Ship's chronometer from HMS Beagle

This chronometer was used on a number of Royal Navy ships during the nineteenth century, but perhaps most famously it was among the timekeepers on board HMS Beagle in 1831.

Also on board was Charles Darwin, whose observations made on this momentous voyage would lead to his theory of evolution.

It was made by Thomas Earnshaw (1749–1828), originally a watch-finisher and escapement maker of extraordinary skills.

Having spent time in a debtors’ prison, by 1780 Earnshaw was running a successful business which allowed him to concern himself with new technology for marine timekeepers to improve their performance. It was perhaps Earnshaw, and his rival John Arnold (1735/6–99), who can be credited with creating an inexpensive marine timekeeper that was simple to make but reliable and accurate enough to be used as a longitude finding instrument.

This chronometer is a one-day duration timekeeper and also contains a temperature compensation balance. This balance was designed to keep the timekeeper running at a constant rate in the varying ambient temperatures encountered at sea.

It is quite a simple mechanism and uses Earnshaw’s spring detent escapement. This technical innovation was also claimed by John Arnold. It became almost universally used in marine chronometers.

Whilst the mechanisms within chronometers were ‘simple’ they were made to very exacting standards and the escapement makers, finishers and adjusters were among the highest-skilled in the clock-making world.

The dial indicates hours, minutes and seconds. The whole machine is housed in a simple wooden box with mounts to keep it level irrespective of the motion of the ship.