We all like fountains, we as a species delight in the play of water, and perhaps we are harking back to the warm comfort of our aquatic ancestry. From purling brooks to the breaking of waves, looking at water in all its forms makes most of us feel good, and we spend time just looking. Ariston men hudor, said Pindar: Water is best.

Urban architecture is largely ignored. We walk through cities on the urgent business of getting somewhere else, and seldom gaze around us at the beauties large and small; if, of course, we are lucky enough to be in a part of a city that has any beauty.

A fountain in an urban park bridges these two domains. We like to sit by it. We like the way everything is a little bit greener or more peaceful around it, peaceful even as it makes its constant splattering sound, because it's probably drowning out cars, a mandala of brown noise smoothing our perceptions and slowing down our lives, giving us some time.

Russell Square is in London, near the British Museum. Surrounded on all sides by elegant dark Georgian houses, it's a large grassy square with trees and flower-beds and a statue of someone nineteenth century, but in its centre is a fountain composed of nothing but jets of water.

No tritons spouting, no shell-shaped stone bowls to sit on the lip of, nothing but a flat space with thirteen jets shooting upward. The one in the centre is highest, reaching 5 or 6 m, the eight around it in a square reach perhaps 2 m, and there are four beyond those, perhaps a metre high. The water drains where it falls.

My passing thought when I first saw it was how wasteful it was spouting here where so few people saw it, then I immediately realised it could be constantly cycled. Then I wondered how much energy would be required to force water to those heights; and in seconds the whole absurdity of these thoughts fell away, and I stopped going where I was bound, and sat and watched the fountain.

I had been going to the British Museum, one of the world's great repositories of what we as a species value in ourselves: seeing what we made in the past, choppers from Olduvai Gorge, the turquoise double serpents of Central America, Benin and Yoruba bronzes, the gold and garnet clasps buried at Sutton Hoo, the red-figure ware of Athens, the hasty perfect sketches of Claude Lorrain...

It's a four-day holiday to celebrate our Queen's Golden Jubilee. I'm in a park, maintained by gardeners, with temporary palings all around the beds to protect the new grass. Then I'm going to Fortnum & Mason and Paxton & Whitfield to indulge myself with the finest tea, chocolate, and cheese I can find.

The fountain and all these others are what we do it for. The sordid world of business, cars, money, necessity, building, time, that's so far away when you're in a park watching a fountain. They are the servants, not the masters, of civilisation. Some of "us" are spending our whole days trekking through heat to carry water from polluted wells, so I'm keenly aware of what a painful privilege it is to say "we" look at Korean celadon and drink Assam for pleasure. But this is what we do it for.

A fountain is emblematic of our highest desires bonding with our deepest nature.

The one and two metre high jets look like solid columns, the smaller ones writhing like undines dancing in diaphanous costume, the larger ones like candles boiling at their peaks, sending down molten slurries, which branch out and fall away from the main body. But allowing for the speeding up of time, these are like solids surrounded by an outer halo of fluid activity.

The largest one never gave the impression of solidity, barely even in its lowest metre (I found out why later, I think). That was like a great thick laser, light made fluid, but for most of its height the large jet was equations brought to life, a tracery of the fiendishly complex Navier-Stokes equation for fluid flow which, according to legend, makes God despair. Metaphors of trees and candles and anything else tangible break down, and you see that this behaviour is vaster and more eternally changing than any attempt to grasp it.

I've seen one waterfall close up, England's highest, High Force in Yorkshire, but that's surrounded by so much natural beauty and ruggedness that you take it in in a different way, without concentrating on the pure glittering fall of water alone. And the television screen flattens out what documentary makers are trying their hardest to show you in Angel Falls, Mosi-ao-Tunya, Niagara. Surf storms breaking on the rocks would be closer, where I've admired the infinite variety of shapes the ropes and pearls of spume can make.

The pressure upward made the great jet take on forms different from those you'd expect. No downward rains of spray, no rainbows in the mist, but globules starting like blood corpuscles and distorting and splitting like the lava in the lamp. Gravity, I suppose, could not yet compete and turn them into teardrops. As I got up and walked round it a bit looking for an angle where I would see rainbows --

it shut off abruptly.

The jets lowered, and fell to almost nothing. This continued long enough that I went on my way. When I came back from the Museum several hours later they were almost flat, just bubbling over the edge of their mouths. I saw there were eight small sources closely circling the main jet, which is probably why its base had been much less solid. So I realised I had been lucky to see the fountain in its full splendour, and had I hurried through Russell Square at a different time I would have had none of these thoughts.

Yet it was now closer to lunchtime and there were more people in the square, in the central part of it, on the benches arranged for viewing the fountain, and they were viewing the water, shallow as it was, with the same pleasure we all get from the flow of water, when it calls to us to stop, and reconnect.

Part of JudyT's Golden Jubilee celebration of Britain.