This octant was made during the period 1845-1876. It has an ebony frame and reinforced brass index arm. The ivory scale is graduated every 20 minutes from -2o to +107o and read by vernier and tangent screw to single minutes of arc.
Ref: Deborah J. Warner, "American Octants and Sextants: The Early
Years," Rittenhouse 3 (1989): 86-112, on 108.