Mostyn clock, Thomas Tompion

This year-going spring-driven table clock was made by Thomas Tompion for King William III (reigned 1689–1702). It is believed to have been commissioned in 1689, the year of the coronation of William and Queen Mary. On the King's death, the clock was part of the legacy of 'the contents of the King's bedchamber' which passed to Henry Sydney, Earl of Romney, who was Gentleman of the Bedchamber and Groom of the Stole. It then passed by descent to the fifth Lord Mostyn.

The clock is a spectacular work by Britain's most celebrated clockmaker. The case is made of ebony veneer, decorated with applied silver and gilt-brass mounts. The dome bears the Royal shield of arms and Britannia with a shield bearing the combined crosses of St George and St Andrew. At the corners are the rose for England, the thistle for Scotland, and the lion and unicorn.

The dial shows hours and minutes but also has a sector aperture at the top which displays the days of the week, each with a figure personifying its ruling planet. The year duration is achieved by using six-wheel gear trains which are driven by relatively large barrels and fusees. The clock has a verge escapement controlled by a short pendulum. It strikes the hours in passing and has a pull-quarter repeat system (that is, it strikes the last hour and quarter on pulling a cord at the side of the case).

A silver plaque on the dial is inscribed 'T Tompion Londini Fecit' ('Thomas Tompion of London made it').