Jan Morris sifts through the myths of a city where history was as clear as mud

Venice and Antiquity: the Venetian sense of the past by Patricia Fortini Brown, Yale University Press, pounds 45

This is an academic work, by an associate professor at Princeton, dressed up in the Yale manner as a sumptuous coffee-table book. As such it will give pleasure to all classes of Venetian aficionado, except perhaps those who believe that Venice is ill-served without some element of careless rapture. If you weary of the style (Professor Brown is no John Julius Norwich) you will certainly enjoy the lavish, varied and elegantly disposed pictures. If you find yourself overwhelmed by the mass of incidental evidence, the bold main theme of the book is sure to interest you.

It concerns, au fond, the Venetian relationship with time. It starts with a plaque on a pulpit at Torcello illustrating Kairos, the classical wheeled figure of Opportunity, being seized by the hair by a youth; it ends with a sculpted figure on a wall of Ca' Bembo, in the sestiere of Canareggio, of a primitive holding a solar disc, a figure of eternity. Professor Brown's thesis, developed with an infinity of scholarly allusion, is that the Venetians more or less made up their own time. A people of opportunism from the very start, by trial and error, fraud and imagination, by grabbing the passing chance and exploiting even the most improbable allegory, they created in the end the myth of a City-State that seemed eternal. "Venice has preserved her independence during eleven centuries", Voltaire wrote, "and I flatter myself will preserve it for ever".

The original problem was that Venice started from scratch, as a community of people driven out of the mainland into the islands of the lagoon in the aftermath of the Roman Empire. It could claim no classical origins, like most Italian cities. It was a mixed bag of refugees, with no civic claim to descent from Rome. This at least meant that it had no pagan roots either, so the obvious thing to do was to project itself as a Christian city of God. Chroniclers learnt to emphasise that the Venetians had been driven into the lagoon not by the Christian Lombards, but by the impious pagan Attila, frequently portrayed with horns: and presently the miraculous discovery of the relics of St Mark, bursting from a column in the Basilica, proclaimed Venice a genuinely apostolic city. It had been founded on the day of the Annunciation - March 25, also the day when God made Adam - and there was obviously something providential about its peculiar situation and ever-growing prosperity there in the watery wastes.

The Renaissance, seducing Venetians as it seduced all other Italians, changed the perspective rather, and made paganism fashionable again. Petrarch was all the rage in Venice. Now Venetian aristocrats liked to claim Roman pedigrees, and it was often claimed that Venice was actually older than Rome, having been founded in an earlier form on the mainland. Like many another European people - even the distant Welsh - the Venetians toyed with the idea of having Trojan origins: did not the very word "Venetia" stem from "Aeneas?" They began to see antiquity in humanist terms - "shepherds, nymphs, and verdant groves," in Professor Brown's words - and to imagine the earliest lagoon settlers as people of Arcadian virtue.

Finally, in the heyday of the Republic, the Venetians became less interested in their past than in their magnificent present, the terrific opportunities always waiting to be seized, the promise of everlasting splendour. They had made good use of antiquity, in creating a national myth, and they were to live upon its accretions until, 200 years ago this year, it all collapsed in ignominy. "J'ai occupe ce matin". General Louis Baraguey d'Hilliers reported to Napoleon, "la ville de Venise, avec la 5e demi- brigade de bataille": and that was that.

This is a pitifully inadequate and simplistic attempt to reflect what Venice and Antiquity is about. The book is a prodigious assembly of facts, conjectures and illustrations concerning what psychiatrists might call Venice's efforts to find itself - and to project itself. Professor Brown may not be the most lucid of expositors, but she is a scholar of stunning range and force, and she seems to grab the very idea of Venice, its very stones in fact, to shake the truth out of it.

She finds evidence of the Venetian concern with antiquity in every aspect of the place. She scours the city for remains of the booty, brought from Byzantium, from the Aegean, from the Italian mainland, by which the Serenissima tried to demonstrate its pedigree: not just the familiar figures, marbles and Byzantine treasures of San Marco, but a mass of Roman tombstones, unnoticed pillars and reliefs. She describes the growth of antiquarianism in the 14th century, the gradual absorption of classical forms and images into the Venetian aesthetic. She pursues her purpose through literature, art, architecture, cartography, genealogy, even astrology, and if sometimes one is inclined to forget just what the purpose is, and she occasionally has to pause and remind us (and perhaps herself), nevertheless the virtuoso nature of the pursuit itself is a marvel to watch.

She is at her best, I think, when she is discussing the particular Venetian truncations of time - an essential process, I suppose, in the evolution of civic eternity. I have always thought, for example, that the portal of the Arsenale was just a flamboyant expression of Venetian self-esteem, a hodge-podge of motifs tumbled rather raffishly together. Professor Brown dissects it more exactly. It includes a date in Roman numerals recalling the city's legendary foundation in 421, a deliberate copy of a Roman arch at Pula, two pairs of Byzantine columns, a standard 15th-century Venetian lion of St Mark, a scallop shell, a couple of decorative urns and sundry statues of pagan import, all providing a ceremonial entrance to the centre of Venetian naval power. In short, it turns out to be a kind of architectural index of Venetian historical pretensions.

Or take Carpaccio's glorious Entombment Of Christ, beautifully reproduced here in the whole and in detail. Professor Brown uses this picture as an example of "contemporising antiquity" - muddling time again. Classical ruins are scattered about the picture, along with skulls and bones to represent the distant past and a culture long gone: but on a ridge in the background, together with a flock of sheep, are two purely contemporary figures, dressed anachronistically in the 16th-century manner. One is a shepherd boy playing a pipe, the other a bearded scholarly-looking figure, a philosopher perhaps, or a Renaissance poet, in attitudes so motionless, yet so alive, that they might be interpreted as timeless.

As it happens, timeless is what Venice always was to me. Despite the tourists and the motor-boats, the film festival and Harry's Bar, the old city still seems to inhabit a moment that is eternal. Professor Brown, in her remarkable and loving book (for she clearly does love Venice, even if she does talk about influencing the articulation of its urban environment) demonstrates throughout how Venice's concept of time is reflected in its art: and nothing is more purely Venetian, to my mind, than the silent hush, so curiously still, confident and deliberate, that is illuminated for ever by the lightning-flash in Giorgione's The Tempest.