Umberto’s Truffle-Hunt : FOUCAULT’S PENDULUM<i> by Umberto Eco, translated by William Weaver (A Helen & Kurt Wolff book/Harcourt Brace Jovanovich: $22.95; 671 pp.) </i>

Umberto Eco’s new novel is an artichoke with 641 leaves and not much heart. There is some real pleasure in artichoke leaves, but what with the work and the scratchiness you probably wouldn’t undertake one unless you thought there would be a heart in it.

True, in “Foucault’s Pendulum” the absence is part of the point. Its elaborately branched story about the search for an ancient secret whose possession will confer world mastery is both an illustration and a parody of the contemporary literary doctrines of semiology and structuralism.

Eco is a professor of semiology and a novelist, author of “The Name of the Rose.” His new book is a bravura series of variations and reverses on the structuralist view that the important reality in a text is the text itself. “Hamlet,” for example, is not “about” a melancholy prince and the difficulty of action. It is “about” Shakespeare’s telling the story. He might have told it differently.

In one of many witty passages in “Pendulum,” in fact, Belbo, a nervous and suggestible book editor who is jealous of his authors, imagines himself coaching Shakespeare. Don’t set it in France, he advises; set it in a less distinctive place, say, Denmark. How about having the ghost come in at the beginning instead of the end? And couldn’t we improve on: “To act or not to act, that is the problem”?

Belbo, in other words, is “deconstructing,” showing the inner flux and instability of monuments. Structuralism, like modern science--which it aims to be--divides the visible into subatomic particles.At that level, a chair and the thumbtack placed upon it are much the same. So are Superman and Keats. In physics, a lot of energy has been liberated. In literature, a lot of scholarly activity has been liberated, but perhaps less energy.

To tell what “Pendulum” is about, itself requires a certain deconstruction. The plot is the least of it, being, essentially, an invention of the characters. Only at the end does the invention appear to come true, although this “coming-true” also is an invention. Through it, Eco makes his point--satirical, serious and above all ambiguous.

Essentially, then; or rather, unessentially: Belbo, Diotallevi and the narrator, Casaubon, three Milan intellectuals of varying stripes and edginesses, work for Garamond, publisher of a few serious books and of a great number of vanity items, paid for by their authors.

One of these, a comically paranoid and right-wing former Foreign Legionnaire who goes by the name of Colonel Ardenti, turns up with a book about a mystical secret known to the Knights Templar. The Templars fought in the Holy Land, came back to Europe, but became rich and powerful, and eventually were broken and dispersed in the 14th Century by the Pope and King Philip of France.

Ardenti claims to have found part of the secret in the form of a coded message. The Templars, he asserts, hid it and purposely got themselves suppressed in order to go underground until, through a 600-year line of initiates and adepts, it should be time for it to become known. The time is now, he insists; by publishing what he knows, others will come forward and complete the chain. Instead, Ardenti vanishes.

Garamond, for reasons that will become unpleasantly clear at the end, thereupon orders a line of books on similar themes. Manuscripts pour in on such assorted topics as the Rosicrucians, the Masonic lodges, the Bavarian Illuminati, and all kinds of alchemical and other lore from the long Western tradition of hermetic and underground belief.

It is all so far-fetched and arbitrary that Belbo, Casaubon and Diotallevi decide they can do better. They feed the manuscripts and masses of other research into a computer. And they proceed to invent their own elaborate theory about the Templar secret.

It consists, they decide, of a map through which can be found a place where the earth’s magnetic currents form a kind of node. Power over this node will allow vast tectonic displacements. Japan, for instance, could be dumped into the Gulf of Panama. Such a power, of course, would make its holders the monarchs of the world.

Using scraps of history, and reams of mystical and cabalistic writing through the ages, the three construct the secret’s genealogy. The Templar tradition, they decide, has been fragmented and passed in bits into the hands of any number of rival secret societies.

Such figures as Roger and Francis Bacon, Descartes, Voltaire, Frederick the Great of Prussia, and Adolf Hitler had connections with them.

The trio keeps reminding itself that all this is an invention; a text, as it were. But they get sucked in. And all kinds of ominous events and pseudo-events begin to take place; various enigmatic figures begin to turn up. An incautious and invented hint that they possess the Templar map brings on a bomb attempt and blackmail threat. Finally, Belbo is kidnapped and taken to the old Templar church of St. Martin des Champs in Paris, now the science museum that houses the pendulum of the title, among other things. There, at midnight, a disorderly gathering of Satinists and Occultists and an assortment of secret orders demand the map. He refuses and is killed.

At the end of the book, Ditallevi is dying of cancer and regrets that, as a devotee of the Kabbala, he has played with “The Word.” And Casaubon is in hiding, expecting a visit at any moment from the “They” who eliminated Belbo.

It is a double semiological joke. If reality is simply a text, then the Milan trio can, simply by writing it, proclaim the secret of the universe. But if, to reverse things, text is reality, it can materialize and gobble them up.

Eco hovers astride the joke, neutrally and with evident glee. At the end, the narrator reflects that the only reality is in such things as the taste of a peach and the sound of a trumpet. And, after all, Belbo has died rather than reveal the details of a map that is pure invention. Is it Eco’s message that there is a reality worth dying for beneath the text? But it is text to say so.

Eco’s jokes are splendid. There is Belbo’s beloved computer. Casaubon struggles with elaborate numbers and combinations to get into it, but the machine keeps repeating: “Have you the password?” Finally, Casaubon gives up and writes: “No.” That, of course, is it.

There is the imaginary college that the trio devise in their spare time, offering courses in Potiosection (slicing soup) and Mechanical Avunculogratulation (building machines to greet uncles). There is Lia, Casaubon’s wonderfully sage girlfriend, who examines the Templar message--upon which the whole phantasmagoric structure rests--and concludes that it is simply a medieval merchant’s list of things to do that day.

We could have used more Lia. There are any number of good things in “Pendulum,” but a novel is not a truffle-hunt. Its central paradox is neat and springy, but a novel is not a mousetrap either.

Eco’s conceits are buried in a vast verbiage, a wearying thicket of occultist lore and scholarship. A whole leaden section is devoted to a visit to Brazilian voodoo ceremony; another to a tired occultists’ orgy. There is a seeming infinity of false leads and circling suppositions.

Eco’s game demands quite a few more reader candles than it is worth. “Name of the Rose” may have been the best-liked half-read novel of its day. “Pendulum,” I suspect, will be well-liked too; and only one-quarter read.