Time to make history

Harrison's memoirs are almost impenetrable

John Harrison's solution to the problem of longitude was a technological triumph.

More than 200 years ago, he realised that for a mariner to accurately determine his east-west position, a near-perfect timekeeper was necessary. By comparing the precise time on the boat with that in the sailor's home port, he knew it would be possible to calculate the distance travelled.

H4: A piece of history

But the trick was in making an accurate chronometer - one that could cope with the rocking motion of a boat and resist the great range in temperatures that might be encountered on a long voyage.

If the clock lost more than a few seconds a day, the ship could sail many kilometres off course.

It took him a lifetime, but Harrison worked through these problems one by one. And in his H4 "pocket watch" managed to produce a clock that lost just fractions of a second a day - a truly remarkable machine.

His work saved countless lives and the achievement is justly considered to be one of the great scientific advances of the 18th Century.

But what really grabs you about Harrison is his story - how he struggled for more than 40 years to build a machine that would satisfy his obsessive demand for perfection, and how leading astronomers conspired to deny him his place in history and a parliamentary reward of £20,000.

It took the intervention of the George III to get him what he deserved.

International bestseller

A clock had to resist the rocking motion of the boat

It is no wonder Dava Sobel's book on the subject, Longitude, became an international best seller: it is a good, old-fashioned story, simply told. It is also the stuff of good television and the BBC will screen a special documentary on the subject in the UK in the New Year.

The Horizon programme tries to take Sobel's book a step forward by using an actor, Peter Malahide, to bring Harrison's work alive. The script has borrowed heavily from the clockmaker's own memoirs that are now deposited in several libraries.

"[Dava] hadn't made much use of them in her book," says the programme's director Peter Jones, "and when we turned to them, we could see why. They are immensely difficult to follow and are almost impenetrable."

But by reading and re-reading, Jones and his team were able to see elements that would make the basis of useable dialogue - elements built on by Malahide and his knowledge of the Yorkshire accent.

H1 can be seen at the Old Royal Observatory, Greenwich.

We are shown Harrison at work, his experimental methods are explained and we hear about his meetings with scientists in London. He made scathing comments about the astronomers, who thought perfect timing could only be found in the Moon and stars.

"It comes across as almost some kind of class conflict between himself and the academics from Cambridge and Oxford, as he refers to them," says Jones.

Harrison enthusiasts

The four revolutionary timepieces Harrison built between 1735 and 1759 - simply referred to as H1, H2, H3 and H4 - can still be seen at the Old Royal Observatory, Greenwich. Their value prevented the clocks from being used as props. This meant the filmmakers had to use replicas.

"There is a small group of wonderfully dedicated clockmakers who came to our rescue," says Jones. "They are immensely skilled and totally absorbed in the work of Harrison and are at various stages of reconstructing his timekeepers."

Andrew King is one of these enthusiasts. He is currently building a replica of one of Harrison's early wooden longcase clocks. Perhaps surprising for a Harrison devotee, he is understanding of the derision heaped on his hero by the astronomers.

Andrew King: The astronomers could not cope with the idea of a near-perfect clock

"It was a system of solving the longitude problem they couldn't really cope with. They could understand an astronomical problem but the very idea of a mechanical timekeeper that was so good was just too good to be true - they couldn't accept it."

Jonathan Betts, the Curator of Horology at the Old Royal Observatory, agrees. "There was no particular vendetta against Harrison - these people were far too boring for that kind of exercise. They really believed that their solution was best and that these tick-tock clocks simply could not be believed."

But their cynicism could not hide the facts. When H4 was taken on a testing voyage across the Atlantic in 1764, it established the longitude of Barbados to within 9.8 nautical miles (18.1 km) of what we know it to be today. Of course, late 20th Century satellites can now pinpoint a location to within centimetres. Yet, 200 years ago, H4's accuracy would have seemed, as it did to the astronomers, simply unbelievable.

Longitude - A Horizon special - is broadcast on BBC Two on 4 January, 1999, at 2130 GMT. It is a Green Umbrella production for the BBC and WGBH Boston. It has already aired in the US on the Nova series on the PBS network.