Compass: a story of exploration and innovation, by Alan Gurney

Hooked on the fish that points north
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When it Came to metal fish which pointed north, the Chinese were, as usual, ahead of the game. In the 11th century, writers noted that thin, magnetised iron strips would, if floated in water, swivel round with their fishy faces towards the Arctic. "Very interesting, but it'll never catch on," was the general reaction to this party trick.

When it Came to metal fish which pointed north, the Chinese were, as usual, ahead of the game. In the 11th century, writers noted that thin, magnetised iron strips would, if floated in water, swivel round with their fishy faces towards the Arctic. "Very interesting, but it'll never catch on," was the general reaction to this party trick.

By the 12th century, both Chinese and Western scribes reported that sailors were using magnetised needles for direction-finding. As Alan Gurney puts it in the final sentence of Compass, the world was now their oyster; an oyster "ready and waiting to be opened, not with a sword but with a compass needle". The rest was just details, but the kind of details which meant the difference between enjoying a safe voyage and being benighted up the creek without a paddle-steamer. Your dead reckoning could easily leave you just dead.

By the 15th century, scientists noticed that the magnetic north was not in the same place as the real or "geographic" north. This did not matter if you were, for example, in the Atlantic near the Azores, since the two norths lined up there, but as you sailed east or west, a gap or angle of variation opened between them.

Worse, this variation was not uniform throughout the globe. Even worse, it altered over the years. In 1580, the magnetic variation in London was 11 degrees to the east, but by 1850 it had swept like a vast, slow-moving windscreen-wiper to 22 degrees west.

Failure to factor in this variation was one cause of the Royal Navy's worst shipwreck. In 1707, thousands of men and four ships were lost because the Scilly Islands turned up in an unexpected place. Like Compass, Dava Sobel's Longitude also features this maritime disaster. The two books are part of the same literary fleet. Gurney, though, has a much longer and more sprawling tale to tell.

Even wooden ships, he explains, had metal fittings - cannons in the case of warships - and this ironmongery played havoc with sensitive compasses. A vertical nail would become magnetised; the top would acquire a southern polarity in the northern hemisphere, and would get a northern polarity in southern regions.

Iron-hulled ships were a magnetic anarchy. An early vessel found its compass was out by over 50 degrees. Another was discovered to affect even compasses on the nearby quay, with its bow tugging needles in a different direction to its stern. It was a floating magnet, and compasses now needed new protection to insulate them.

In a few points, this intriguing book left me all at sea. Speaking as an unscientific landlubber, I would have welcomed a basic explanation of why a compass needle actually seeks north. Why does the magnetic variation change over the years? And why didn't the publishers anglicise the book's spelling?

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