NavList:

There is a detailed but enigmatic passage in _HMS Surprise_ that was discussed on this list a few years back:

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HMS Surprise; Norton 1991 (paperback); pp. 361-362

The instruments stood on a carefully leveled patch of sand, and as the great moment approached the tension could be felt even from the tree. A deadly hush fell over the group, broken only by Jack's voice reading off figures to his clerk.

'Two seven four,' he said, straightening his back at last. 'Mr Storton, what do you find?'

'Two seven four, sir, exactly.'

'The most satisfactory observation I have ever made,' said Jack. He clapped the eyepiece to and cast an affectionate glance at Venus, sailing away up there, distinct in the perfect blue once one knew where to look. 'Now we can stow all this gear and go back aboard.'

He strolled up the beach. 'Such a charming observation, Stephen,' he called out as he came near the tree. 'I am sorry to have kept you so long, but it was worth it. All our calculations tally, and the chronometers were out by twenty-seven mile [sic]. We have laid down the island as exactly...

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The reference to Venus and precise timing imply that they determined their chronometer error (and thus longitude) from a transit or occultation. The reference to a "carefully leveled patch of sand" speaks to an effort to determine true north or south by meridian transit. Beyond that... well, O'Brian was a scholar of Royal Naval history, and of period medical practice, but not so much of celestial navigation. This passage may be a pastiche of tidbits he picked up, and which frames other actions that take place on the island.
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