Once David Marks and Julia Barfield mention the Palm House at Kew, its connection with their Ferris wheel, due to be erected this week, becomes obvious. When they entered Burton and Turner's masterpiece after its 1980s refurbishment, before replanting, they were astonished by the lightness of the structure. Marks describes it as 'the perfect symbiosis of architecture and engineering', the quality aimed for in the design of the wheel. They had found it earlier in a very different building, Frank Lloyd Wright's Falling Water, though Barfield points out that the engineering there is 'implicit'.
Barfield remembers the Alhambra, visited together many years ago, as a magical place, not just for its architecture but also for the landscape, the light, the use of water. 'People tend to think of cities being characterised by buildings,' says Marks, 'but other things define cities, such as bridges and parks.' Waterloo Bridge, with its magnificent views, is a perfect example. He regrets that Thomas Telford's design for a single-span London Bridge was never built: 'If it had been, it would have been by far the most marvellous structure in the city'. Another unbuilt structure that has fascinated him since his student days is Buckminster Fuller's floating city, a sphere so large that the warm air inside it would keep it permanently airborne.
'That's where we're heading,' says Marks, as he and Barfield prepare for lift off.