The use of locks on canals may be said to date from this time. It is related that in the canalisation of the Mincio, Pittentino so regulated the rise and fall of the river that boats could ascend to Mantua and descend to Po, the depth being so equally maintained that the river was navigable for about twelve miles. This must have involved the employment of locks, however rude.[96] 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.