The nub of it is in that title: "At Sydenham". This is about the Crystal Palace Mark Two. Most people are aware of the tale of the original Crystal Palace - essentially a huge prefabricated greenhouse - being designed and built in double-quick time for the Great Exhibition of 1851 in Hyde Park. The story of how the Duke of Devonshire's gardener/designer and self-taught glasshouse specialist, Joseph Paxton, came to produce such a wonder is one of the great moments of the 19th century. Paxton, untrained as either an architect or engineer, ranks alongside his contemporary Brunel. And like Brunel, he was not content to sit back on his laurels. Just as Brunel had to design the more ambitious S.S. Great Britain after the success of the S.S. Great Western, so Paxton's second Crystal Palace - at Sydenham, not far from the Dulwich Picture Gallery - was a far grander affair.

The Palace could not stay in Hyde Park once the Great Exhibition had finished. Being prefabricated, it could easily be dismantled. A number of possible new locations for it were considered, as they always are after world expos. Such plans seldom come to anything. At one point it was going to go to New York. But then a group of entrepreneurs, Paxton among them, bought a tract of land high on the south London ridge at Sydenham, and there they rebuilt the Palace, re-using the original components but adding to them extensively. For me, this was the revelation of the show. I knew it had grown, but I did not realize by quite how much.

The Palace became much longer, acquired two barrel-vaulted end transepts, as well as the original central one. Connecting these was a longitudinal vaulted nave - also missing from the Great Exhibition version. It was 50 per cent bigger in volume, was two tiers taller, and used twice as much glass. That made it 1608 feet long, 312 feet wide, and 168 feet high to the crown of the central transept. It was better proportioned. In fact, it was better all round than the Mark 1 Crystal Palace, in everything except commercial success.

Its park with its extraordinary (and very temperamental) fountains, designed by Paxton at full stretch, was considerably more spectacular than anything Hyde Park could provide. Brunel was conscripted to build two great conical-hatted water towers, standing at each end of the Palace like pagodas, to provide the pressure. This was to be a great place of public education and entertainment, envisaged as a commercial rival to the emerging museums district of South Kensington which, ironically, the profits from the Great Exhibition were funding. All in all, it was prodigious. And - true to the spirit of the times - it was fast. Crystal Palace Mark Two opened in 1854. John Ruskin described it as "a cucumber frame between two chimneys". He would.