The image below shows an Astronomical Transit sitting on what appears to be a wooden box covered with material made in the same manner as the stone at Calais.  I am sure that some of these were made from concrete at hundreds of Coast Survey observatories around the world.  Yes, concrete did exist in 1857, there is a concrete pad, or rather 'beton' under the Epping East Base stone. Are there any Astronomical Transit's still in existence?  Probably not, as they were probably modified into Davidson Meridian Transits.  The Davidson transit could also make star observations for Latitude, negating the need for the Zenith Telescope such as used at Calais.  The Coast Survey was very frugal, and often times instruments were built and rebuilt as technology changed. The Davidson Meridian Instrument was replaced later by Bamberg and Wild T-4 astronomical instruments, so called 'broken back' instruments, as you looked into the end of the trunnion to a mirror which was in the main telescope I believe that the crank below may have been used to 'transit' the instrument, that is, flip it around 180 degrees so that reverse readings could be made.  Obviously, when a star passed the Calais Meridian, you could not 'transit' the instrument and observe on the same star.  Therefore some stars were observed in Direct mode, and some in Reverse mode.  Note the two oil lamps that shown via mirrors into the telescope tube. The Zenith Telescope shown here, No. 4, IS the one used at Calais in 1857.  Latitude observations were not done in 1866, that was just the serious business of finishing the longitude work.  The Smithsonian may have only the photograph. . Also attached are some images of an Astronomical Transit in the Smithsonian, again showing the crank apparatus on bottom. I tossed in one photo of Skip Theberge and the Wild T-4, the last instrument used by C&GS for astronomic observations.  This replaced the Bamberg Transit. It would be interesting if the Smithsonian would loan the Zenith Telescope No. 4, and an Astronomical Transit in the future, perhaps at the dedication of Meridian Park.........sorry folks, I have yet to find a good image of a Hardy Clock, but there has to be some Chronographic Registers out there. Please look for 3 indentations in the top of the stone that would accommodate the footpads of the Zenith Telescope.  They should be equally spaced around a circle. The Astronomic transit should have 4 indentations.  Both instruments were probably anchored down with plaster of paris. They might have made some small drill holes to help hold the instrument in place.

Geodetic Transit   (Astronomic Transit)A
Catalogue number: PH*307210

Inscriptions: "Troughton & Simms, London, 1849" and "U. S. C. S. No. 6"

Dimensions: telescope 3 inches aperture, 46 inches long; striding level 26 inches

Discussion: Troughton & Simms made this instrument for the United States Coast Survey in 1849. It has a double-cone horizontal axis, two lamps for illuminating this axis, a large striding level, two small vertical circles attached to the telescope tube, and a solid iron base. By 1880, The Coast Survey had added the eccentric cam apparatus to the base, to facilitate the reversal of the telescope.

Ref: "Determination of Time by Means of the Transit Instrument," United States Coast and Geodetic Survey Report (1880): 205-227 and pl. 62.

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.