A nautical chronometer made by Thomas Earnshaw (1749–1828), and once part of the equipment of HMS Beagle, the ship that carried Charles Darwin on his voyage around the world, is held in the British Museum. The chronometer was the subject of one episode of the BBC's series A History of the World in 100 Objects. Meticulous naval inventories show that HMS Beagle carried a total of at least 34 recorded chronometers on its three main survey voyages from 1826 to 1843, and 22 on the second voyage with Darwin on board, when they had a dedicated cabin. Some were Navy property and others were on loan from the manufacturers, as well as six on the second voyage owned by the captain, Robert FitzRoy. Both the two known survivors from the second voyage are owned by the British Museum (the second is registration No. CAI.1743). Background Nautical chronometers were of great importance in the 18th and 19th centuries as aids to navigation. Accurate measurement of time was needed for the determination of longitude. Earnshaw was not the first to make such chronometers, but he was one of the first to make them cheaply enough that they started to become essential equipment for a ship at sea. By the time the Beagle set sail, it was being reported in The Nautical Magazine that the price of chronometers was dropping rapidly while the same quality was being maintained. Earnshaws' chronometer had a novel escapement mechanism, the spring detent escapement, and a bimetallic strip for temperature compensation so that it would continue to maintain accuracy in all climates around the world. ===Beagle'''s chronometers=== The Beagle was sent in 1831 on a survey mission which involved circumnavigating the globe, a journey which lasted until 1836 and described by the naturalist on board the ship, Charles Darwin, in his book The Voyage of the Beagle. It was on this journey that Darwin began forming the ideas published much later as On the Origin of Species. The Beagle carried twenty-two chronometers, an unusually large number, but necessary to ensure accuracy of the survey. Three would have been commonplace on ships of the time, as this is the minimum number required to easily identify one that has gone faulty. The Admiralty started a general issue of chronometers to H. M. Ships from 1825, but between about 1800 and 1840 availability could not keep up with demand. The Admiralty