RGO6/22/99: Royal Society Council minute for 29 July 1819: 'Resolved that the Council do recommend the purchase of Mr Johnston's clock for the use of the Royal Observatory, the apparatus relating to the length of the seconds pendulum which accompanied it having been paid for by the pendulum committee.'
An undated inventory (RS MS/372/171) implies the following clock by Johnson was acquired in 1819. (This appears to be confirmed by a Royal Society Council minute for 18 December 1819 in RGO6/22/100)
'An eight day clock by Johnson with a microscope & apparatus by Mr Dollond for trying experiments on the Seconds Pendulum'
This may have been the clock by Johnson that replaced one by Molyneux & Cope as the Transit Clock on 28 November 1822. The clock Johnson was replaced as the Transit Clock by the clock ‘Hardy’ on 4 November 1823.
Bill on account 'To Grimaldi & Johnson for an 8 day regulator £120' (RGO6/22/100) forwarded on 27 Jan 1820 to be paid by the Admiralty.
Note: Bill on account for £26. 12s. to Johnson ordered to be paid in 1826 (RGO6/22/174).
1840 inventory described as 'clock Grimaldi & Johnson' and listed as having been moved to the magnetic building in May 1838 (RGO6/54/75)
See also RGO39/1/19/(20) for inventory for same year (clock set to sidereal time, pendulum a replacement)
In Upper Magnet Room and Ante-Room in 1893 (RGO7/67) set to sidereal time.
In Upper Magnet Room in 1909, at junction of eastern and southern arms, but no longer in use (ceased to be used when upper declination magnet and its theodolite were removed) (Results of the Magnetical and Meteorological Observations made at The Royal Observatory, Greenwich, in the year 1909 p.iv).