The Lake Maggiore is the source of the Tesino, which in its course is divided into several streams, which, however, are reunited before it enters the Po, near Pavia. For the whole distance it is navigable, although at Pan Perduto, where the fall is considerable, it is sometimes hazardous. Immediately below this spot commences the canal to Milan, which at Abbiate divides into two channels. The entire length of the excavation is about 32 Italian miles, and its breadth 70 Milanese cubits.
The Canal della Martesana, by some supposed to have been executed by Leonardi da Vinci, was made in the year 1460, under the Duke Francis Sforza. Leonardi da Vinci joined the two canals some time during the reign of Francis I. The Canal della Martesana, which is drawn from the Adda, is 24 miles in length, and in width about 18 cubits; but when constructed at first, the water it contained was barely sufficient for navigation for more than two days in the week, and this only when all the openings for the purposes of irrigation were closed.
One of the branches of this canal was carried for several miles by a stone dyke, and afterwards passed through a deep cutting. The other branch had its course through the rock, after which it was supported on one side by a lofty embankment, where it crossed the Molgara river by an aqueduct of three stone arches.
Early in the thirteenth century, Bassanallo had a canal 11 miles long, which was navigated by the vessels that brought building stones to Venice. One of the several canals in the lagunes, on which the latter city is built, is 36 miles long. Between Padua and Venice, again, there is a canal some twenty miles in length, which has a fall of 50 feet, to overcome which four locks are provided.
Milan, like Venice, is the centre of a network of canals. Here unite the great canal of Tesino and the branch from Pavia; the Muzza Canal, which commences at Cassano and ends at Castiglione, after traversing a distance of 40 miles; the canal of Abiato, made in the thirteenth century, which has a top breadth of 130 feet, and a bottom breadth of 46 feet; and the canal which connects Buffolaro, Biagrasso, and Arsago with Milan.
Nor is Piedmont less rich in monuments and resources of the same description, having more than half a dozen canals which communicate with the Po at different points. Most of these canals are, however, of limited extent, the longest, called the Naviglio d’Inea, being 38 miles in length.
The canals, large and small, in the Papal States, are so numerous that it would be wearisome to enumerate them. None are of great length, and most of them have been constructed rather with a view to drainage or irrigation than to navigation.
Pagnani has left us an account of the levels and other operations of art, undertaken by former engineers, to ascertain whether some navigable canals might not be projected in Lombardy; and, above all, to determine the practicability of joining the Lake of Como with the neighbouring lakes. In the first place they found that the surface of the lake of Como was 48 braces lower than the surface of the lake of Cevate, 62 braces lower than that of the lake of Pusiano, and about 100 braces below that of the lake of Lugano; further, that the lakes of Como and Lugano are, at the point of their nearest approximation, in the valley of Porlezza, about six miles distant from each other; and that they are separated by a very high ridge, which would render any attempt at a navigable canal very arduous, even independently of the very great difference in the levels. The general map of Lombardy will, on a slight inspection, show these several places.
The same engineers found that the scheme of running a canal from the lake of Lugano by the valley of the Olona to Milan was impracticable. It might, however, be possible to render the Olona navigable below Tredate, provided the waters were retained in the last trunk by means of some well-situated locks, and the upper mills were so placed as not to interrupt the bed of the river. In the project to render navigable the Tresa, which is the outlet by which the lake of Lugano discharges itself into the Lago Maggiore, these engineers found difficulties from the deficiency in the body of the water, and from the too great slope of the Tresa; to which it may be added that several torrents which enter it carry into it stones and gravel. It has been considered strange that these engineers never thought of another project, of which the execution would be easy, as well as convenient and useful—namely, to make navigable the Boza, which is the outlet of the little lake of Varese into the Lago Maggiore.
The scheme of conducting a navigable canal from Milan to Pavia is of a much older date, having been designed for the purpose of joining the two canals of Milan with the Tesino, the Po, and the sea. Galeazzo Visconte, the father of Azzon, began its excavation. In 1564, the completion of the work was made the subject of considerable discussion. It was imagined that the expense could not be very great; and that by giving the sluices the common height, a great number would not be required. The enterprise was abandoned afterwards, because the canal of Bereguardo, although it did not reach the Tesino, was found sufficient to keep up the commerce between the two cities of Milan and Pavia. Pagnani, in the Treatise already referred to, mentions some other projects of a similar nature.