The combined action of the two rivers has so advanced the coast-line that Adria is now more than fourteen miles inland, and, in other places, the deposits made within the same period by these and other neighboring streams have a width of twenty miles. What proportion of the earth with which they are charged these rivers have borne out into deep water, during the last two thousand years, we do not know, but as they still transport enormous quantities, as the North Adriatic appears to have shoaled rapidly, and as long islands, composed in great part of fluviatile deposits, have formed opposite their mouths, it must evidently have been very great. The floods of the Po occur but once, or sometimes twice, in a year. [Footnote: In the earlier medieval centuries, when the declivities of the mountains still retained a much larger proportion of their woods, the moderate annual floods of the Po were occasioned by the melting of the snows on the lower slopes, and, according to a passage of Tasso quoted by Castellani (Dell' Influenza delle Selve, i., p. 58, note), they took place in May. The usually more violent inundations of later ages are due to rains, the waters of which are no longer retained by a forest-soil, but conveyed at once to the rivers--and they occur almost uniformly in the autumn or late summer. Castellani, on the page just quoted, says that even so late as about 1780, the Po required a heavy rain of a week to overflow its banks, but that forty years later it was sometimes raised to full flood in a single day. Pliny says: "The Po, which is inferior to no river in swiftness of current, is in flood about the rising of the dog-star, the snow then melting, and though so rapid in flow, it washes nothing from the soil, but leaves it increased in fertility."--Natural History, Book iii, 20. The first terrible inundation of the Po in 1872 took place in May, and appears to have been occasioned by heavy rains on the southern flank of the Alps, and to have received little accession from snow. The snow on the higher Alps does not usually thaw so as to occasion floods before August, and often considerably later. The more destructive flood of October, 1872, was caused both by thaws in the high mountains and by an extraordinary fall of rain. See River Embankments; post. Pliny's remark as to enrichment of the soil by the floods appear to be verified in the case of that of October, 1872, for it is found that the water has left very extensively a thick deposit of slime on the fields. See a list of the historically known great inundations of the Po by the engineer Zuccholli in Torelli, Progetto di Legge per la Vendita di Beni incolti. Roma, 1873.]