List of chronometers on HMS Beagle
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.
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.
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 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.
[22] On its first voyage (1826–1830), the Beagle accompanied the Adventure, which led the expedition under the command of Phillip Parker King.
His mission was to chart the coast of South America from the southern shore of the Río de la Plata to Tierra del Fuego.
King was also ordered to use his chronometers to accurately determine the longitude of several points, mostly Atlantic islands, on the outward journey to South America.
It was the custom for Navy chronometers to be run for six months or so at the Greenwich Observatory and the rate checked before issuing to a ship.
Since the rates were regularly checked and variations allowed for in the calculations, the magnetic effect of the ship was automatically taken into account.
However, its manufacture was only completed two days before the Adventure set sail, leaving no time to check and settle the rate.
It was used by King as a journeyman watch, transferring readings to and from the place of measurement, which allowed the principal chronometers to be left undisturbed.
Secondly, he was to establish a chain of reference points with known longitudes that could be used by future mariners to accurately set their chronometers.
[33] Using chronometers, the crew of Beagle succeeded in providing a chain of longitude distances that completely circled the globe for the first time.
The sum of the differences in local noon between each location should have amounted to 24 hours since the Beagle completed a circumnavigation of the globe.
The total discrepancy was only 33 seconds but Fitz-Roy considered this too large to be explained by chronometer inaccuracy alone and none of his results disagreed excessively with earlier surveys.
He suggested that sailing mostly in the same direction relative to the Earth's magnetic field was having a small unexplained effect on the chronometers.
Others were lent by chronometer makers, who were keen to gain the prestige of having had instruments aboard major, well-publicised expeditions.
Fitz-Roy employed an instrument maker, George James Stebbing, to look after the chronometers and ensure they were regularly and properly wound.
The Admiralty conceded that Stebbing could be fed from the ship's stores – a favour not extended to Darwin who paid £500 (inflation adjusted £57,300) for his keep.
The first wave slowed the ship sufficiently that she could no longer make way and the second turned her broadside to the third.
However it is quite clear that the chronometers remained a key part of the mission; frequent stops were made throughout the voyage to check the rates.
Despite his wounds and profuse bleeding, Stokes considered the chronometer to be valuable enough to save; he carried it while he staggered back to his comrades barely able to walk and still being pursued by his attackers.
