Towards the middle of the twelfth century, the flood-waters of the Po were retained on their left or northern side by dikes near the small city of _Ficarolo_, which is about 19,000 _metres_[390] to the north-west of Ferrara, spreading themselves southwards over the northern part of the territory of Ferrara and the _Polesine_ of Rovigo, and flowed through the two formerly mentioned canals of _Mazzorno_ and _Toi_. It seems perfectly ascertained, that this change in the direction of the waters of the Po had been produced by the effects of human labours; and the historians who have recorded this remarkable fact only differ from each other in some of the more minute details. The tendency of the river to flow in the new channels, which had been opened for the more ready discharge of its waters when in flood, continually increased; owing to which the two ancient chief branches, the _Volano_ and _Primaro_, rapidly decreased, and were reduced in less than a century to their present comparatively insignificant size; while the main direction of the river was established between the mouth of the Adige to the north, and what is now called _Porto di Goro_, on the south. The two before-mentioned canals of _Mazzorno_ and _Toi_ becoming insufficient for the discharge, others were dug; and the principal mouth, called _Bocco Tramontana_, or the northern mouth, having approached the mouth of the Adige, the Venetians became alarmed in 1604; when they excavated a new canal of discharge, named _Taglio de Porto Viro_, or _Po delle Fornaci_, by which means the _Bocco Maestra_, was diverted from the Adige towards the south. During four centuries, from the end of the twelfth to that of the sixteenth, the alluvial formations of the Po gained considerably upon the sea. The northern mouth, which had usurped the situation of the _Mazzorno_ canal, becoming the _Rama di Trimontana_, had advanced in 1600 to the distance of 20,000 _metres_[391] from the meridian of Adria; and the southern mouth, which had taken possession of the canal of _Toi_, was then 17,000 _metres_[392] advanced beyond the same point. Thus the shore had become extended nine or ten thousand _metres_[393] to the north, and six or seven thousand to the south[394]. Between these two mouths there was formerly a bay, or a part of the coast less advanced than the rest, called _Sacca di Goro_. During the same period of four hundred years previous to the commencement of the seventeenth century, the great and extensive embankments of the Po were constructed; and also, during the same period, the southern slopes of the Alps began to be cleared and cultivated.