Complex sentence: Several other watch makers most notably Thomas Earnshaw had started to copy Arnold's work, the latter around 1780 modifying his detent escapement by mounting the detent on a spring and creating the spring detent escapement.
Split into simpler sentences: Several other watch makers, most notably Thomas Earnshaw, had started to copy Arnold's work. Around 1780, Earnshaw modifyied his detent escapement by mounting the detent on a spring to create the spring detent escapement.