This form of sextant was introduced in the 1790s and was still popular in the
middle decades of the nineteenth century. W. & S. Jones described it as being "so framed, as not be liable to warped or strained, with adjusting screws, telescopes, and other auxiliary apparatus, the most proper for taking distances accurately, to determine the longitude at sea, &c." This example belonged to Colby College in Waterville, Maine, and may have been acquired when that school was founded in 1813. The silvered scale is graduated every 10 minutes from 0o to +130o and read by vernier with tangent screw and swinging magnifier.
Ref: George Adams, Geometrical and Graphical Essays, 2nd edition
revised by William Jones (London, 1797), pp. 271-284.
Encyclopaedia Britannica (Edinburgh, 1797), vol. 12, art.
"Navigation," p. 737.
W. & S. Jones, A Catalogue of Optical, Mathematical, and Philosophical Instruments, appended to G. Adams, An Essay on Electricity (London, 1799).
