leveling screws, as seen in the image above. It has an independently rotating azimuth scale with small handles and a locking screw Brass construction with brushed nickel finish on scales and grey krinkle-enamel. The needle is 4 inches long. Both the altitude circle and the azimuth circle are of 2 inch radius, and are graduated to single degrees of arc. There is a bubble level on the back (see image). The case has a diameter of 6 ½ inches, and both front and back glazed windows may be opened (see image).
An HSU inventory decal ( marked out with red felt pen) is on the top of the case: State of California/Humboldt College/17619. The company logo is just above the index for reading the azimuth scale.
The online inventory of the Science Museum Collections group lists a "Dip circle in glazed case, by W. & J. George & Becker Ltd" with a serial number of 3934 in their Science Teaching collection. There was no image when accessed 7 August 2013. According to the Collections Online web site of the Science Museum Group (London), W & J George and Becker, Ltd. was active from 1944–1954 and was: "Founded from take over of F.E. Becker & Co. by W.J. George Ltd. [ in 1897, the] former continued to trade under the original name until 1950s when W. & J. George & Becker Ltd., was adopted. The company was succeeded by Griffin & George when W. & J. George & Becker Ltd merged with Griffin & Tatlock." **
According to the 1987 HSU inventory the instrument was acquired in December of 1959.
* "Immediately after leaving Europe, Humboldt began taking regular measurements of the earth’s magnetic field, using a kind of vertical compass called a dip needle. During the course of his journey, he made 124 magnetic observations ranging over 115 degrees of longitude and 64 degrees of latitude, also recording in every case the geodetic coordinates, height above sea level, and distance from any mountains or prominent rocks that might influence the results. As he traveled south toward the geographic equator, he noted with increasing excitement a steady decrease in the earth’s magnetic field. Even after Humboldt crossed the geographic equator in Ecuador, the magnetic dip—the angle with which the magnetic needle was attracted downward, toward the earth, continued to decline. But now, as the party traversed the Cajamarca Plateau, he finally registered a dip of zero: He had located the magnetic equator—the line where the vertical component of earth’s magnetic force is zero—at 7 degrees, 27 minutes south latitude and 81 degrees, 8 minutes west longitude. ... Humboldt’s discovery of the magnetic equator on the desolate Cajamarca Plateau was a landmark achievement, not only proving that the earth’s magnetic field varies predictably with latitude but pinpointing the exact location where there is no vertical dip at all. 'I have considered the law of the decrease of the magnetic forces from the pole to the equator as the most important result of my American journey,' Humboldt wrote."
Helferich, Gerard (2004). Humboldt's Cosmos: Alexander von Humboldt and the Latin American journey that changed the way we see the world. New York : Gotham Books (Kindle Location 4263-4280). Tantor eBooks. Kindle Edition.
** http://collectionsonline.nmsi.ac.uk/detail.php?type=related&kv=63431&t=people Downloaded 7 August 2013
© R. Paselk
Last modified 22 August 2013