Chronometers were formerly used for the accurate determination of longitude by ships at sea. By measuring the time of local solar noon compared to the time of noon at a reference point the difference in longitude can be directly found. For this system to work, a timepiece showing the time at the reference point must be carried to the measuring point. A timepiece intended to remain accurate while subjected to the motions of a ship at sea and through extreme changes in environment, especially temperature, is called a chronometer. These were first built in the 18th century and were used extensively by mariners in the 19th century and into the 20th century, even after the widespread use of radio for time signals – the time signal was used to set the chronometer, but the instrument was still necessary to display the time.[1]
HMS Beagle was an Admiralty survey ship sent on three major expeditions. The first (1826–1830) was to survey the coast of South America in company with HMS Adventure. The second expedition (1831–1836) was to build on the work of the first in South America and then to go onward to establish a chain of linked reference points encircling the globe. The third expedition (1837–1843) surveyed the coast of Australia.[2]
A chronometer was first carried on a survey ship by James Cook on his second voyage in 1772. This was Larcum Kendall's K1, a copy of John Harrison's H4, the first practical chronometer. From 1818 the Admiralty began issuing chronometers to naval ships. From 1825, this became standard and ships were issued a second chronometer if the captain personally provided a third. Three chronometers was the minimum necessary to be able to identify whether one had become unacceptably inaccurate. By the time of the Beagle voyages, the use of marine chronometers had become routine and the ship carried a large number – an unprecedented 22 on the second voyage. This had become possible because the price of chronometers had decreased steadily as manufacturing increased. H4 had cost over £20,000 (inflation adjusted £3,870,000) to develop. Kendall's K1 cost £500 (now £88,000) and his cheap model, K3, cost £100 (now £15,900), but by the time the Beagle voyages were over the cost of a good chronometer had fallen to under £40 (now £5,000).[3]
Beagle required large numbers of chronometers because some would inevitably break down on long voyages and the chronometers were essential for the mission of the ship. Further, the rates of all chronometers vary with time. Although this can be allowed for by interpolating between the regular rate checks, there is no guarantee that the changes are linear, and often they are not. Averaging the readings from a large number of chronometers, especially if they are of a variety of types, will tend to cancel out such errors.[4][5]
To reach all points designated by the Admiralty for longitude measurements, it was sometimes necessary to take chronometers ashore, across inlets and up rivers too shallow for the ship. This put the chronometers at risk and the disturbance of moving them affected their accuracy. This was another reason for survey ships to carry a large number – most of them could be kept permanently in a safe, well-cushioned place on the main vessel.[6]