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.
The history of the 8-foot Achromatic Zenith Telescope is somewhat 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. But all is now what it seems: either the date is incorrect, 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’. To date, no further references to the 8-foot achromatic 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.
Howse (1975) appears to have unearthed more information, though its accuracy cannot be verified. According to him, the Zenith Telescope carried the 2¾-inch Object Glass (OG) from Bradley’s Transit Instrument which had been dismounted in 1816. He gives the date of the telescopes 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 and goes on to say that 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.