An elevation is produced in the soil of Egypt, at the same time that this extension of its surface takes place, and the bed of the river rises in the same proportion as the adjacent plains, which makes the inundations of every succeeding century pass far beyond the marks which it had left during the preceding ones. According to Herodotus, a period of nine hundred years was sufficient to establish a difference of level amounting to ten or twelve feet. At Elephantia[94], the inundation at present exceeds by seven feet the greatest heights which it attained under Septimus Severus, at the commencement of the third century. At Cairo, before it is judged sufficient for the purpose of irrigation, it must exceed, by three feet and a half, the height which was necessary in the ninth century. The ancient monuments of this celebrated land have all their bases more or less buried in the soil. The mud left by the river even covers, to a depth of several feet, the artificial mounds on which the ancient towns were built[95]. The delta of the Rhone is not less remarkable for its increase. Astruc gives a detailed account of it in his Natural History of Languedoc; and proves, by a careful comparison of the descriptions of Mela, Strabo and Pliny, with the state of the places as they existed at the commencement of the eighteenth century, taking into account the statements of several writers of the middle age, that the arms of the Rhone have increased three leagues in length in the course of eighteen hundred years; that similar additions of land are made to the west of the Rhone; and that a number of places, which were situated, six or eight hundred years ago, at the edge of the sea or of large pools, are now several miles distant from the water. Any one may observe in Holland and Italy, with what rapidity the Rhine, the Po, and the Arno, since they have been confined within dikes, raise their beds, advance their mouths into the sea, forming long promontories at their sides; and judge, from these facts, how small a number of ages was required by these rivers to deposit the low plains which they now traverse. Many cities, which were flourishing sea-ports at well known periods of history, are now some leagues inland; and several have even been ruined, in consequence of this change of position. The inhabitants of Venice find it exceedingly difficult to preserve the _lagunes_, by which that city is separated from the continent; and notwithstanding all their efforts, it will be inevitably joined to the mainland[96].