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.

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index of the valve up; the key will run in on the axis of the barrel, when the clock is wound, the key is only to be withdrawn till a circular scribe on the shank becomes visible - then the index of the valve is to be brought back to its downward position, before the key is to be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".

Last Revised: 2009 November 5th