The London Eye is the biggest observation wheel in the world.

At 135 metres it is 30 metres higher than the previous tallest observation wheel, in Yokohama Bay, Japan.

Towering over Thameside it looks up only to Canary Wharf, the NatWest Tower and the Telecom Tower in London.

The Eye is a testamant to computer aided design and engineering, but its roots are a good deal more humble.

"A great cartwheel"

The idea of riding a wheel into the skies for fun is not a new one.

The first historical record of a funfair wheel comes from Bulgaria.

Peter Mundy, an English traveller, described how at the Feast of Biram at Phillippopolis on 17 May 1620 he saw children riding on vertical wheels "like a great cartwheel, on whose circumference are fastened little seats, wherein the children being sat, the wheel is put about".

There are accounts of similar rides being seen in England in the 17th Century.

Their appearance was closely linked to the development of trade fairs where goods and merchandise could be advertised and sold.

People got into "flying coaches who insensibly climbed upwards, knowing not whither they were going, but being once elevated to a certain height, they came down again according to the circular motion of the sphere they moved in".

Similar rides appeared all over the world, in India, Siberia and Romania. But it was in the USA that the funfair wheel took a giant leap forward.

Big wheel turning

In 1893, in Chicago, a bridge builder called George Ferris created the largest wheel ever seen.

Supported by two 140 foot steel towers, its 45 foot axle was the largest single piece of forged steel in the world at the time.

The wheel itself had a diameter of 75 metres (250 feet). It was powered by two 1,000 horsepower reversible engines and had 36 wooden cars that could each hold 60 people.

Built for the Chicago Fair, the Ferris Wheel grossed $726,805.50 during its short time in operation, at 50 cents a ride.

The wheel was dismantled in mid-1894, after the fair, and reused at the St Louis exposition in 1904 but was scrapped in 1906.

Since then the original has spawned imitations all over the world.

High-tech support

Strictly speaking the Eye is an observation wheel, not a Ferris Wheel.

Unlike its illustrious predecessor, the Eye's 32 passenger capsules are fully enclosed, they are positioned on the outside of the wheel, and the entire structure is supported on one side only.

The design of the Eye is also very different. At twice the height of the original Ferris Wheel it would collapse under its own weight if the same design had been used, so the Eye has been made as light as possible using computers.

Eighty wire cables support the rim like a vast bicycle wheel - albeit 200 times larger. Each cable is pre-stressed so the rim does not sag.

To prevent the apparently fragile structure from shaking itself to bits in high winds, each cable is monitored by sensors, and computers control dampers which iron out any vibration.

To stay upright, the wheel depends on a massive central hub and spindle. The spindle is 23 metres long, around the height of a church spire, and weighs 330 tonnes, more than Big Ben.

Once operational the London Eye is expected to carry 800 passengers a trip, or 15,000 a day - enough to fill Concorde 160 times over.

The more firma, the less terror

Some things probably won't change. Standing in glass capsules with "uninterrupted views" of London, passengers on the Eye may feel a sneaking sympathy for those who bravely rode the original Ferris Wheel 107 years before.

The Pittsburgh Commercial Gazette recorded that on its inaugural turn "a fourth of the way up the wheel stopped. The passengers gasped in unison and looked at each other with smiles more or less sickly.

"The wheel then climbed steadily upward and the passengers grew bolder. Some of them looked over the edge of the car and at once became less bold.

"Then all gave three cheers to the inventor and drank to the health of his pretty wife with immense enthusiasm."