Things continued in this state for several centuries; during which period, it is probable that the inhabitants of these lands were often, by various catastrophes, disturbed in the enjoyment of them, though not discouraged. But in 516, by which time these people were become very numerous, more than 600 of them perished by one of the concurrences of fatal circumstances already defined. It was then that they undertook the astonishing enterprise of enclosing these lands. They dug ditches around all the _marsches_, heaping up on their exterior edge the earth which was taken out; and thus they opposed to the sea, dikes of eight feet in height. After this, comprehending that nothing could contribute more to the safety of their dwellings, than to remove the sea to a greater distance, they undertook, with that view, to exclude it from the intervals between the islands, by uniting, as far as should be possible, those islands with each other. I will describe the process by which they effected this, after I shall have recalled to attention some circumstances leading to it. From all that I have already said of the _fore-lands_, and of the manner in which they are increased, it may be understood, that the common effects of the _waves_ and of the _tides_ is to bring materials from the bottom of the sea towards the coasts; and that the process continues in every state of the sea. The land winds produce no _waves_ on the coasts, which can carry back to the bottom of the sea what has been brought thence by the winds blowing against the shore; and as for the _tides_, it may have been already comprehended (and shall soon be proved), that the _ebb_ carries back but very little of what has been brought by the _flood_. So that, but for some extraordinary circumstances, the materials continually impelled towards the shore, which first form islands, would at last unite against the coast in a continuous soil. The rare events, productive of great catastrophes, do not carry back these materials towards the bottom of the sea; they only, as it has been said before, ravage the surface, diminishing the heights, and destroying the effect of vegetation. These, then, were the effects against which it was necessary to guard.