Thomas Cummins Senior, London, No.18-23
A full description of this rare chronometer has been provided by Jonathan Betts, Curator Emeritus, Horology, author of 'Marine Chronometers at Greenwich' and is reproduced verbatim herewith:

The movement:
Full plate, four pillar, chain-fusee movement, jewelled to the fourth wheel and engraved on potence plate: "Thos Cummins INV.T, London, No.18-23" with a degree scale engraved under the balance for reading amplitude. The movement has later (probably mid-19th century) machine-spotting over the surface. The fusee has an early version of the 'sliding bar' stop-work, where the end of a spring-loaded bar under the fusee cap is pushed in by the last turn of the fusee chain at the top of the fusee, the other end of the bar then meeting a fixed post on the potence plate of the movement. Over-sprung two-arm bimetal compensation balance with separate bimetal rim segments clamped at ends of the arms, with brass timing and compensation nuts, and terminating in brass spherical compensation weights. Blued steel helical balance spring with terminal curves. Special spring detent escapement with duplex escape wheel, combining the advantages of both Arnold's and Earnshaw's escapements. The locking is on the lower set of teeth, with the detent in tension, moving towards the wheel centre when unlocked, as in Arnold's escapement. However, the impulse is delivered on the tips of the upper escape wheel teeth, with minimal sliding on the impulse pallet, as in Earnshaw's.

The dial:
The regulator-type silvered-brass dial has blued steel hands and is signed as per the movement. It has the seconds dial above the centre, marked with batons at five-second intervals for clear reading, and a 24 hour hours dial below the centre, with XII noon at the top, and marked A / M / P in the upper half, for Ante / Meridian / Post, and with a small engraved star in the lower half of the dial representing the night hours of GMT.

The case:
The movement is contained in a brass bowl intended to be tightly dust-sealed and has the convex glass in a fixed bezel over the dial (secured with six screws) originally with a baize seal at the junction. The extended winding square projects through the rear of the brass bowl, but this was originally through a form of 'stuffing box' to ensure a good dust seal. Holes in the winding square and key suggest the winding key was originally permanently attached to the square. The brass bound mahogany box contains standard brass gimbaling.

This chronometer, No.18-23, was sent to the Royal Observatory Greenwich in the Spring of 1824 to take part in the Premium Trials that year. It remained on trial until October when it was withdrawn, having too large a mean daily rate. It returned to Greenwich the following year, but was not purchased by the Admiralty (photocopies of the published rates are with the lot).

Thomas Cummins Snr, son of Thomas Cummins, a carpenter of Wey Hill in Hampshire, was apprenticed to Christopher Beck, watchmaker of Bell Alley London on 24 July 1799. He was made free of the Clockmakers Company on 13 October 1806. He took on his son, also Thomas, as apprentice on 6 July 1829 at 3 Holland Street Blackfriars, and another son Charles Cummins on 9 January 1832, who would in the 1840s be established as a watchmaker in 148 Leadenhall Street, London. Clutton & Daniels recognise Thomas Cummins Senior as an exceptionally fine watchmaker, and one who is known to have employed a version of Massey's lever escapement in his watches in the early 1820s. It would appear that the clever form of the spring detent escapement seen in this chronometer was also developed by him - it is probably the invention referred to in the "INV.T" associated with his signature. An identical, but later example of this escapement is found in a mid-nineteenth century two-day marine chronometer by Edward Watson of London, though it is uncertain whether there was any connection between him and Cummins. In the early years, Cummins' double numbering system for watches and chronometers seems to be based on the serial number and year, hence this instrument would be the 18th and made in 1823, but this does not remain consistent in his later numbering so there remains some uncertainty.
The case 19cms (7.5ins) wide