This 6-foot mural circle was first installed at the Observatory in 1812. Full circle measuring instruments became possible and popular after the development of the dividing engine towards the end of the 18th century. Maskelyne, the astronomer royal at the time, first requested a mural circle for the Observatory in 1792. Interest was renewed in the early 19th century when John Pond made measurements from his home in Somerset demonstrating the inaccuracy of the Observatory's current apparatus. This mural circle, the Observatory's first, was ordered from Edward Troughton and installed shortly after Pond became Maskelyne's successor.

A mural circle is a telescope mounted on a circular frame which is then mounted on a wall aligned north-south. The telescope is clamped to the circle and both pivoted at the centre of the circle while the circle's circumference carries a scale from 0 to 360 degrees. This instrument allows the observer to measure the angular height of the star he or she is observing.

Telescope: The telescope is clamped to the circle at both the object glass end and the eye end. The object glass has an aperture of 102mm (4 inches). The tube length is the same as the diameter of the circle, 1.83m (6 foot).

Stand: The mounting is made up of a 6-foot circle rotated on a conical axis. The circle was originally mounted on a pier now the east pier of Airy's Transit Circle. Today the circle is mounted on the wall between the Bradley transit instrument and the Airy Transit Circle. The circle was read by 6 micrometers spaced at intervals around the circle. The micrometers are now missing.