When William Hutton of Birmingham walked along Hadrian's Wall in 1801, he wrote: "I am the first man that ever travelled the whole length of the Wall, and probably the last that ever will attempt it".
Nothing about his experience of the 73-mile Roman frontier, running through the wild landscape of Cumbria and Northumberland, hinted to him at what it would become: one of Britain's most famous ancient monuments, a hugely popular pilgrimage for walkers, and, on Friday and Saturday night, the site of a hi-tech art installation by the New York digital arts collective YesYesNo.
As dusk fell east of the Roman fort at Birdoswald, technicians and volunteers surrounded a line of weather balloons standing tall on poles along the wall, each inflated by a man with the kind of leaf-blower you see in public parks. Curious bullocks wandered up, one of them taking an experimental lick at a balloon. "You may be on cow-scaring duty," someone said tentatively to the man with the leaf-blower.
As the balloons were finally adjusted into place, they began to glow: pale mauve morphing into deep pink, apricot into yolky yellow, duck-egg blue into aquamarine. A bright, almost full moon shone, looking down on its colourful imitators, dulling the stars in the clear sky. The colour of each balloon changed as if following some internal logic of its own. Which, in a way, it was.