Transit Clock by Thomas Earnshaw

The transit clock, Earnshaw's first ever long-case clock, is described by him in his "Appeal to the Public" in 1808, where he states his claim to a national reward for his contribution towards the solution of the determination of longitude problem. In manufacturing the clock, Earnshaw employed devices used in chronometer making, among which were the use of high numbers, small teeth, large amount of jewelling, and a small angle of escape, 0.5 degrees in the (Graham) escapement. On the advice of the then Astronomer Royal, Revd. N. Maskelyne, he made the case as nearly air-tight as possible. The clock originally had a 9- bar gridiron pendulum of alternate steel and brass rods which was replaced in 1830 by a mercurial pendulum made by Mr Sharp, Sen., of Dublin. Compensating barometers were added to the pendulum in 1832 to endeavour to correct for the effects of varying atmospheric pressure. These barometers were removed in 1835.

Earnshaw Clock Movement

Extracts from M129: "The clock has an 8-day movement, with finely jewelled pallets and pivot holes. The winding is effected through a valve in the glass front, which when its index points downwards is closed air-tight; when the key with a bit of buff leather on its shank is put into the winding hole, and a compressing spring from the case brought to bear on it by turning the index of the valve up; the key will run in on the axis of the barrel, when the clock is wound, the key is only to be withdrawn till a circular scribe on the shank becomes visible - then the index of the valve is to be brought back to its downward position, before the key is to be intirely removed. The case is made extremely strong and with great care and attention to make it as airtight as possible - which is effected by laying all the necessary opens with waxed cloth, and screwing all home by a great many screws with milled heads".

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


8 day English regulator in hermetically sealed mahogany case, by Thomas Earnshaw,London, cl79O.

Maximum external dimensions (cms): Height:195 Width:49.5 Depth:3O.5

Substantial and highly finished, five pillar movement with high quality, four Wheel train with all pivots jewelled with endstones except barrel arbor and hour wheel. (Jewelled holes to front ‘scape and centre). Jewelled dead beat escapement and Harrison's maintaining power. Now with Mercury compensation pendulum with glass jar, and with resilient connection to crutch pin. Separate heavy cast brass pyramidal support for pendulum behind movement, now cut into at the sides. Kg brass cased weight on a double line.

11" square silvered brass regulator dial with Arabic five minute figures and five second figures, and Arabic numerals for the reversed 24 hour dial, this dial signed in the centre:"Earnshaw London .

Counterpoised, blued steel hands. The seconds dial has 'observatory marks' at five second intervals, probably added in the 19th century.

Solid, veneered mahogany case with all openings hermetically sealed with 38 knurled brass screws, closing hood, hood door and trunk door. (Note: Screws for trunk-hood are numbered '1' and screws for hood- backboard are numbered '2', these being engraved on the brass heads of the screws and also painted in gilt on the casework), There is a rotating brass valve in the hood door glass for winding key access to the dial, The rectangular trunk door has a (later) glazed opening showing the pendulum etc. The trunk has bombe sides to accommodate the swing of the pendulum bob. The plain mahogany base has a rect- angular panel and a double skirting below.

Historical Notes

In terms of precision horology, this clock ranks among those of the highest importance and value. The story of how Thomas Earnshaw (1749- 1829) of London came to supply this clock and 'No.2' (the circle clock) to Armagh is well known and well documented in the Armagh archives and in Earnshaw's "Appeal to the Public,.."(1808). Suffice it to say that it was fixed to a solid stone pillar by Earnshaw himself on August 18th 1794, next to the first transit instrument, and it was rated to maintain sidereal time.

However, from a technical point of view there are one or two mysteries as yet unsolved,

One concerns the escapement of this clock which according to early correspondence from the Astronomer Royal to J.A.Hamilton, (16.2,90), was, at the design stage, to be 'detached' (i.e, not a conventional ‘dead beat'escapement, but a more refined, accurate design). Such an escapement, designed by Earnshaw, is shown in his patent of 1784 and this might well have been what he had in mind, But having studied the movement of this clock very closely, and having read the contemporary correspondence, descriptions and 19th century records, I have come to the conclusion that the detached escapement idea was dropped at a very early stage, almost certainly before the clock left Earnshaw's workshop, in early 1791, and that it has always had the present dead beat escapement. There are several unused threaded holes in the back-plate of the clock, but these almost certainly relate to missing parts associated with rigidly attaching the movement to the seatboard and to the pendulum support.

The clock has not now got its original pendulum fitted. Originally it would have had a nine bar gridiron pendulum which, from the description of 1794 in the Armagh archives, is probably that pendulum now in store. Earnshaw's patent of 1784 also shows his idea for fine rating of a pendulum by using a secondary rating ball below the bob; a feature of the existing pendulum and described in the 1794 document.

Curiously there is another identical pendulum on the regulator signed Archibald Buchanan in the cathedral, and it is not known where this pendulum fits in, In my opinion it is not the original for the Buchanan and one theory is that because the first pendulum for the transit clock was under-compensated (we know this from the correspondence) Earnshaw made another, tested in London and sent over, rather than cause the great expense of going over himself to alter/adjust the first. (The transit clock, complete, cost 150gns and Earnshaw's expenses to go and set it up 100gns). If this is what happened, it is curious that no mention is made of it in the records, and that the clock is described as having been running for many years after setting up without interruption.

The clock's present pendulum, described by T.R. Robinson (see RAS memoirs Vol,5 p.125), was fitted on September 11, 1830 by the Dublin clockmaker Christopher Sharpe and had the refinement of barometric compensation. Unfortunately the barometer tubes used for this compensation are now missing from the pendulum. Other alterations were also carried out at this time; a thermometer and a pendulum scale were added inside the trunk (both unfortunately now missing), and the glazed opening was cut into the door so that these could be read without disturbing the clock. The resilient crutch pin was also fitted at this time and the pendulum support was probably cut open at the same time to make access to the crutch easier. This was probably when the extra support brackets on the backplate, and the original solid covering plate, which sat right across the top of the pendulum support and movement, were removed and discarded.

See also:
Thomas Earnshaw
Earnshaw's Second Clock
The Scientific Instruments of Armagh Observatory
A Short History of Armagh Observatory

Last Revised: 2014 November 10th