An exceptionally rare early 19th-century English spring detent chronometer, housed in a specially made silver case. The full-plate gilt key-wind fusee movement features Harrison’s maintaining power, a chased and engraved cock with a bust depicted on the table, and a diamond endstone. It is fitted with an Arnold “Z”-type compensation balance and a freesprung helical blued steel hairspring. The adjustable polished steel stud is fixed to a gilt cock with two steel screws and blued steel washers.

This chronometer employs a very rare form of spring detent escapement, combining features of Arnold’s and Earnshaw’s designs. The detent unlocks toward the centre of the escape wheel in the Arnold manner, with a jewelled locking stone, while impulse is delivered via the flat face of the escape wheel teeth as in an Earnshaw system. Unusually, the impulse is transmitted to a solid sapphire roller by a delicate polished steel escape wheel. The movement is fully jewelled, with screwed-in endstones for the escape pivots.

The signed white enamel dial includes a subsidiary seconds dial, Roman and Arabic numerals, gold hands with heart-shaped tips, and a slotted steel arbor for the seconds hand. The substantial plain silver Irish open-face case bears the engraved Scottish Pollock clan crest and motto, along with Dublin hallmarks for 1820 and the maker’s mark “AON” in a rectangle. The chronometer is accompanied by a fine three-tier mahogany deck box with brass fittings and locking key.