Sextant: A Voyage Guided by the Stars and the Men Who Mapped the World’s Oceans. David Barrie

master apparently thought he was 600 miles from land.⁷ Such an error would be quite understandable after a long ocean passage, especially since the prevailing westerly winds in the southern Indian Ocean generate an east-going current that would have been hard to detect. To confirm this, however, it would – at the very least – be necessary to know whether the master was using a chart on which the position of the reef was correctly recorded, and if so where he believed his ship was in relation to it. It is equally difficult to assess the reasons for the loss of the British warship Ramillies, which occurred as late as 1760. Having entered the English Channel, she was caught in a gale off the south coast of Devon and embayed. She tried to anchor but was driven ashore with the loss of all but twenty-seven of her crew of 800. Assuming that her commander knew the latitude correctly, was he mistaken in his longitude? Was the chart he was using in error? Or did poor visibility mean that those on watch failed to notice the approaching coast before it was too late? We have no way of knowing, but the impossibility of accurately determining their longitude put offshore navigators under a heavy handicap.

      Commodore George Anson’s circumnavigation of the world provides the clearest illustrations of the navigational difficulties that sailors faced before the longitude problem had been solved. England being at war with Spain, Anson (1697–1762) was dispatched in 1740 with a squadron of ships and more than 1,900 sailors and soldiers⁸ to harass the enemy’s colonial settlements in the Pacific. Long delayed by contrary winds, the squadron reached Madeira – their first port of call – in October, having already been at sea for forty days. The island’s longitude was laid down on contemporary charts – as it happens, quite accurately – in 17° West ‘of London’, but Anson and his men placed it somewhere between 18° 30' and 19° 30'.⁹ Their DR was out by at least 75 miles. Five months later, in early March 1741, having passed through Le Maire Strait, north-east of Cape Horn, Anson’s ships struggled round into the Pacific, facing a succession of terrific gales: men were injured or lost overboard, some lost fingers or toes to frostbite, the ships began to leak heavily and both sails and rigging were frequently damaged. Typhus and dysentery had already weakened the squadron on the voyage south from Madeira, but scurvy too now began to take its toll.¹⁰ On Anson’s flagship, the Centurion, the crew were so much weakened that they were unable to throw overboard the bodies of their dead shipmates. One old soldier, who had fought at the Battle of the Boyne in 1690, discovered as he lay dying that his long-healed, fifty-year-old wounds were reopening.¹¹

      On 3 April the Centurion ran into a particularly severe storm:

      In its first onset we received a furious shock from a sea which broke upon our larboard quarter, where it stove in the quarter gallery, and rushed into the ship like a deluge … to ease the stress upon the masts and shrouds we lowered both our main and fore-yards, and furled all our sails, and in this posture we lay to for three days …¹²

      Despite the appalling conditions Anson and his men were convinced that they had not only rounded Cape Horn but had also made good westerly progress out into the Pacific. Judging that it was now safe to head north, they were in for a nasty surprise. On the night of 13–14 April, when they thought they were hundreds of miles out to sea, the leading vessel caught sight of rocky cliffs – probably the western end of Noir Island off the south-west coast of Tierra del Fuego. She fired a gun and showed lights to warn the ships astern of her of the impending danger:

       [the land] being but two miles distant, we were all under the most dreadful apprehensions of running on shore; which, had either the wind blown from its usual quarter [south-west] with its wonted vigour, or had not the moon suddenly shone out, not a ship amongst us could possibly have avoided …¹³

      One officer recalled seeing the cliffs rearing up ‘like two black Towers of an extraordinary height’, but every ship managed to get clear.¹⁴

      Anson’s own log gave the Centurion’s longitude on 13 April as 87° 51' W, while another surviving log gives a longitude of 84° 12' W just before land was sighted.¹⁵ The difference between these estimates is itself an indication of how difficult it was to determine longitude reliably by DR. In fact the longitude of Noir Island is about 73 degrees West, which means that Anson’s estimate was out by nearly 14 degrees – a distance of almost 500 nautical miles in this latitude. The authorized account of the voyage plausibly places the blame for these very large errors on the unexpected strength of the ocean currents in this locality:

      It was indeed most wonderful that the currents should have driven us to the eastward with such strength; for the whole squadron esteemed themselves upwards of ten degrees more westerly than this land, so that in running down, by our account, about nineteen degrees of longitude, we had not really advanced above half that distance.¹⁶

      The squadron – from which two ships had already separated – faced further battering by storms as it struggled northwards. By the end of April, as the death toll from scurvy rapidly mounted, each of the surviving vessels found itself alone. Having barely survived a hurricane at the end of May off the island of Chiloé,¹⁷ the Centurion headed for a planned rendezvous at the island of Juan Fernández [now Robinson Crusoe Island] off the coast of Chile where desperately needed fresh provisions could be found. However, to save vital time, Anson ‘resolved, if possible, to hit the island upon a meridian [of longitude]’.¹⁸ In other words, rather than heading north up the coast of Chile and then running down the island’s latitude in the time-honoured fashion, he took the chance of sailing directly for it. On 28 May they had nearly reached the latitude in which the island was laid down and ‘had great expectations of seeing it: But not finding it in the position in which the charts had taught [them] to expect it’ they were afraid that they might have gone too far to the west.¹⁹ Though Anson himself was ‘strongly persuaded’ that he had glimpsed the island, his fellow officers were unconvinced and, following ‘a consultation’, it was agreed that they should head back to the east. They sighted the distant, snow-capped peaks of the Chilean cordillera on 30 May:

      Though by this view of the land we ascertained our position, yet it gave us great uneasiness to find that we had so needlessly altered our course when we were, in all probability, just upon the point of making the island; for the mortality amongst us was now increased in a most dreadful degree, and those who remained alive were utterly dispirited by this new disappointment …²⁰

      It took nine days to regain the ground they had lost and it was not until 10 June that the Centurion, whose crew at full strength would have numbered between four and five hundred men, at last reached Juan Fernández ‘with not above ten foremast men in a watch capable of doing duty’. This single navigational error had cost the lives of ‘between seventy and eighty of our men, whom we should doubtless have saved had we made the island [on 28 May], which, had we kept on our course for a few hours longer, we could not have failed to have done’.²¹

      Once the remnants of his squadron had gathered at Juan Fernández and the surviving members of the ships’ crews had recovered their strength, Anson carried out a raid on a small Spanish coastal settlement in Peru, and also captured a few merchant vessels. He next tried to intercept a Spanish treasure ship that was expected to sail from Acapulco, but news of his presence had reached the Spanish authorities and the ship remained safely in port. Disappointed but undaunted, in May 1742 he set out across the Pacific with his two surviving ships.²² In the course of an agonizingly slow voyage, again greatly complicated by unreliable charts and uncertainties about their longitude, one ship had to be abandoned and the Centurion, with eight or ten men dying every day ‘like rotten sheep’, was barely afloat when she reached Tinian in the Marianas Islands in August.²³

      Anson’s determination now, at last, paid off. He was able to reach Canton (modern Guangzhou) where the Centurion was repaired and then succeeded in capturing the treasure ship, Nuestra Señora de Covadonga, off the Philippines. The Covadonga was carrying more than 1.3 million pieces of eight and 35,682 ounces of silver. The exact value of the loot Anson finally brought home in 1744 is uncertain, but as a ship’s captain as well as commander-in-chief, he would have received three-eighths. His share of the treasure from the Covadonga alone may have amounted to the vast sum of £91,000. For comparison the pay due to