400 PHYSICAL GEOGRAPHY. 



The shrill cicalas, people of the pine, 



Making their summer lives one ceaseless song, 

 Were the sole echoes, save my steed's and mine, 



And vesper bells that rose the boughs along." 



Cuvier has given an extract from the researches of M. de Prony on the hydraulic 

 system of Italy, who was employed to remedy the disastrous inundations of the Po, which 

 contains an account of the enlargement of that part of the coast occupied by its mouth. 

 According to the statement, no exact detail can be given of the successive progress of the 

 changes, and more especially of their precise measures, during the ages which preceded 

 the twelfth century of our era. One fact however is certain, that Adria, which gives its 

 name to the Adriatic, a confederate city of the Romans, and a municipium, was a sea-port 

 town, between the mouths of the Po and the Adige, and a station for the Roman fleet 

 under the emperors. This city has a modern representative upon the old site, and by 

 this we not only attain a known fixed point upon the primitive shore, but the means of 

 measuring the extent of alteration- that has occurred. The following results have been 

 clearly established : that in the twelfth century the shore had been already removed to 

 the distance of 9000 French metres, or between five and six miles, from Adria ; that by 

 the sixteenth century, or in four hundred years, it had been extended farther to the 

 medium distance of 18,500 metres, or nearly eleven miles and a half, giving from the 

 year 1200 an average yearly increase of the alluvial land of 25 metres, or rather more 

 than 27 yards ; that by the present century it had advanced to near 33,000 metres, or 

 about twenty miles, whence the average annual progress is about 70 metres, or 761 yards, 

 during the last two hundred years, being greatly more rapid in proportion than in former 

 times. The precise date cannot now be ascertained when the waves of the Adriatic 

 washed the walls of Adria, but they certainly did in the time of Augustus, and the nearest 

 part of the present coast, at the mouth of the Adige, is at the distance of fifteen miles and 

 a half, while the extreme point of the alluvial promontory formed by the Po is upwards 

 of twenty miles. In consequence of the melting of the snows in the Alps, the Po is 

 periodically flooded in the summer months, and its inundations, now guarded against by 

 immense embankments, have in past ages raised the level of the country subject to them, 

 by the deposition of alluvium, especially towards the sea-coast. The level of its own 

 bottom has, at the same time, been so much raised, that the surface of its waters is now 

 higher than the roofs of the houses in Ferrara, and both the Adige and the Po are higher 

 than the whole tract of country between them, which would be invaded by their waters 

 but for the artificial bulwarks constructed along their course. In the same manner the 

 delta of the Nile has been altered by elevation as well as extension, the cultivated soil 

 having increased vertically seven or eight feet since the time of the Ptolemies, at the 

 rate of about four inches in a century, the bed of the river rising in proportion. 



Where the sea acts alone upon a low coast, without the aid of rivers, considerable 

 alterations are effected in its line and aspect, but of a far less beneficial nature than those 

 just noticed. If the bottom is sandy, the waves then drive the sand towards the shore, 

 which becoming partially dry at every reflux of the tide, a certain quantity .is heaped up 

 on the beach by the action of the wind. Around stones, bushes, and tufts of grass, the 

 sand accumulates, and becomes a continuous ridge where these arresting obstacles are 

 contiguous, or where they are thinly scattered, a number, of small hillocks are produced, 

 which increase into mounds of considerable height. These formations are called downs, 

 or dunes, which occupy an immense extent of coast, and frequently penetrate to a con- 

 siderable distance inland. In situations where the habitual direction of the wind is from 

 the sea, the particles at the surface of the sand hills or ridges are carried forward by it, 

 giving a gradually increased breadth to the downs ; but where the wind blows generally 



