Humboldt's Latin American Expedition, 1799–1804

Richard Paselk, Curator

Vernier Sextant

In his own worrds in his Personal Narrative of Travels…. Humboldt carried "A sextant by Ramsden, of ten inches radius, with a silver limb, with telescopes which magnify from twelve to sixteen times." How does that compare to the instrument on display, made a hundred years later than his? First, Humboldt's instrument is significanly larger, though most likely of the same accuracy (readable to 10" of arc by vernier). Jesse Ramsden first constructed a dividing engine in 1766 enabling him to divide sextants etc. more quickly and accurately and with reduced radii making them easier to handle. With his second, improved engine in 1776, which divided directly to 10 minutes of arc, (published in 1777 and sold to the Board of Longitude so other instrument makers could have their instruments divided by Ramsden, who maintained control of it) engine division soon became universal, and sextant radii gradually 'standardized' over the eighteeenth century to around seven inches.*

On-line examples of eighteenth-century Ramsden sextants that may be similar to Humboldt's include:

1. at the Smithsonian NMAH: http://amhistory.si.edu/navigation/object.cfm?recordnumber=1060170

2. at the Royal Museums, Greenwich,

   1. http://collections.rmg.co.uk/collections/objects/43428.html

   2. http://collections.rmg.co.uk/collections/objects/43318.html

   3. http://collections.rmg.co.uk/collections/objects/43319.html

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).

Sextant Description

Brass construction with black oxidized frame, index arm, mirror and filter mountings (all now with a light green patina), bright lacquered telescopes and fittings (the lacquer finish is pitted to varying degrees, except in the case of the long telescope in which the finish is entirely missing), and a polished rosewood handle. The hand engraved, 7 3/8” radius, inlet silver scale is graduated from -5°—145° by 10’ of arc. The extended scale, rather than just from 0°-120° as expected for a sextant (1/6 of a circle), is needed to accommodate the vernier. The scale may be read to 10 arc seconds with the vernier scale, using the clamp & screw fine movement, and swing out magnifier. The instrument has three horizon glasses (filters) and four index mirror filters built onto the frame. The long telescope (7 1/4” closed) and short wide field telescopes have complete optics. There is also a short telescope barrel (no optics), a shade tube (eyecup missing?), brass ball tipped adjusting pin, and a folding hand magnifier in a tortoise shell (?) case (lens badly chipped). There is an empty place for a telescope eyeshade. The accessories etc. can be seen in the photo of the sextant in its case. The instrument is engraved "english" at the far left of the scale arc, and G.E. Butler, San Francisco. (a nautical instrument dealer still in business in San Francisco, California in the 1980's) near the center of the arc. The accessories etc. can be seen in the photo of the sextant in its case. The instrument is engraved "english" at the far left of the scale arc, and G.E. Butler, San Francisco. (a nautical instrument dealer still in business in San Francisco, California in the1980's) near the center of the arc.

Case Description

The sextant is in its original hand-dovetailed mahogany case. The hook catches for the lid are chemically darkened brass, while the handle is age polished. The key is a replacement. Inside the green felt lining is water damaged, the hinge has a red oxide patina, and the lid has been owner modified (gouged out) to allow the wide field telescope to remain in place on the sextant during storage.

There were two trade stickers in the lid of the case, one over the other. The upper sticker was removed without damage to reveal an earlier one below:

* This dividing engine is in the collection of the Smithsonian NMAH (it is currently [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 2013, Last modified 19 December 2020