Thirty-Hour Marine Chronometer, No. 574

This chronometer is most likely one of the first chronometers made by the partnership of John Roger Arnold and Edward John Dent. It is a classic Arnold chronometer but lacks any of the Dent features developed by him in the following decade. This chronometer includes Arnold’s father’s spring detent escapement and blued steel helical balance spring, as well as Harrison’s maintaining power. The movement also has an unusual “U+O” balance, a feature John Roger Arnold patented in 1821. This temperature-compensating balance is comprised of a steel ring and three small, movable weights. It was used later by Charles Frodsham (1810–1871) who took over John Roger Arnold’s business after his death in 1843.