One in the eye for Lord St John

The Millennium Wheel will provide what London conspicuously lacks, a good vantage point from which to survey the city. In my view, its size is a virtue

"All that one must do to understand our case," wrote a group of distressed aesthetes in 1887, "is to imagine for a moment a dizzily ridiculous tower dominating Paris like a black and gigantic factory chimney, crushing beneath its barbarous mass Notre-Dame, the Sainte Chapelle, the Louvre, the dome of the Invalides, the Arc de Triomphe, all our humiliated monuments, all our raped architecture disappearing in this stupefying dream." The object of this trill of alarm was the Eiffel Tower, which was already rising on the Champs de Mars, in preparation for the centenary exposition of 1889.

These days, those who set out to protect the chastity of the national heritage are a little more bureaucratic in their language. "Wholly unsuitable for this part of London," says Lord St John of Fawsley, chairman of the Royal Fine Art Commission, responding to a proposal by architects David Marks and Julia Barfield to build a 500ft big wheel on the South Bank of the Thames. It would, he complains, have "a damaging visual impact on the Royal Parks, the neighbouring Grade I listed buildings and the world heritage site of Westminster Abbey and the Houses of Parliament". Not quite raped, then, but certainly affronted.

As it happens, Lord St John knows something about damaging Royal Parks. As chairman of the Royal Fine Art Commission, he gave his blessing to the Queen Elizabeth gates, a truly hideous confection of wrought iron and poster paint which now disgraces the south-eastern exit from Hyde Park. They are probably the most tawdry addition to London's landscape since the Elephant and Castle shopping centre was painted pink, but they escaped Lord St John's fastidious disdain for one overriding reason - they had royal connections, and Lord St John is a man who melts before pedigree.

The wheel, on the other hand, is unabashedly democratic in spirit, a structure that proposes to elevate any Tom, Dick or Harriet with the admission price (and quite a few without - concessions will be offered). You may feel, as I do, that that anything that gets up Lord St John's nose is worth building on those grounds alone. But the wheel will offer far more than that.

It will be vast - more than three times the height of Tower Bridge and 120 feet taller than St Paul's - thus providing what London conspicuously lacks, a good vantage point from which to survey the city. In my view its size is a virtue - a smack at the offend-nobody good manners of most urban planning. Sheer scale doesn't have a good name in architectural circles, being associated mostly with megalomaniacs - commercial or political. But Eiffel knew that it had its place in symbolic buildings. "There is an attraction and a charm inherent in the colossal," he wrote, "that is not subject to ordinary theories of art. Does anyone pretend that the Pyramids have so forcefully gripped the imagination of men through their artistic value? What are they after all but artificial hillocks?" In its height the wheel represents an unrestrained exuberance which has been sadly lacking in alternative millennium proposals. It is, in the best sense, a structure of refined vulgarity - an idea of obvious popular appeal carried out in the purest geometry. Eiffel dressed up his tower with decorative arches, to conceal the alarming novelty of his technology, but the wheel makes no such concessions to human vertigo, hanging over the river on a cantilevered bearing, an awesome testimony to modern British design.

If it is extravagant in imagination and flair, though, the wheel is quite the opposite in its operation. Sixty per cent of the energy needed to operate it will come from the river beneath, harnessed by turbines. It makes no claim on the Millennium Fund, having already secured a commercial sponsorship deal which will underwrite its construction costs (around pounds 9.5m) in return, presumably, for a cut in the wheel's revenue. It is engineered for symbolic performance, too, a literally revolutionary device provoking ideas of human time, universal orbits, the rise and fall of fortune. It will be the first high-rise structure that could not conceivably be dismissed as phallic.

The objectors to the Eiffel Tower described it as "useless and monstrous". So is the wheel, but in the most uplifting way imaginable. It is the perfect response to the timidity and aridity of spirit that Heritage worship has inflicted on us - a reminder we shouldn't go into the next century tugging our forelocks to the past but rather celebrating the possibilities of the future.