This telescope was erected as a replacement for the unsuccessful 9½-foot zenith telescope that had been set into the rear of the stone pier that carried the Troughton 6-foot Mural Circle in 1812. Its purpose (like that of its predecessor) was presumably to determine the zenith point of the Mural Circle by comparing observations of γ Draconis.
According to Howse (1975), the Zenith Telescope carried the 2¾-inch Object Glass (OG) from Bradley’s 8-foot Transit Instrument which had been dismounted in 1816. Unfortunately, Howse gives no information about the source of his information. He gives the date of its erection as 1816, although elsewhere he states that the OG wasn’t removed from the Transit Instrument until 1820. He gives the Maker as P & J Dollond, London.
Pinning down the true origins of the 8-foot Achromatic Zenith Telescope has proved elusive. An inventory of instruments dated 26 June 1818 is preserved in the Royal Society Archives (RS MS372/170). Compiled for the Visitors by the First Assistant, Thomas Taylor, it appears to show that at that time, the 9½-foot zenith telescope was still in situ and that Bradley’s Transit Instrument, although dismounted, still contained its object glass. Either the date is incorrect (which seems unlikely as the inventory was compiled for the visitors, or Taylor compiled the list in a very sloppy manner as there is no mention of either the α Aquilae or the α Cygni fixed telescopes which were both installed in 1816. A later inventory (RS MS372/171) that appears to date from the 1820s (1824?) states that the 9½-foot instrument had been removed to the Transit Room and ‘an eight feet achromatic substituted’. It also states however that the OG was still present in Bradley’s Transit Instrument. The 1831 inventory (RGO6/54/) mentions a zenith tube by Dollond on the west side of the pier, but no details as to dimensions or type are given. The same inventory also lists Bradley’s Transit Instrument as still being in the Transit Room, but interestingly no longer makes specific mention of its OG. The 15 May 1840 inventory (RGO6/54) compiled by Airy states that the Ys supporting the 5-foot transit instrument being used as a collimator for the 10-foot Troughton Transit Instrument, were ‘said to belong to the old 8-feet transit’, which itself was suspended on the western wall as a relic, but without its object glass, suggesting that Howse may well be correct in most of his assertions about the Zenith Telescope.
To date, no further references to a telescope described as the 8-foot achromatic Zenith Telescope have been found in any primary source or in contemporary publications (such as Greenwich Observations) during research for this web-site.
Although it may have happened earlier, it seems most likely that the telescope was dismounted in 1848 when preparations began to convert the Circle Room into the room for the new Transit Circle. It also seems probable that the instrument was subsequently hung on one of the walls of the Transit Circle Room with the other historic instruments. It should be noted however that although Airy and Christie gave a list of the instruments that had been hung from the walls in their introductions to the volumes of Greenwich Observations no mention was ever made of the 8-foot achromatic.
The instrument is preserved in the collections of the National Maritime Museum (Collection ID: AST0997). The telescope was put on display in Flamsteed House in 1960 and moved to a place close to its original position in the Meridian Building in time for its opening in 1967. It is not currently on display.