Oh, You Leonardo! : FICTION : THE MEMORY CATHEDRAL: A Secret History of Leonardo da Vinci,<i> By Jack Dann (Bantam Books: $22.95; 496 pp.)</i>

Leonardo’s curse was that he remembered everything,” Jack Dann solemnly announces in the prologue to “The Memory Cathedral.” We are told that memory, not sight, was the primary sense of the great Leonardo, that his fertile, inventive mind--”a holly house of memory, from which nothing could be lost”--is replete with a thousand adventures, thoughts and dreams. And so a dramatically insightful autobiography follows, one filled with a rich and exact narrative of artistic and scientific disclosures, right?

Wrong. What we are given is a historical novel, which, often descending from fancy into farce, travels back over colorful Renaissance terrain less by way of the Master’s notebooks--to which this long novel is mainly (and admittedly) indebted--than by Dann’s own imagination. The novelist explains in an afterword how he has frankly gone about his business “revising the myth of Leonardo.” He tells us how he “nudges history” in writing it. He candidly confesses, “I set my story between the cracks of known history to explore Leonardo’s character and the moral ramifications of his brilliant ideas and inventions.”

It is no secret that one of the book’s major ambitions is to save Leonardo from charges, essentially Freud’s, of homosexuality (Dann admits “I took a compelling minority view . . .”) and so we shouldn’t be surprised to find red-blooded Leonardo in scene after sweaty, savage, urgent scene, when not fashioning ornithopters and devising flame-throwers, bonking every fair maiden that comes into view. Especially vile, nevermind utterly misbegotten and slanderous, is the scene where beautiful Simonetta Vespucci--the chaste subject of poets in her day as well as the young model for Botticelli’s Primavera and Birth of Venus--fellates Leonardo, with dialogue right out of TV’s “Baywatch.”

But the love of Leonardo DaVinci’s life was, according to Dann, Ginevra de Benci. Dann bases the story of this consuming passion on nothing but the fact that Leonardo was a friend of her illustrious family and had once painted her, the lovely pie-faced, red-headed portrait which, of course, now hangs in Washington, D.C.

Half of “The Memory Cathedral” deals with Leonardo’s apostrophes to this lost love--she marries a sympathizer to the Pazzi conspirators and is killed for it--and the other half to his mercilessly unending peregrinations in the East, another of Dann’s historical “nudges,” for Leonardo is kidnapped from Florence by a ruthless warlord, the Devatdar of Syria and by way of his inventions made to serve the Caliph’s revenge on the Turk. Parading through this book, along with people like Ginevra and Kait Bay, Caliph of Egypt, are Christoforo Columbus, Amerigo Vespucci, Ludovico Sforza, Pico della Mirandola, Lorenzo the Magnificent, and so forth. Everybody appears in this book! At one point in the novel, the painter Sandro Botticelli, breathless with news, comes running to Egypt to tell Leonardo, who is sitting on a camel, that Niccolo Macchiaveli is safe!

We are ludicrously given to believe that only after various amorous but unsuccessful misadventures, comprising essentially the first half of the novel, can the great Leonardo, as if as an alternative, turn to invention--as a consolation! “He would not be reminded of Simonetta . . . of Ginevra,” Dann writes in all seriousness. “Instead he became obsessed with mathematics and invention and anatomy. . . . He had fashioned a cold, hollow place for himself.” Thwarted in love--sort of like Emile DeBecque who turns to war thinking he lost Nellie Forbush--Leonardo turns to mechanics, and “The Memory Cathedral” suddenly becomes a picaresque tale of adventure, with scenes shifting to the mysterious East, where, in spite of the fact that Dann has humbly conceded he is fictitiously “entering the cracks of known history,” Leonardo Da Vinci seems no more at home as a character than Mother Teresa of Calcutta in a Broadway musical.

I was alone struck by Dann’s knowledge of 15th century Italy, its customs and culture. He has done his homework; one can only imagine the amount of his collateral reading and research. Dann knows things, such as the medicinal value of Agnus Scythicus; details of plagues and torture machines; the softness of Siena stone for sculpting; the colors of mourning (“shades of mulberry, green and brown”); the detailed ballets of famous crimes and assassinations; the lore of jinns and racing camels and Bedouin habits; Turkish weapons and Mamluk soldiers.

The scene of the Pazzi attack against Lorenzo de Medici (wounded) and his brother Guiliano (killed) on Easter Sunday in 1478 in the great cathedral of Santa Maria del Fiore is exceptionally graphic and well done, as are Dann’s minor disquisitions on Leonardo’s endless fascination with, ideas for, and sketches of such mechanical ingenuities as winches, cranes, spherical projectiles, lens polishers, submarines, transportable siege bridges, bombs made of hemp and fish glue and of course flying machines and the whole idea of flight itself. We are treated with great exactitude to a balloon flight and to no end of Leonardo’s dreamy speculations on the nature of soaring, in every sense.

A pity that “The Memory Cathedral” wasn’t shorter, for it is long-winded and at times painfully self-indulgent, the inevitable hazard, to my mind, when the very legitimate fancy in the novelist of asking “What if?” hasn’t the concomitant urge to answer with “What if not?”