The last few years have seen a vogue for building devices from old designs: Renaissance flying machines, Roman plumbing systems, medieval trebuchets. Last year a particularly ingenious example was exhibited in Newcastle: artist Rod Dickinson's reconstruction of a machine first described in 1810. It resembled a mutant pub bar, with hoses running out of barrels into a large oak-panelled cabinet surmounted by brass cylinders. Levers enabled an operator to blend the contents into mixtures before releasing them as a ray, which could be directed at a target.
The effect resembled that of a bar, too. It produced befuddlement, helplessness and occasional hilarity. But the substance was more ethereal, the experience less enjoyable, and the underlying intent more sinister. The machine's purpose was to destroy hopes of a peace with France by controlling the minds of British politicians, using such techniques as "fluid-locking", which froze the tongue, "kiteing", which raised an idea into the mind to undulate there for hours, and "lengthening the brain", which twisted serious thoughts into absurd ones. It could also "lobster-crack", killing the victim. Of course, this "Air Loom" and its gang of evil operators existed in the mind of only one man: James Tilly Matthews, an inmate of Bedlam.
Or did it? As Mike Jay shows, the whole elaborate delusion can be seen as a co-creation by Matthews himself and the apothecary whose book about him became a medical classic: John Haslam. Haslam bent his patient's condition to his own purposes, selecting and magnifying the most spectacular of Matthews's fantasies and presenting them as the definitive picture of his mental state - which might now be diagnosed as paranoid schizophrenia. Thus Haslam made his name, humiliated colleagues who had not thought Matthews insane at all, and left his patient to posterity as the Air Loom man.
In fact, Matthews's mental condition was wondrously fluid. He was sometimes sane, sometimes adrift in a border zone, sometimes the victim of quite different obsessions. To complicate matters further, at one stage he was genuinely mixed up in affairs of diplomacy and espionage, during most of which (not all) he was compos mentis.
Jay's purpose is not merely to trace the vagaries of Matthews's illness, but to undermine the Haslamesque practice of interpreting an entire mental life in terms of a single high-definition snapshot. In a quietly persuasive way, The Air Loom Gang grows from a biographical study into a meditation on time, change and truth. It focuses (or unfocuses) our attention on the hazy border between delusional reality and real reality, sensations and their causes, lived experience and later interpretation. And it sets everything in its historical context: this was wartime, when fears of conspiracy and control played in even the sanest minds.
The result is gripping as well as profound. Jay's narrative is structured for maximum effect, and themes woven in strand by strand until they run into a multi-coloured fabric. Jay proves to be as dextrous a weaver of thoughts as "Glove Woman", the most skilled of the Air Loom's operators. He leaves the reader's brain feeling well and truly kited.
Sarah Bakewell's 'The Smart' is published by Vintage