Inside the Big Mind : Umberto Eco’s dazzling blend of science and fantasy comes perilously close to pedagogy : THE ISLAND OF THE DAY BEFORE, <i> By Umberto Eco (Harcourt Brace: $25; 515 pp.)</i>

The old Aristotle decided that a love of wonder was the beginning of wisdom; for this reason he loved myths, which were full of marvels. In “The Name of the Rose,� Umberto Eco imagined Aristotle’s lost book on laughter and managed to create a comic thriller around the philosophical issue of rationalist irony and skepticism. “The Island of the Day Before� takes up another godless, philosophical quest: the 17th century’s new science and its hunger for knowledge, whatever the cost in magic, murder, fraud, destruction. Some of the wonders in this book are natural--tropical flora and fauna, the living coral of the Great Barrier Reef, the stars of the Southern Hemisphere, the wheeling of the heavens, the multitude of atoms in air and light. Eco can rhapsodize--solo--with the energy and volume of a Handel chorus.

It is 1642, the year the historical Abel Tasman sailed past Australia and missed it. The background to his adventures governs Eco’s plotting, for in the novel the empires of England, France and Holland are racing one another to discover how to measure longitude; the secret will give one of them mastery of the sea. Roberto della Griva sails as a French spy on an English ship bound for the legendary islands of Solomon. In the hold, he finds a wounded dog, who groans in agony when, in faraway Paris, a man applies salt to a rag dipped in the animal’s blood. Using this terrible hocus pocus of the so-called Powder of Sympathy, Dr. Byrd, the scientist on board, can tell the time at the ship’s port of departure, and can then calculate accurately how far they have traveled west.

Vividly described in the novel, all this is of course playful occult nonsense, the kind Eco enjoyed so expansively in his last novel, “Foucault’s Pendulum.� An accurate timepiece was not to arrive until John Harrison invented the escapement and gimbals so that his clock would not stop, however foul the weather. It was Harrison’s marine chronometer that Captain Cook used when, more than 200 years after Eco’s time frame, he navigated these same seas.

A shipwreck destroys the magic experiment, and Roberto, now a castaway, fetches up at his original destination just the same, but on board a phantom ship. There, like another Robinson Crusoe, he discovers wonderful creatures and an “intruder� who leaves a footprint: Father Caspar Wanderdrossel, who has also been looking for ways to plot longitude but by charting the eclipses of the moons of Jupiter and other recondite astronomical means. He engages Roberto in passionate debate, all conducted in a crazy macaronic of Latin-German (brilliantly rendered in English by Eco’s translator, William Weaver) and reveals to Roberto that the island they can see a little way off, beyond the reef, lies on the other side of the meridian which divides today from tomorrow. From their point of view, island time will always be in the past. Because neither he nor Roberto can swim, Father Caspar decides to walk there through the sea, and so improvises the first diving suit complete with metal boots. Unfortunately, when he walks out of the book, some of its comic energy leaves with him.

Like a hero in the traditional epics he relishes, Eco has set himself innumerable--perhaps insuperable--tasks. In “The Name of the Rose� he had fun parodying the whodunit and cloister gothic; in “Foucault’s Pendulum,� he joined in the ever-proliferating arcana about the treasure of the Knights Templar, combining information systems--Cabbala and computer--in a games-playing frenzy.

In “The Island of the Day Before� he tries other narrative genres he must have loved as a boy, mimicking the chivalric epic (Cervantes) and the adventure yarn of the high seas (Stevenson) as well as the nesting boxes of Arabian Nights storytelling. But the form he tests most severely here is the Romance, with a princesse lointaine whom Roberto loves and can never attain and--as if this weren’t already enough playfulness--an Evil Double who expresses Roberto’s darkest desires.

The two tales--the feverish fantasy of derring-do in Roberto’s head as he remains marooned and the huge novel dreamed in Eco’s giant brain--flow together but gain no narrative shore. The novel stops (closes it refuses to do) with Roberto adrift forever on the parallel of longitude, approximately on today’s international date line, which separates today from tomorrow.

In a recent article about the new rise of fascism, Eco has called for vigilance on two fronts in particular: regard for truth and precision of language. Both principles are fully served in this novel: He celebrates fecundity of discussion, thought and language with exhilarating zest throughout. But this novel also belongs in a new strain of postmodern fiction that comes perilously close to pedagogy (like Jostein Gaarder’s “Sophie’s World,� for instance). In fictional worlds, narrative conventions, like the marine chronometer, provide orientation.

Eco’s “Island of the Day Before� is dazzling in its range, its linguistic fireworks (“Babelizing� as Eco calls it) and sheer learning. But it has to be admitted that the book is all too easy to put down. The author exults in ways of knowing--in mapping, naming, archiving, enumerating, classifying; he is passionate about the scientific renaissance, its invention of tools still in use today--the museum, the globe, the telescope, the anatomy theater.

But, fatally, he’s much less interested in passions or in people. Roberto is a colorless vehicle for Eco’s own concerns and ruminations, and the few relationships he makes all die prematurely in the book. Juggling narrative devices, demystifying romance and expatiating on le gai savoir and its Faustian risks can tire; the cleverest conjuring tricks can pall after too long.

“The Island of the Day Before� is available on four audiocassettes from Audio Renaissance, ($24.95).