The frequent inundations caused by the Po and the numerous changes of its bed, by spreading the alluvium all over the country, have raised the whole of the plains to about the same level. But now, when all the arms of the Po are confined within embankments, most of the alluvium brought down by the floods is deposited on the coast of the Adriatic. The land, therefore, gains much more rapidly upon the sea than it did formerly. The series of dunes marking the ancient shore now lies fifteen miles inland, and the new land formed annually is estimated at 280 acres. In exceptional years the quantity of solid matter carried by the river into the sea amounts to 3,531,000,000 cubic feet; on an average it is 1,623,000,000 cubic feet, sufficient to form an island ten square miles in area in ten feet of water. The Po, next to the Danube, is the most active geological agent amongst all the rivers {213} entering the Mediterranean.[66] The Rhone is inferior to it, and so is the Nile. At the present rate of progress, the Po, in the course of a thousand years, will throw a tongue of land six miles wide across the Adriatic, converting the Gulf of Trieste into an inland sea. Northern Italy, in addition to these numerous rivers, possesses one of the most extensive systems of canals in the world, which has served as a pattern to all the rest of Europe. Lombardy, portions of Piemont, the Campagna of Turin, the Lomellina on the Ticino, and the Polesinas of Ferrara and Rovigo possess a wonderful ramification of irrigation, which carries fertile alluvium to the exhausted fields. In the Middle Ages, when the remainder of Europe was still shrouded in darkness, the Lombard republics already practised the art of irrigation on the vastest scale, and drained their low-lying plains. Milan, after she had thrown off the yoke of her German oppressors, towards the close of the twelfth century, constructed the _Naviglio Grande_, a ship canal derived from the Ticino, thirty miles distant—probably the first great engineering work of the kind in Europe. In the beginning of the thirteenth century the superabundant waters of the Adda were utilised in filling the Muzza Canal. The same river, at a subsequent period, was made to feed another canal, the Martesana, which was constructed by the great Leonardo da Vinci. The art of surmounting elevations of the ground by means of locks had been discovered by Milanese engineers about a century before that time, and was applied to the construction of secondary canals. Amongst works of more recent date are the _naviglio_ from Milan to Pavia; the Cavour Canal, fed by the Po, below Turin; and the Canal of Verona, derived from the Adige.[67]