"You don't get a three-and-a-half-year circumnavigation voyage out of thin air," she says. "The antecedents are in two basic directions – one is the hydrographic works that's been done by especially British and American hydrographers." The US led the way in terms of deep sea until the 1860s. Then the Royal Navy, no longer having to support troops in Crimea, filled the vacuum left when the US fell into civil war.

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