During the HMS Beagle's return from its five-year voyage (December 1831 - October 1836), Captain Robert FitzRoy made a deliberate detour from Ascension Island back across the Atlantic to Bahia (Salvador), Brazil, in August 1836 before continuing to Cape Verde and eventually England. This seemingly circuitous routing was driven by FitzRoy's meticulous commitment to hydrographic accuracy and verification of his chronometer measurements.
<-- more -->The Critical Measurement Discrepancy
When the Beagle had first arrived in Rio de Janeiro in 1832, FitzRoy discovered that a difference exceeding four miles of longitude existed between the meridian distance from Bahia to Rio as determined by the French expedition under Baron Roussin and his own measurements via the Beagle. This was not a minor discrepancy—in maritime navigation and hydrographic surveying, such differences could represent significant inaccuracies that would undermine the entire chain of measurements across the globe.
FitzRoy's early response had been pragmatic: he returned to Bahia from Rio de Janeiro in 1832 to double-check his measurements. However, by 1836, when homeward bound via Ascension Island, FitzRoy remained concerned about whether his original measurements or the French measurements were correct. This uncertainty could not be left unresolved, as the primary mission of the Beagle was not primarily scientific exploration (though that was Darwin's contribution) but rather the creation of an accurate chain of meridian distances and determinations of longitude that could be established across the Pacific, Indian and Atlantic Oceans.
Why Chronometer Accuracy Mattered
The heart of FitzRoy's concern lay in his 22 chronometers—instruments that were essential for determining longitude with precision. Unlike latitude, which could be determined relatively easily through astronomical observation, longitude required maintaining extraordinarily accurate time across months and years at sea. FitzRoy had carried far more chronometers than the Admiralty initially recommended; Captain Francis Beaufort had suggested 18 would suffice, but FitzRoy, anticipating potential losses or failures, brought 22, many purchased at his own expense. These chronometers formed a system of redundancy—if one or two began to drift, the agreement among the remaining 20 would reveal the error.
The Verification Voyage of 1836
Rather than proceed directly to England via Cape Verde, FitzRoy took the Beagle back to Bahia and Pernambuco in August 1836 to retake measurements and check the readings of his chronometers. This was not explicitly ordered by the Admiralty, but FitzRoy was an independent thinker who prioritized accuracy above official instructions. The voyage was vindicated: FitzRoy's original measurements proved correct, confirming that his chronometers had been working well throughout the voyage and that the French measurements under Roussin had contained the error. As one source notes with evident satisfaction: "So FitzRoy had been correct, and his extra trip reaffirmed that his chronometers were working well. He must have felt extremely satisfied to have his initial".
The Hydrographic Mission
This detour exemplifies the core purpose of the Beagle's voyage. While Charles Darwin's presence aboard and his subsequent evolutionary theories have dominated historical memory, the Beagle was fundamentally a surveying vessel on a mission to chart South American waters and establish reliable navigational coordinates for global navigation. FitzRoy's willingness to sacrifice time and resources to verify measurements over a four-mile discrepancy in longitude demonstrates the exacting standards of 19th-century hydrographic work and his personal commitment to precision. The "chain of meridian distances" established through such careful measurement would ultimately benefit maritime navigation globally for decades to come.
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