m Rivers and Canals. 605 
derable, and do more damage in the interior parts of a 
country, than towards the mouths of moft rivers. 
In the Po, for example, the height of the banks made 
to keep in the waters diminilhes as the river approaches 
to the fea. At Ferrara they are twenty feet high ; whereas 
nearer the fea they do not exceed ten or twelve feet, al- 
though the channel of the river is not larger in one place 
than in the other. 
44. The mouths of rivers, by which they difcharge 
their waters into the fea, are liable to great variations, 
which produce many changes in them. 
1 ft, The velocity and direction of the current at thefe 
mouths are in a continual variation, caufed by the tides, 
# 
which alternately retard and accelerate the ftream. 
2dly, During the flowing of the tide, the current of the 
river is firft flopped, then turned into a direction intirely 
contrary throughout a confiderable extent; if we may 
believe M. de buffon, there are rivers in which the 
effect of the tides is ienfible at 1 50 or 200 leagues from 
the fea. 
3dly, This ftate of things is a caufe of a great quan- 
tity of fand, mud, See. being precipitated and accumu- 
lated in the channel near the mouth. This continually 
raifes and widens the bed, and at laft changes it intirely 
into a new place, or at leaft opens new mouths to dif- 
Vol. LXIX. 4 K charge 
