In his own words in his Personal Narrative of Travels…. Humboldt carried " A suffbox sextant by Troughton, of two inchs radius, with a nonius divided into minutes, telescopes which magnify four time, and an artificial horizon of crystal. This small instrument is very useful for travelers when forced in a boat to lay down the sinuosities of a river, or take angles on horseback without dismounting." (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.) From his description Humboldt's snuffbox sextant was very similar to the one on display, but had an additional small telescope that could be attached. His instrument would also likely have been in the laquered brass standard of the day.
On-line examples of 18th- and early 19th-century Box sextants that may be similar to Humboldt's include:
The Drum or Box sextant works on the same principles as the vernier sextant but the mechanism is enclosed in a small (usually around 3" diameter) brass cylindrical case. The earliest known description of this form of the sextant is by William Jones, a leading London instrument maker, in 1797. The graduation and manufacture of such small instruments was made possible by the use a Ramsden's dividing engine.*
The sextant is used for the precise determination of latitude and longitude by taking the angle between the sun or star and the horizon or the angular distance between celestial objects such as the moon and a star. The instrument comprises a graduated arc of approximately 1/6th of a circle, which is used to measure angles via a mirror of up to 1/3 of a circle. The sextant was developed to find the longitude at sea via the “lunar distance method” about 1770. It is essentially of the same design as an octant (invented by John Hadley in 1731), but the sextant is more precise (and was thus more expensive) and covers a greater angle (1/3 as opposed to 1/4 of a circle). Box sextants remaineded in production with little change until the early 20th century. Some examples, including Humbolt's, had an accessory telescope, that could be used instead of the peephole sight (often attached by a bracket over the sight), though most examples just have a peephole sight as seen in the specimen on display and those in the collections of Greenwich and the Smithsonian.
A 'contemporary' description of the box sextant 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.***
This instrument has no telescope, observations are made via a pinhole with sliding solar filter. It has a black oxidized brass case with lacquered brass furnishings as seen in the photo. The sextant is 2 ¾ inch in diameter with a 1 ¾ inch radius arc. The silvered scale is graduated every 30 minutes from –5° to 150° and may be read to one minute of arc by vernier (see image of scale and vernier). A magnifier on an arm aids the reading of the vernier (see image of magnified scale and vernier). Two additional filters are on levers (see image of filter) that may be moved out of the optical path though a slot in the back of the case (see image of closed and open slot). Reversed and mounted on the back the sextant, the bayonet mounted cover acts as a handle. An image of the sextant with the cover mounted as a handle is shown here. There is a small brass handle on the cover. As seen in the photo, there is a table of natural tangents on a card pasted in the lid. The sextant mechanism with labeled parts is shown in this image.
The instrument has a fitted leather case (see image) that is in good condition except for the strap which has been repaired in one spot with staples.
* F. Barker & Sons was a well known maker of pocket compasses.
** Ramsden's original prize-winning dividing engine is in the collection of the Smithsonian NMAH (it is currrently [July 2013] on display at the Smithsonian Air & Space Museum) and can be seen on-line at: http://amhistory.si.edu/navigation/object.cfm?recordnumber=694508
*** 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 28 January 2014