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.