Nuclear-powered imagination fires novelist, Winter's Tale, by Mark Helprin. New York: Harcourt Brace Jovanovich. 659 pp. $14.95.

Mark Helprin is a young man with thin features, an air of amusement, and an imagination that must be nuclear-powered. His new novel, ''Winter's Tale,'' features (among other things) a flying Brooklyn milkhorse; warrior clam-diggers from Bayonne Marsh; a newspaper - the New York Ghost - with a dry-cleaning critic; and Boonya, a Norwegian cook whose specialties include Turkish calendar cake and Rhinebeck hot pots with fresh armando.

Time travel is possible via a wall of white cloud. A rich heiress lives in a tent on her roof. Characters have Dickensian names like Bat Charney, Daythril Moobcot, and The Ermine Mayor. Everyone is awaiting the arrival of a perfect age.

Seems confusing, you say? Winter's Tale does occasionally sound cacophonous. But by the time this ambitious, entertaining book is over, the author has most of its elements marching in order and whistling in tune.

Helprin is a writer of great natural talent. He is perhaps best known for his short stories, which tend to be brief, myth-like tales of courage or renewal; ''Ellis Island and Other Stories,'' his latest anthology, was nominated last year for both an American Book Award and a P.E.N. Faulkner prize.

''Refiner's Fire,'' his only previous novel, was published in 1977. A semi-precious gem of a book, it chronicled the picaresque life of Marshall Pearl , a US foundling destined to become an Israeli soldier.

For ''Winter's Tale'' Helprin has raised his sights considerably. While his first novel was sprawling, his second is vast. It is long, convoluted, and deals with the nature of time, the reason for evil, and man's search for justice.

''Justice,'' writes Helprin, ''(comes) from a fight amid complexities, and (requires) all the virtues in the world merely to be perceived.''

It is almost impossible to summarize the book's plot without its sounding silly. It opens in the years just preceding World War I. Peter Lake, a burglar and mechanic with unparalleled skills, attempts to rob the mansion of the wealthy Isaac Penn - and falls in love with Beverly, Isaac's beautiful but sickly daughter.

They marry. She dies. He departs on an involuntary journey through time. One hundred years later, he reappears, and with the help of younger Penns and various hangers-on leads the city of New York through the horrible waning hours of the 20th century, into the justice of the third millenium.

Through it all Helprin plays with words the way others play with blocks, or tennis balls. But Helprin at times misses the mark, like a natural athlete who tries for a spectacular shot when a safe one will do. His musings on time and justice sometimes ramble on and tend to repeat themselves. Metaphors occasionally travel in confusing swarms, and the book slows near the end. It would have profited from some trims.

On the whole, Winter's Tale contains breathtaking verbal pyrotechnics - but readers not familiar with Helprin's work may find his ''Refiner's Fire'' more moving.

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