What if . . .

Fortune Is A River: Leonardo Da Vinci and Niccolo Machiavelli's magnificent dream to change the cour

I took up this book with avidity, since the blurb promises one of those famous meetings which so enliven history, especially if they are one-off encounters such as those between Wellington and Nelson, or Franco and Hitler. By common consent, the near contemporaries Machiavelli (1469-1527) and Leonardo da Vinci (1452-1519) were two of the greatest figures of the Italian renaissance, and the idea that they might have collaborated is irresistible. The author claims that his narrative is the fruit of historical detective work but the story he unfolds is a mystery and scarcely in a way desired by the author.

Let us deal first with the solid ground. It is a historical fact, mentioned among others by Vasari in his famous Lives of the Painters, that Leonardo was employed by the government of Florence in 1502-06 and that he devised a scheme to divert the river Arno, thus at once turning Florence into a seaport and depriving the rival Tuscan city-state, Pisa, of its water supply. During the same period, Machiavelli was entrusted by the Florentines with many diplomatic missions, and both he and Leonardo took orders from Florence's then leading light, Piero Soderini. Among variegated tasks performed for Florence's ruling elite, Leonardo was commissioned to paint a giant fresco in the Great Council Hall of the Palazzo Vecchio.

It is also known that both the damming of the Arno and the fresco were abandoned, that Soderini was discredited (in part because the great renaissance man whom he had hired failed to live up to expectations) and that Leonardo thereafter sought royal patrons who were not subject, as Soderini was, to the winds of political change. Why did the Arno project fail, despite employing 3,000 men (2,000 workers and 1,000 troops to guard against attacks from Pisa) and having 7,000 ducats spent on it? Masters identifies a number of reasons. Not enough money was spent to see Leonardo's idea through to fruition, and Soderini proved to be too weak politically to impose his will on the Florentine oligarchy. The choice of engineer for the scheme, Colombino, was a disaster, since he changed Leonardo's design, substituting two separate diversions of the Arno for the one Leonardo proposed, and cutting corners by trying to complete the project in a mere three weeks; Leonardo had called for a six-month commitment, and then only if a digging machine (which he designed) was built. Instead of a permanent weir, Colombino, trying to do the job on the cheap, failed to dig the diversionary ditches deep enough; it was classic false economy, since the Arno simply resumed its original course.

Thus far historical fact. But Roger Masters, versed in the history of the Italian renaissance, has gone (if I may use the expression) a bridge too far in trying to tell this story as if it were a collaboration between Leonardo and Machiavelli. Unfortunately, he cannot find a scrap of documentary evidence to sustain his case, so proceeds to talk airily of the dangers of discussing matters by letter in the turbulent era of Cesare Borgia, since the authorities habitually intercepted correspondence. The blurb speaks confidently of a meeting between Machiavelli and Leonardo, and Masters actually has a chapter entitled "The Meeting" - a classic example of Sherlock Holmes's dog that barked in the night, for there is no meeting. Instead we have Masters' unsatisfactory statement that there is "circumstantial evidence" that his two principals met in 1502. If history was inference, Masters would be home free; but it is not.

Having established the "fact" of the meeting, Masters then proceeds to write a potted biography of both men. What seems to have happened is that, noting the plethora of Machiavelli and Leonardo biographies, he proposed that new fad, a "dual biography", and his editor asked for an organic link between the two. Masters then employed those old stand-bys, "must have", "might have" and "could have". Yes, it would be marvellous if Leonardo had met Machiavelli, and many historical novels are written around this sort of premise, with Odysseus meeting Moses, Gordon meeting Mahdi or even (as in Schiller's Maria Stuart) Elizabeth I of England meeting Mary, Queen of Scots. But to rely on circumstantial evidence in a work of history is to employ a notoriously false friend. To take one example: in 1916 Bertrand Russell was deeply embroiled in difficult relationships with Wittgenstein and D H Lawrence, and met both men on numerous occasions. The historian working from inference or circumstantial evidence would be entitled to conclude that Lawrence and Wittgenstein met. But in fact they never did. Masters' failure to make out his case for the encounter between Leonardo and Machiavelli explains why, despite my eagerness when I took up the book, I laid it down severely disappointed.

Frank McLynn's most recent book, "1066: the year of three battles", is published by Jonathan Cape, £18.99