This octant has an ebony frame, reinforced brass index arm, and telescopic
sight. The ivory scale is graduated every 20 minutes from -2o to +111o and read by vernier and tangent screw to single minutes of arc. This instrument was presumably made after 1857 when the Blunts began offering nautical instruments "of American manufacture," and before 1866 when the firm became Blunt & Nichols.
A brass nameplate reads: "George Davidson. Philadelphia, Pa." George Davidson (1825-1911) was a surveyor, geodesist and astronomer affiliated with the U.S. Coast Survey.
Ref: Deborah J. Warner, "American Octants and Sextants: The Early
Years," Rittenhouse 3 (1989): 86-112, on 91-92.
"George Davidson," American National Biography, vol. 6, pp.