This is one of four zenith telescopes that Troughton & Simms made for the United States
Coast Survey, and that was used for the determination of latitude by the Talcott method. It
arrived in the United States in 1848. In 1891, in preparation for the variation of latitude (polar
motion) program organized by the International Geodetic Association, this telescope was
extensively remodeled. The new features included: an objective lens made by Brashear; diagonal
and straight eyepieces made by Kahler; two levels; a striding level; a micrometer with horizontal
and vertical parallactic motions; a larger axis for the telescope; a and heavier base and vertical
axis; wyes to support the telescope; and an improved clamp for the telescope.
Ref: C. A. Schott, "The Variation of Latitude at San Francisco, Cal., as determined from
Observations made by George Davidson, Assistant Coast and Geodetic Survey, Between May,
1891, and August, 1892," United States Coast and Geodetic Survey Report for 1893, Appendix
No. 11, p. 449.