IN THE age of the GPS, we already take for granted a mastery of time and space at sea, keying in a waypoint and leaving it to technology to bring us within metres of the wreck we want to dive.
But until comparatively recently many shipwrecks occurred simply because mariners out of sight of land could do little more than guess their position. They relied on dead reckoning, with 'dead' too often the operative word.
As long as the skies were clear, determining latitude was no problem. Sailors had been able to follow parallels using the sun and stars since ancient times. Longitude was another matter. So ships tended to sail in straight lines, and keeping to narrow corridors made them easy prey for pirates and rivals.
Dava Sobel's little book Longitude is the story of Robert Harrison, inventor of the marine chronometer. It has taken the publishing world by storm, topping Britain's hardback non-fiction lists with sales of 10,000 copies a week, after spending much of 1996 - incredibly, in view of its British subject matter - among the US top ten best-sellers.
When four warships of the British fleet sank off the Scillies in 1707, with the loss of 2000 lives, it was because the admiral, having hanged a sailor for being impudent enough to suggest that they were off course, had proved unable to determine the fleet's position. The disaster led to the setting up of a Longitude Board in Britain and the offer of £20,000 to anyone who could find a way of guiding a ship to within half a degree of its destination.
This led to bizarre bids - such as the use of wounded dogs (it would take too long to explain) - but primarily to a struggle between astronomers and mechanics. The stargazers swore that only the heavens held the key, and worked to produce complicated and ultimately flawed lunar tables. The mechanics, like country-bumpkin watchmaker Harrison, believed that as degrees of longitude equalled degrees of time, the answer could be engineered.
But clocks and the sea had never been compatible, and Harrison had to find a way to overcome 'those irregularityes in time, that naturally arise from the different degrees of Heat and Cold, a moist and drye Temperature of the Air, and the Various Agitations of the ship.'
Harrison struggled for decades, not only against technical difficulties but against his arch-enemy, the Astronomer Royal Nevil Maskelyne. His weird and wonderful solutions, including H-4, 'the Mona Lisa of horology', revolutionised navigation and are still to be seen in Greenwich.
Dava Sobel, an award-winning former New York Times science reporter, knows how to keep things simple - which is more than you could say about Harrison, the convoluted workings of whose clocks proved so difficult to imitate. But she also writes with an elegance and precision very much in keeping with the subject matter. It is an easy read, and anyone interested in wreck-finding and navigation will find it absorbing.
Steve Weinman