A box (or pocket) sextant works on the same principle as a traditional
sextant, but here the mechanism is enclosed in a brass box of about 3 inches
diameter. William Jones, a leading instrument maker in London, introduced the
form in 1797. The German explorer, Alexander von Humboldt, had an early example
that he described as being "very useful for travelers when forced in a boat
to lay down the sinuosities of a river, or take angles on horseback without
dismounting." By the mid-nineteenth century, box sextants were said to be
particularly useful for military reconnaissance. They were still available in
the early twentieth century.
Ref: William Jones, "Description of a New Pocket Box Sextant," in
George Adams, Geometrical and Graphical Essays, 2nd ed. by
William Jones, (London, 1797), pp. 283-285.
Alexander von Humboldt, Personal Narrative of Travels to the Equinoctial
Regions of America During the Years 1799-1804 (London, 1822).