Astronomical Clock by J. Crosthwaite

Astronomical Clock by J. Crosthwaite, Dublin. c.1780s.

The clock was constructed for Dr Ussher of Dunsink Observatory. On Ussher's death it was purchased for Armagh Observatory for 40 guineas. The clock has rubied pallets, an eight-day movement, and rated at mean solar time. This clock was attached to a stone pillar in the passage to the transit room. (see Earnshaw's Astronomical Timepieces, M129)

From "A Report on the Precision Clocks at Armagh Observatory" by Jonathan Betts, 1989.


8 day Irish regulator in mahogany case by John Crosthwaite, Dublin, C. 1785.

Maximum external dimensions (Cms): Height: 179 Width: 42 Depth: 23

Six pillar movement with four wheel train, with pinions of twelve, and jewekked dead beat escapement. Harrison's maintaining power. Separate cast iron mounting for spring pendulum suspension, mounted on backboard. The pendulum itself now missing, The movement has had brass plates, sliding in groves across the edges of the plates of the movement, to protect the movement from dust and dirt. All three are now missing. The movement is secured to the seatboard with two hooks.

12.5 inch square silvered brass dial with conventional hour and minute hands (plain blued steel pokers), and Roman hour numerals with Arabic five minute figures. Signed: "John Crosthwiate Dublin" above the VI. The seconds dial has "observatory marks" at five second intervals, probaly added in the 19th century.

Mahogany veneered, crossbanded and boxwood strung mahogany case with flat top hood. The rectangular trunk door has a shaped top, and the square base has a low skirting around.

Historical Note

This clock was purchased for 40 Gns at the same time or just before the Earnshaw clocks were set up at Armagh, from Dunsink Observatory, on the death of the Director, Dr Usher. It was originally set up at Armagh "against a stone pillar in the passage to the transit room", and was used to maintain Mean Time. According to descriptions from Dunsink, the clock originally had a pendulum condtructed of glass, and suspended on a steel point sitting on a plate of flat diamond mounted on the backboard of the clock. Unfortunately, the pendulum is now missing, (the glass pendeulum probably got broken some time before) and the existing cast iron pendulum support is a later replacement, designed to take a spring suspension. All trace of the original suspension arrangement is therefore now gone unfortunately. A description of the pendulum suspension can be found in the 'Repertory of Arts' of 1795 which took the description from the Transactions of the Royal Irish Academy in 1788. Apparently Crosthwaite also made a Journeyman clock for Dunsink. It is possible the mercurial pendulum, without a jar, now in store is the replacement pendulum for this clock.

Last Revised: 2014 November 10th