We know, from the testimony of Strabo, that Ravenna stood among lagunes in the time of Augustus, as Venice does now; but at present Ravenna is a league distant from the shore. Spina had been built by the Greeks at the edge of the sea; yet in Strabo’s time it was ninety stadia from it, and is now destroyed. Adria in Lombardy, which gave name to the Adriatic sea, and of which it was, somewhat more than twenty centuries ago, the principal port, is now six leagues distant from it. Fortis has even rendered it probable that, at a more remote period, the Euganian Mountains may have been islands. M. de Prony, a learned member of the Institute, and inspector-general of bridges and roads, has communicated to me some observations which are of the greatest importance, as explaining those changes that have taken place along the shores of the Adriatic[97]. Having been directed by government to investigate the remedies that might be applied to the devastations occasioned by the floods of the Po, he ascertained that this river, since the period when it was shut in by dikes, has so greatly raised the level of its bottom, that the surface of its waters is now higher than the roofs of the houses in Ferrara. At the same time, its alluvial depositions have advanced so rapidly into the sea, that, by comparing old charts with the present state, the shore is found to have gained more than six thousand fathoms since 1604, giving an average of a hundred and fifty or a hundred and eighty, and in some places two hundred feet yearly. The Adige and the Po, are at the present day higher than the whole tract of land that lies between them; and it is only by opening new channels for them in the low grounds, which they have formerly deposited, that the disasters which they now threaten may be averted. The same causes have produced the same effects along the branches of the Rhine and the Meuse; and thus the richest districts of Holland have continually the frightful view of their rivers held up by embankments at a height of from twenty to thirty feet above the level of the land. M. Wiebeking, director of bridges and highways in the kingdom of Bavaria, has written a memoir upon this subject, so important as to be worthy of being properly understood, both by the people and the government, in all countries where these changes take place. In this memoir, he shews that the property of raising the level of their beds is common in a greater or less degree to all rivers.