The Tiber.—In Italy another great undertaking has been agitated, namely, to render the Tiber navigable from Ponte Nuovo, below Perugia, to the entrance of the Nera, from which the navigation begins to be free and without interruption, to the sea. MM. Boltari and Manfredi reported on an inspection which they made of the Tiber in 1732. In this report they laid it down as a first principle, derived from experience, that to navigate any river with facility, particularly against the stream, it is requisite that the slope should not exceed 3 Roman palms per mile (a Roman palm is about 8½ English inches).
Now, as the fall of the Tiber is 8 or 9 palms, they calculated that it would be very difficult to steer the boats down the river, and still more difficult to conduct them up against so rapid a stream, especially in some places where the fall was even greater, and where, consequently, the stream must, they held, remain impassable. They, moreover, pointed out the difficulties and the dangers which must be encountered in adopting the different expedients that had been proposed for reducing the excessive slope by weirs, for removing the detached stones by manual labour, for blowing up the obstructing rocks by mines, and for removing the bed, in certain places, by changing its course, or by contracting or enlarging its dimensions.
The schemes proposed for rendering the bed of the Tiber navigable having been thus discredited, the same engineers inquired whether a canal for boats of a moderate size and suitable burden might not be formed parallel with the river; observing the nature of the soil through which the canal must pass, the different crossings that would be required from one side to the other, the number of dykes and sluices that would be wanted, and the other works that would be necessary to secure the navigation against all accidents, and particularly those from floods. This undertaking they regarded as very difficult of execution, and they advised that it should not be attempted. They next examined the plan of making the Tiber navigable to Rome, proposed by the engineer Chiesa, in a report printed in 1745, but nothing came of these proposals.
Within the last two years, a new project has been brought forward with the view of rendering the Tiber navigable to the sea, and it is possible that this work will before long be attempted.
The Villoresi Canal.—The water for this canal is derived from the Ticino, at a place called “Rapida del Pamperduto,” by means of a weir thrown across the river. This weir is 290 metres (951·2 feet) long, and 24 metres (78·72 feet) broad, and of sufficient height to raise the water in the Ticino 3·75 metres (12·30 feet) above the ordinary low-water level. Below the right abutment the river-bank is protected by a wall for a distance of 50 metres (164 feet), whilst up stream, on the same side, an embankment, partly in masonry and partly in earthwork faced with stone pitching, has been constructed for a distance of 600 metres (1968 feet), in order to confine the river to its present bed. At right angles to the weir is a lock, with a drop of 6 metres (19·68 feet), the largest in Italy, which serves for the passage of boats from a channel below, 10 metres (32·8 feet) wide, and about one kilometre (0·62 mile) long, from the canal to the Ticino. The channel is supplied with water from the basin below the measuring weir by means of four sluices 0·80 metre by 1·20 metre (2·62 feet by 3·93 feet) placed in the wall which separates the basin from the canal. On the side of the basin, opposite the weir, are two buildings, the first containing the sluices, which admit 8 cubic metres (282·52 cubic feet) per second of water into a canal belonging to the Visconti family; and the second, which forms the entrance to the Villoresi Canal, serves to regulate and maintain the level of the water in the basin constantly at 0·90 metre (2·95 feet) above the crest of the weir. It consists of a three-storied building, in the lower part of which are six sluices, 2·30 metres (7·45 feet) wide, and 3 metres (9·84 feet) deep, with iron gates, worked by suitable mechanism from the floor above. The headworks, which are on the left bank, consist of a building 67 metres (219·76 feet) long, 6 metres (19·68 feet) wide, and 12·80 metres high, provided with thirty sluices, each of 1·50 metre (4·92 feet) clear width, and 3·25 metres (10·66 feet) high, the cills of which are placed at 2·75 metres (9·02 feet) below the level of the crest of the weir. These sluices are capable of admitting 190 cubic metres (6710·13 cubic feet) per second into the canal from the river, of which 70 cubic metres (2472·15 cubic feet) per second is the amount granted by the concession to the Villoresi Canal. The remaining 120 cubic metres (4237·98 cubic feet) per second have to be returned to the Ticino by a specially constructed measuring weir established at 600 metres below the headworks, in order to respect the existing rights of others further down the stream. The passage of boats from the Ticino to the canal is provided for by means of a channel with a lock 8 metres (26·24 feet) wide.
The Canals of Venice.—In speaking of the canals of Italy, it would be unpardonable to omit due reference to those which give to Venice, the “mistress of the Adriatic,” her peculiar and pre-eminent position. Founded in the year 452, soon after Attila invaded Italy, Venice is built upon a number of small islands, and is divided into two nearly equal parts by the “Grand Canal,” 1200 yards in length, and 100 feet in breadth. Many smaller canals branch off from the Grand Canal. These are crossed by some five hundred bridges, many of them of considerable architectural pretensions.
The construction of the canals of Venice was a work that would be naturally unlike that of laying out a canal in the ordinary course. The whole city, built on a number of small islands, is more or less constructed on piles; there is an almost dead level throughout; and the waterways would, no doubt, in the majority of cases, be naturally formed, at least to a partial extent. There is, however, very little information extant as to the circumstances under which the work of adapting the canals to the requirements of the population was carried out.
Irrigation Canals.—It would hardly be proper to pass from the canal system of Italy without making some remarks on the excellent system of irrigation canals that has been provided in Lombardy and Piedmont. Navigation canals take priority over irrigation canals in Lombardy in point of origin, but not to a great extent. The Vettabbia Canal, which is supposed to have been used for navigation previous to the eleventh century, is claimed as the oldest existing canal in Lombardy. In the latter part of the twelfth century, the Cistercian monks of Chiaravalle obtained possession of this canal, and applied its waters to irrigation purposes. Not very long afterwards the same order of monks constructed the Ticinello, a canal derived from the Ticino at Tornavento, and it was used exclusively for irrigation until 1177, when it was enlarged and partly opened for navigation. In 1257, the same canal was so far enlarged as to connect Milan with Lake Maggiore, and the waterway is now known as the Naviglio Grande.
One of the most important irrigation canals in Italy, which may be briefly described as illustrative of the system generally, is that of the Cavour Company, in Piedmont, which is derived from the left bank of the Po, near the town of Chivasso, and was constructed for the purpose of irrigating the provinces of the Vercellese, Novarese, and Lomellina. It was Francesco Rossi, a land surveyor of Vercelli, who, in 1844, first proposed to employ the waters of the Po for irrigation purposes. It was a good many years later, however, before the project was undertaken. The head works of the Canal Cavour are situated about 400 metres below the bridge over the river, on the road which connects Chivasso with the military road from Turin to Casale. The full discharge of this canal is 110 cubic metres per second, and its supply is obtained by means of a temporary dam of timber carried across the river. The sluice-house for regulating the supply of water to the canal is built across the canal, which is 40 metres in width, and consists of twenty-one openings separated by granite piers. Each opening is provided with three sluice-gates, which work in grooves cut in the granite piers, and can be easily raised or lowered by the sluice-keeper by means of a lever. The remainder of the building is constructed principally of dressed stone and bricks, and the contrast between the granite used for the quoins and the red brickwork has an excellent effect. Another sluice-house, placed at right angles to that of the main canal, communicates with that of the “Scaricatore,” or discharge channel, by means of which the surplus waters in times of floods may be discharged into the Po, and any deposit of gravel and sand on the floors in front of the entrance to the main canal can be effectually swept away by the velocity of the water discharged into the “Scaricatore,” which has a rapid fall, and enters the Po again, about 2 kilometres below the headworks.
The quantity of material used in the construction of this important work was:—