The frame of this sextant is brass. The silvered scale is graduated every 10
minutes from -5o to +175o and read by vernier with tangent screw and swinging magnifier to 10 seconds of arc. There is a small bubble level on the index arm. William Harkness, the American astronomer who introduced this feature, said that it "proved a very great convenience and saved much time and trouble." This example belonged to Columbia University in New York and probably dates
from the 1870s.
Ref: "Report of Professor William Harkness, U.S.N.," in B. F. Sands, ed., Reports on Observations of the Total Eclipse of the Sun, August 7, 1869 (Washington, D.C., 1870), p.30.
Deborah J. Warner, "American Octants and Sextants: The Early Years," Rittenhouse 3 (1989): 86-112, on 108-109.