The London EyeThe London Eye is a cantilevered(带悬臂的) observation wheel on the South Bank of the River Thames in London. It is Europe's tallest cantilevered observation wheel, and is the most popular paid tourist attraction in the United Kingdom with over 3.75 million visitors annually, and has made many appearances in popular culture. Sir Richard Rogers, winner of the 2007 Pritzker Architecture Prize, wrote of the London Eye in a book about the project: The Eye has done for London what the Eiffel Tower did for Paris, which is to give it a symbol and to let people climb above the city and look back down on it. Not just specialists or rich people, but everybody. That's the beauty of it: it is public and accessible, and it is in a great position at the heart of London.The structure is 135 metres (443 ft) tall and the wheel has a diameter of 120 metres (394 ft). When it opened to the public in 2000 it was the world's tallest Ferris wheel. Its height was surpassed by the 160-metre (525 ft) Star of Nanchang in 2006, the 165-metre (541 ft) Singapore Flyer in 2008, and the 167-metre-tall (547.9 ft) High Roller (Las Vegas) in 2014. Supported by an A-frame on one side only, unlike the taller Nanchang and Singapore wheels, the Eye is described by its operators as "the world's tallest cantilevered observation wheel". The London Eye used to offer the highest public viewing point in London until it was superseded by the 245-metre-high (804 ft) observation deck on the 72nd floor of The Shard, which opened to the public on 1 February 2013.The London Eye was formally opened by the Prime Minister Tony Blair on 31 December 1999, but did not open to the paying public until 9 March 2000 because of a capsule clutch problem. The London Eye was originally intended as a temporary attraction, with a five-year lease. In December 2001, operators submitted an application to Lambeth Council to give the London Eye permanent status, and the application was granted in July 2002. On 5 June 2008 it was announced that 30 million people had ridden the London Eye since it opened.The wheel's 32 sealed and air-conditioned ovoidal passenger capsules(胶囊；舱), designed and supplied by Poma, are attached to the external circumference of the wheel and rotated by electric motors. The capsules are numbered from 1 to 33, excluding number 13 for superstitious reasons. Each of the 10-tonne (11-short-ton) capsules represents one of the London Boroughs, and holds up to 25 people, who are free to walk around inside the capsule, though seating is provided. The wheel rotates at 26 cm (10 in) per second (about 0.9 km/h or 0.6 mph) so that one revolution takes about 30 minutes. It does not usually stop to take on passengers; the rotation rate is slow enough to allow passengers to walk on and off the moving capsules at ground level. It is, however, stopped to allow disabled or elderly passengers time to embark and disembark safely. On 2 June 2013 a passenger capsule was named the Coronation Capsule to mark the sixtieth anniversary of the coronation of Queen Elizabeth II.16.How many capsules are there on the wheel?