Thomas Earnshaw

Thomas Earnshaw was a London chronometer maker who, alongside John Arnold, is credited with bringing the spring detent escapement to the simplified, robust form that made the marine chronometer a reproducible commercial instrument rather than a unique masterpiece. Trained in the London trade, Earnshaw worked for a period finishing movements for other retailers before establishing his own name, and he pursued Board of Longitude recognition and reward for his chronometer work through the 1790s and 1800s.

Earnshaw's escapement design, and his cheaper, faster method of producing compensated balances, allowed chronometers to be made in greater numbers at lower cost than the earlier bespoke instruments of Harrison's generation, a development of real practical consequence for the Royal Navy and merchant shipping during the Napoleonic era. His long and bitter public dispute with John Arnold (and after Arnold's death, with Arnold's son) over priority in the spring detent escapement is among the best-documented priority disputes in English horology, aired partly through Earnshaw's own polemical pamphlet, Longitude: An Appeal to the Public.

This encyclopedia treats the Arnold–Earnshaw priority question as genuinely contested rather than settled; both men's supporters have argued the case, and modern historians generally credit both with distinct, overlapping contributions to the same practical goal rather than assigning sole invention to either.