Baltic, and a great part of it is built upon piles. The inundation, therefore, becomes part of the sea, and great waves roll through the town, and beat upon the houses as on stranded ships.

Three great inundations are recorded within the last one hundred years, those of 1726, 1777, and 1824, of which that of 1824 was the most disastrous. A violent west wind raised a barrier of water at the Neva's mouth, and the river rose until it had attained a height of from six to twelve feet in every quarter of the city. Immense waves raised by the hurricane beat with fury into the midst of the town. Entire streets were destroyed, and the miserable cottages of the suburbs were carried away, and their wretched inhabitants either buried under their ruins or engulfed in the Neva. Not only were boats swept into the midst of the town, but at Cronstadt, the port, a ship of a hundred guns was carried into the midst of a square, overthrowing all the buildings in its way. The number of victims was considerable, for there was no rising ground to which they could escape. In one of the barracks the soldiers sought refuge upon the roof. In a short time the waves had shaken the building, so that it fell to pieces, and all these unhappy men disappeared beneath the waters. These scenes of horror were repeated during the small number of hours which the inundation lasted. Happily the wind calmed after three hours, and in a short time the town was free from water.*

Nothing like this could ever happen in the case of the Tiber, even if Rome were situated at the apex of the Delta or built on the site of Fiumicino, so as to be more within the influence of the sea. The river is scarcely wider at its mouth than it is for many miles higher up, so that a wave would not increase in height as it ascended the stream; and the wave at starting would not be more than about four feet above the sea level, which was the greatest height observed

Voyage en Russie, par Léon Renouard de Bussière. Lettre IV.

by Gambarini and Chiesa during the most violent storms.* Again, the Tiber, instead of flowing into a funnel-shaped bay like the Neva, has pushed out a tongue of land into the sea, so that all the conditions on which floods produced by gales of wind depend, are here reversed. No force of wind, even if an Oriental cyclone were cyclone were combined with a West Indian hurricane, could on this shallow coast heap up the water in the manner supposed by the advocates of the Scirocco theory of inundations. Were it possible for such a wall of water to be raised, it would not be confined to the Tiber's mouth, but extend along the whole coast, and flood far into the interior the low lying country which stretches north and south from eighty to a hundred miles. Yet such an effect has never been observed. The greatest floods, also, rise but little in the Delta of the Tiber, where, according to the theory which I am controverting, they ought to be the greatest. From Ostia to Torre San Michele, and thence to the sea, the rise is trifling.

But there are some who say that if the wind does not heap up the waters of the sea, and impel great waves into the mouth of the river, it may, when it blows against the course of the stream, retard by its friction the velocity of the current, and thus gradually raise the level of the water, until, either alone or aided by rain, it causes the river to overflow its banks. Theoretically, one would not expect that the friction of the wind on a nearly smooth and horizontal surface would produce any appreciable effect. But this is a question to be determined by observation and experiment. It is not found

that other rivers are flooded, or sensibly raised by winds blowing against the course of their streams, unless their estuaries are of the form which I have described. No inundations of the Po are produced by easterly, or of the Rhine

*Off the Cape of Good Hope waves are sometimes forty feet in height. In our seas, those about England, only ten and, in exceptional cases, eighteen feet (Land and Sea); but half this height is depression below the mean level of the sea.

by northerly winds, however violent they may be; and in the case of the Tiber, it is a remarkable fact that the days on which the greatest floods have occurred during the last thirteen years, have either been quite calm or with a light breeze from the north-east. The answer usually given by the advocates of the theory which attributes these floods to wind, when their attention is directed to this fact, is "that a heavy gale may be blowing at the mouth of the Tiber while a dead calm prevails at Rome." This is a purely gratuitous assumption, and so improbable-considering the short distance as a bird. would fly, scarcely fifteen miles from Rome to the sea-as not to deserve a serious reply. Gambarini and Chiesa report that the effect of the wind in retarding the current of the river is extremely slight. This they ascertained by comparing the velocity of a piece of wood when the day was calm, and when there was a gale of wind blowing up the stream, and they ridicule the notion that the wind has any considerable effect in causing the rise of the Tiber.

To those who are not acquainted with Rome it may appear a waste of time and paper to argue against notions so absurd; but these opinions are held by many persons well known in the city, both Romans and English residents in Rome, and by diverting the public mind from the true causes of the floods of the Tiber, they prevent any effectual precautions being taken against them.

In the time of Bonini and Bacci, or three hundred years ago, it was the fashion to ascribe these inundations to the great quantity of water brought down by the Velino, which, along with the Salto and Turano, drains, as will be seen from the map, a large area of country; and Bacci speculates on the feasibility of diverting it from the Nera, and turning it through a rocky district abounding in caverns and chasms, which he fancied would be able to swallow it up. The same authors ridicule the apprehension which prevailed in those times that the " "muro grosso," or great dike of the Chiana, which kept up a head of water in that river, would give way in time of floods, and liberate a mass of water by which Rome

would be submerged. These notions are mentioned along with the popular fallacies of the present day, to shew the tendency of the human mind to look for the cause of a phenomenon in every direction but that in which it is likely to

be found.

CAUSES OF THE INUNDATIONS.

Having shewn that the popular theories are insufficient to explain the great and sudden risings of the Tiber, I proceed to state the real causes of these floods. The inundations of a river depend:

1. On the area and form of its basin.

2. On the rainfall within that basin, and its distribution among the months of the year.

3. On the permeability of the soil traversed by its affluents. 4. On the number of its tributaries.

I. AREA OF BASIN.

If a fall of rain extends over the whole basin of a river, the quantity of water which finds its way into the river and swells its stream, will, cæteris paribus, be proportionate to the area of the basin. On the other hand, the larger the basin, the less likely is a fall of rain to be simultaneous over its whole extent. It seldom happens that all the tributaries, even of the Tiber, whose basin has an area of only seven thousand five hundred square miles, are swelled at the same time, and in the same degree, and rarely, if ever, that those of the much larger basin of the Loire, of which the superficies is forty-five thousand square miles, are simultaneously flooded, though the same kind of weather may prevail over the whole centre and south of France. This is truer still of the Danube, which drains an area of more than three hundred thousand square miles.

The affluents of the Mississippi and the Amazon, which are equal to rivers of the first magnitude on the continent of Europe, belong to different climates, and different meteorological conditions; so that, while some are in high flood, others are at their lowest level. Hence there are two great risings of the Mississippi in the year, corresponding to the