There are something like 2,500 museums in the UK, and around 100 million visits were made to them last year. At times - particularly half-term holidays - it seemed as if all of them were trying to get into the Natural History Museum's dinosaur section at once. The Earth Galleries - designed to appeal principally to 15-year-olds - are if nothing else an attempt to spread the load a little. Similarly the HLF has set up a £7m fund to help lesser-known museums put themselves about a bit with temporary exhibitions, touring shows and the like. The public's love-affair with museums and galleries is fickle: if new sensations are not provided at frequent intervals, visitor numbers can drop off alarmingly as rival attractions kick in. Even a relatively new museum such as Manchester's Museum of Science and Industry - established only in 1983 - must keep expanding to stay ahead of the game. Accordingly it has just opened a new "Futures" gallery dealing with technological change from the arrival of the railways to the arrival of the computer. It is in the world's oldest railway bulding of 1830, restored by Ken Moth of BDP's Manchester office.

The need for renewal is the real reason that the V&A's director Allan Borg still wants to build Libeskind's "Spiral" - he knows that new architecture, as at the Louvre in Paris, means clicking turnstiles. The Science Museum nearby is investing heavily in new displays and is already building its new Wellcome Wing, designed by Richard MacCormac: the V&A dare not get left behind.

We can expect a new flood of visitors to Edinburgh following St. Andrew's day this November, for instance, when the long-awaited £32.5m Museum of Scotland opens (nearly the same money as the V&A's British Galleries refit). The Museum of Scotland started out as a semi-autonomous extension to the existing Royal Museum, which dates from the same period as South Kensington. But the craggy, competition-winning design of the big new bit by architects Benson and Forsyth, placed prominently on an important corner, has given it a completely separate identity. This is the architecture of civic and national pride: building it helped Edinburgh to regain some of the cultural kudos it had lost to its old rival Glasgow, while the fact that its contents will be purely Scottish appeals to the rising nationalist instinct.

So the stately dance of the museums goes on. The National Maritime Museum in Greenwich is nearing the end of its £14m rebuild by BDP with Rick Mather: new galleries and public space beneath Europe's largest clear-span glass roof. Mather will then repeat the trick at London's Wallace Collection, while the cranes are already at work on Sir Norman Foster's rather larger "Great Court" project at the British Museum. Lottery or no Lottery - and some of these projects were planned years before anyone had thought of reviving that ancient fund-raising idea - this process of keeping up with the Joneses isn't suddenly going to stop in 2001. Museums may look permanent, but they're not. Despite the supposedly inviolate nature of their collections, the buildings housing them are almost as volatile as airports.