Marine chronometer no. 12 by John Arnold

John Harrison's timekeeper, now known as H4, was tested at sea in 1761-62 and provided the solution to finding longitude at sea. This opened the way for others to perfect and simplify the marine timekeeper. One leading clockmaker who devoted considerable effort to this task was John Arnold (1735/6-99). It was perhaps John Arnold and his rival Thomas Earnshaw who can be credited with creating a marine timekeeper that was inexpensive and simple to make but reliable enough to be used as a longitude finding instrument.This particular machine, signed 'John Arnold Invt et Fecit London No 12', is thought to be one of the earliest to contain John Arnold's newly invented version of the spring-detent escapement. This was a technical innovation also claimed by Thomas Earnshaw and one that was to become almost universally applied to marine chronometers. The chronometer is a one-day duration timekeeper and also contains a temperature compensation balance of John Arnold's invention. The balance was designed to keep the timekeeper running at a constant rate in the varying ambient temperatures encountered at sea. The simple white enamel dial is designed to show the hours, minutes and seconds very clearly and the whole machine is housed in a glazed octagonal mahogany box to protect it from the elements.When this machine was made the ship's master or captain would have had to purchase it with his own money. Nevertheless, these simple machines soon became an essential part of the navigator's equipment.