In his own words in his Personal Narrative of Travels…. Humboldt carried " An artificial horizon by Caroché, of plane glass, six inches inm diameter, with an air bubble level, the divisions of which are equivalent to two sexigesimal seconds." (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 35.) Humboldt's artificial horizon was probably very similar to the one on display, however, the frame was most likely made of brass as seen in the examples at the Greenwich museum, and his may have used clear glass though black is more common. Humboldt also lists a mercurial horizon, and examples from Greenwich are included below for the reader's convenience.
Selected on-line examples of early 19th-century artificial horizons that may be similar to Humboldt's at the Royal Museums, Greenwich, include:
Used with sextants when the horizon is not visible, such as with terrestrial sightings. When using an artificial horizon the image of the object and its reflection are brought into coincidence rather than the object and the horizon. Note that the resultant angle read off the sextant is double the actual altitude of the object and must be divided by two. As a result the finest division that can be read is doubled (a division of one minute of arc becomes two minutes of arc) and thus the accuracy of the reading is halved when using an artificial horizon.
A 'contemporary' description of the artificial horizon and its use from Frederick Walter Simms, A Treatise on the Principle Mathematical Instruments Employed in Surveying, Leveling, and Astronomy, Troughton & Simms, London (1834) is available as a pdf here.***
A 5.2” dia x 0.5” thick disk was cut from a sheet black “Carrara” glass. The disk was set with black silicon rubber sealant into a 1/2" thick ‘mahogany’ frame cut with a fly cutter and jig-saw from wood recovered from a motor cycle shipping pallet. Three holes were bored and then threaded with a 5/16 x 24 NF tap to fit three salvaged brass leveling-screws.
The mounted level glass was recovered from a David White WW II balloon transit. Image of artificial horizon with level glass.
*** The complete text of this work is available online via google books as well as in various reprints from vendors such as Amazon.
© R. Paselk
Last modified 13 August 2013