THEORY OF THE EARTH. 139 



ployed for preventing the devastations occasion- 

 ed by the floods of the Po, he ascertained that 

 this river has so greatly raised the level of its bot- 

 tom, since it was shut in by dikes, that its present 

 surface is higher than the roofs of the houses in 

 Ferrara. At the same time, the alluvial additions 

 produced by this river have advanced so rapidly 

 into the sea, that, by comparing old charts with 

 the present state, the coast appears to have 

 gained no less than fourteen thousand yards since 

 the year 1604, giving an average of an hundred 

 and eighty to two hundred feet* yearly ; and in 

 some places the average amounts to two hundred 

 feet. The Adige and the Po are both at present 

 higher than the intervening lands ; and the only 

 remedy for preventing the disasters which are 

 now threatened by their annual overflowings, 

 would be to open up new channels for the more 

 ready discharge of their waters, through the low 

 grounds which have been formed by their alluvial 

 depositions. 



Similar causes have produced similar effects 

 along the branches of the Rhine and the Maese ; 

 owing to which all the richest districts of Holland 

 have the frightful view of their great rivers held up 



* In the appended extract from the Memoir of M. Prony, the 

 older average yearly increase is stated at 25 metres, or 82 English feet 

 and a quarter of an inch ; and the average of the last 200 years at 70 

 metres, or 229 feet 7 inches and 9-tenths yearly. TransL 



