Don't bank on total accuracy

THE UNDERGROUND MAN by Mick Jackson, Picador pounds 15.99

The Underground Man is, roughly speaking, William John Cavendish Bentinck-Scott (1800-1879), fifth Duke of Portland and one of the 19th century's most notable eccentrics. In the last dozen years of his life, the duke became ferociously antisocial and devoted his wealth (you can deduce its source by comparing the elements of his name with an A to Z of London) to hiding himself from outside view. He had a great subterranean labyrinth constructed beneath the family seat at Welbeck Abbey in Nottinghamshire. He then withdrew from human society, communicating with his servants by means of notes and occasionally venturing to London in a closed railway carriage (he got to the station via a long tunnel, so that he would not be seen).

From the bare bones of this crusty obsessive, Mick Jackson has constructed an altogether more appealing character - shy, certainly, but also warm and affectionate. This duke is a baffled innocent, incessantly trying to explain to his own satisfaction the mysterious workings of the world, of animals, trees, his own body. The largely plotless, rambling book is filled with his bizarrely rational speculations - on the rhythms of the tides, for instance, which he suspects may be caused by the shrinking and swelling of the continents, and on the possible astronomical significance of the arrangement of moles and freckles on his own back. It's filled, too, with images of bodies dissected, reduced to the cogs and springs: skinned rabbits, jars full of anatomical specimens, the parts of a bee smeared on microscope slides. The duke is visited at different times by visions of the soft machinery at work in his own body (swimming, he thinks, like fish inside him) of skeletons stripped of flesh, and of souls stripped of physical form - a luminous white child follows him around.

Along with this fascination with the workings of things, the duke takes a boyish pleasure in maps and diagrams. He pores over Gray's Anatomy, he becomes entranced by a phrenologically labelled plaster head, and his work-room is dominated by a circular map of his own countryside, with Mansfield at the hub. Yet he is constantly lost, even inside his own house - bewildered in the maze of Edinburgh, he realises that "a map is absolutely useless unless one can say for certain whereabouts one is on it"; and the peculiar attraction of this fictional duke is that he never finds his bearings, yet never loses hope.

The Underground Man is a hugely charming book, the duke an unusually lovable protagonist; and there are some enjoyable set pieces (including a detailed account of a fart so vast it catches a candle and sets the duke's bedroom on fire). The qualification is that Jackson's prose isn't up to his imagination. He's often reduced to dull superlatives - "fine", "beautiful" - in his efforts to conjure up the wonder of things.

More damagingly, he's tried to write the book in a kind of mock-Victorianese which he can't sustain: too many adjectives are qualified by the word "totally" - which is, like, totally un- 19th-century - and he uses far too many words and phrases which are clearly modern. At one point, he describes a crow taking off: "Then he spread his oily wings and pumped them; rose, banked and disappeared into the trees." It's a moot point whether "pumped" is an accurate way of describing the fairly ragged movement of a crow's wings; but "banked" is certainly wrong - an aeroplane word, not a bird word (and a swift glance at the OED confirms this: the first recorded use in the context of flying was in 1911). Perhaps you think anachronism doesn't matter too much (and after all, Jackson isn't claiming historical accuracy: he deliberately places his duke's birthdate 30 years after the real one's); but in that case, why try to speak Victorian at all?

The problems aren't all to do with dating; sometimes it's just a question of trimming back some rather shaggy sentences. A couple of pages after the crow, the duke plays with the idea that his body is held together with wires - when he moves, he says, these wires "can be seen twitching under the skin like tense lengths of twine". Wires like tense lengths of twine? It's hardly an illuminating simile. Jackson, it's plain, has an abundance of imaginative intelligence at his disposal; with a competent editor to help him make the most of it, he should be an act to watch.