_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.