A marine chronometer numbered 16285, by Thomas Mercer. It has a two-day full-plate fusee movement with four pillars, all fixed with blued screws. The movemetn is marked with the number 16285. The fusee, with stop-work and Harrison's maintaining power, has a pipe round the winding square. There is an Earnshaw-type spring detent escapement and a two-arm balance with helical balance spring.

The movement is mounted in a two-tier mahogany box with glassed top. There is a flip-catch for the upper half, which has a strut limiting opening to 90°. On the front of the upper half is screwed a circular tablet inscribed ‘THOMAS MERCER Ltd / 16285’ with a broad arrow. The box fittings are standard, if minimal. There are no handles on the box. The box has brass gimbals and a ratchet-winding key. The underside of the box is covered with green baize. Inside the upper half is a brass label-holder (empty).

A silvered-brass dial has Roman numerals and a large seconds dial has Arabic ten-second figures with straight batons at alternate five-second intervals. The dial is inscribed in the centre ‘THOMAS MERCER LTD / ST. ALBANS, ENGLAND.’ In the seconds dial is the number 16285 and the broad arrow. The up-and-down dial, below XII, is marked with Arabic numerals in eight-hour intervals up to 56 and is marked 'UP' above zero and 'DOWN' above 56, with 'WIND' instead of 24. The chronometer is in very good condition.

Thomas Mercer is one of the great names in the history of the marine chronometer, being one of the most prolific makers during the second half of the 19th century and making chronometers for many of the famous retailers across Britain. Over the years, Mercers submitted many chronometers successfully to the Greenwich trials and exhibited at many of the International Exhibitions. Born in Lancashire, Mercer was trained as a watchmaker but moved to London around the middle of the century to work for, and train under, the well established chronometer maker John Fletcher, setting up on his own in 1858 in Clerkenwell and moving to St Albans in 1874. The firm prospered for well over a century at St Albans, manufacturing chronometers and precision instruments long after all other British makers had gone out of business. In recent years the chronometer and clock making wing of the company has changed hands several times and is now based in London.