Londoners will be aware of the debate surrounding the empty plinth at Trafalgar Square. The plinth was built in 1841, but the intended statue of William IV was never realised, due to lack of funds. Over the last two years, a series of temporary sculptures have been erected - the latest being Rachel Whiteread's Monument.

Monument is a brilliant idea - an inverted clear resin cast of the plinth itself. On first sight, I let out a disappointed "oh, is that it?" It's much smaller, far less noticeable than I had imagined it. Being clear, it reflects the available light, a commodity which is in short supply in England. In the dull twilight, it is like the view at British seaside resorts - grey. I should imagine it looks stunning on a lovely day, reflecting the blue sky.

It reminded me of nothing so much as the electricity substation in the middle of Elephant and Castle roundabout. It has that same "Is it glass? Is it metal?" duality. My second thought was that it was a huge clear eraser. It is somehow both glassily hard and squidgy, icy and warm, solid and airy, bulky and imperceptible.

As I walked round the plinth, it came to life, changing colour, becoming deeper, darker, lighter, allowing refracted glimpses of buildings, trees, sky. The longer I spent walking around it, the more I liked it. Which is more than can be said for the plinth's previous tenant, Bill Woodrow's Regardless Of History. The pigeons seem wary of Monument for the moment, but it can't be long till it's covered in bird shit and discoloured by traffic fumes. See it now - preferably on a clear day.