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Zenith Telescope - click to enlarge

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Zenith Telescope

Catalogue number:
PH*316660

Inscriptions:
"Troughton & Simms, London, 1849" and "U. S. C. S. Z. T. No. 4"

Dimensions:
telescope 3.25 inches aperture, 46 inches long; horizontal circle 10.5 inches diameter; vertical arc 5.5 inches radius

Discussion:

This is the last of the 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 1849. When the Survey agreed join in the variation of latitude (polar motion) program organized by the International Geodetic Association and found that no other instruments were available, it decided to have zenith telescopes No. 2 and No. 4 "remodeled at the Survey Office." Edwin Smith, chief of the Instrument Division, explained that "every precaution" was taken to make these instruments "as perfect as possible under the circumstances." The new features included: base and leveling screws; vertical axis; wyes for horizontal axis of telescope, with adjustment for level; larger horizontal axis for the telescope; new micrometer screw and reconstruction of micrometer box and slide; improved clamp to telescope; two fine levels attached to telescope; striding level for the telescope axis. In addition, the whole instrument was polished and bronzed, and provided with electric lamps and batteries "for illumination of telescope field, the reading of levels, etc." Zenith telescope No. 2 was used in Hawaii in 1891-1892, while zenith telescope No. 4 was used at Rockville, Md.

Ref: [E. Smith], "On the Variation of Latitude at Rockville, Md., as Determined from Observations Made in 1891 and 1892 in Cooperation with the International Geodetic Association," United States Coast and Geodetic Survey Report for 1892, Appendix No. 1, pp. 4-5, with illustration.

Further Information:

Zenith Telescope
Troughton & Simms

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