Public view: Adventures in contemporary art No: 16 Snow Sculpture

A single exhibit dominates the main gallery at Stephen Friedman's - a car covered in snow. It shines mysteriously in the dimly lit space, inviting a closer look.

There's a blanket of two inches of snow over the bonnet, the windscreen and the roof. Down the side of the car, the snow clings in a thinner, rippled layer. The windows are opaque but crisply delineated by a narrow grey shadow, and the four door handles protrude. But the relief is very slight, and the only items that really disturb the lines of the typical modern car box shape are the jutting wing mirrors, swollen by their all round covering. Snow has drifted under the vehicle too, filling the gap between its underside and the floor. No metal or glass or rubber is visible, all is snow. Snow and light, actually: the headlights shine dimly through ...

Of course, it's not snow at all, it's a cast. I rub my hand over the shiny white fibreglass bonnet, wondering vaguely how the artist managed to make it. Kerry Stewart's work is usually models of the human body, and there's one of her figures in this show: Follower. A young person, smaller than life-size, crouches in the corridor outside the main gallery, face blank or expressionless. Monochrome pink skin, brown hair, blue eyes; but the girl's blouse has a pattern to it. She might be listening, but there is a lack of self-awareness about the figure. The viewer can look down on her bowed back, or walk behind and consider her unprotected rear. She's innocent, powerless, vulnerable ... I can never see a Kerry Stewart figure without thinking of The Boy from the Chemist's is Here to See You. In that piece, there was a door with a pane of frosted glass between the viewer and the painted model of a physically disabled boy. (How had the callipered figure walked with his high-street collection box to my home? What a plucky little guy! What does he want from me? Oh, God, I'll never be able to give him enough ... ) Guilt inducing, funny-sad work, I'm thinking as I return to the main gallery. Here, there is a blanket of snow and a car body between me and whoever's trapped in there. It's the headlights that seem to infuse the sculpture with life.

If I could scrape away the model snow and open the door of the implied car, what would I find? An old women huddled in the driver seat. "My Aunt from the Nursing Home is Here to See Me."

The last time Aunt Jean drove her car, I was the only passenger. A bad stroke meant she couldn't walk, but could still drive. After a fashion. The trouble was that she couldn't concentrate any more: the car's speed went up and down, and the vehicle veered back and forth across the highway. At one point we got back to our own side just avoiding a head-on collision with a police car. I remember the shocked look on the driver's face. I expected them to turn round and pursue us. But they didn't. Maybe they understood about old people in country areas having somehow to get from one place to another.

In her first year in the nursing home I visited her regularly. I took her out for a drive once or twice, but she wasn't comfortable. Was I sure the handbrake was off? Did that red light mean we had run out of petrol? Where was the intolerable draught coming from? So we stopped going out for drives. Confined to a wheelchair, she would sit in her little room contentedly watching telly all day, and would cheerily chat about this and that whenever she got a visitor.

I only visit her occasionally now. First she lost interest in the outside world, and now she hardly registers what's going on in the nursing home. When I do visit, I'm soon bored, soon yawning. "You don't have to stay, Duncan," she tells me softly. I look at her. Pink face, golden hair, mild and blue eyes; damaged and defenceless. "Off you go," she says, suddenly decisive.

Jean and me, sitting side by side. We sit there in silence staring at the covered window. When her head bows and she dozes off, I open the door long enough to exit, then seal it again with an unavoidable clunk.

I'm outside the hollow space, looking down at it. No sign of the old- young figure inside, muffled and warm. I walk round the magical sculpture, fascinated and saddened by a pair of dimming headlights.

Kerry Stewart: Stephen Friedman Gallery, W1 (0171 4941434), to Saturday.

'Personal Delivery', Duncan McLaren's book on contemporary art, is published by Quartet (pounds 12).