A dowser explains the twitch's spell: National Trust experts have taken unusual advice on tracing 'lost' buildings. Michael Leapman reports

TED FAWCETT dangled a chair castor on the end of a piece of string over the head of a man in his audience, and it moved in a faltering circle.

'What does that mean?' the victim asked nervously.

'It means you are not a woman,' declared Mr Fawcett. To prove the point he moved his pendulum to the head of a female spectator and the castor moved in a straight line.

This was not an end-of-the-pier magic show, but a sober gathering yesterday of archaeologists and related experts in the august boardroom of the National Trust in Queen Anne's Gate, London. Mr Fawcett was introducing them to the theory and practice of dowsing, and its application in tracing the outline of long-vanished buildings and gardens.

Julian Prideaux, the trust's chief agent, said that Mr Fawcett, a garden consultant to the trust and once its public relations officer, had 'slightly badgered' them to allow him to make his presentation there. Yet it attracted a high-calibre audience.

Mr Fawcett admitted that dowsing was controversial but maintained it could be explained scientifically. It was usually performed not with a home-made pendulum but with either a forked or L-shaped stick, which twitched when it passed above buried water, stone or anything that caused a change in the magnetic field.

The first thing to know about dowsing is that the kind of stick does not really matter. 'It is not the sticks, rods or pendulums that move,' Mr Fawcett explained. 'It is the human being who twitches. The stick is like the hands of a watch, indicating something that is happening to you inside.'

The body contained 12 pairs of sensors sited at strategic locations, he said. They responded to changes in the magnetic field and, if the feet and hands were properly spaced, they would cause an involuntary movement in the hands, making the stick move. 'Any old thing will do because it's you who's giving the reaction. You can make two L-shaped rods from a metal coat-hanger.'

Yet while dowsing has a physical explanation there is a mental element as well. Unless you are concentrating on the substance you are trying to locate - water or an old flower bed or whatever - it will not work.

He then showed slides of old garden layouts and building foundations that he and his wife Jane had found at Osterley Park, Avebury Manor, Moor Park, Hatfield House, and even Westminster Abbey. Most of their findings, he said, had been confirmed by subsequent excavation.

Whether you believe in the arcane art depends to an extent on how far you are prepared to take Mr Fawcett's word for it. Yet if any went to Queen Anne's Gate to scoff, most seemed convinced.

'I've taken you to the wilder shores of this extravagant art,' he said, as he called a break for tea and cake. 'Any questions?' An onlooker asked: 'Can you use it for sexing chickens?' Mr Fawcett thought you probably could.

(Photograph omitted)