3. By making a sloping
bank to a certain height and then building a perpendicular wall upon
it.--Now if the 1st of these constructions could be arranged, I have
no doubt that it would be the best of all, because a sea does not
_break_ against a perpendicular face, but recoils in an unbroken
swell, merely making a slow quiet push at the wall, and not making a
violent impact. But practically it is nearly impossible. The 2nd
construction makes the sea to break tremendously, but if the sloping
surface be made of square stone put together with reasonable care
there is not the smallest tendency to unseat these stones. This is the
principle of construction of Plymouth Breakwater. In the 3rd
construction, the slope makes the sea to break tremendously, and then
it strikes the perpendicular face with the force of a battering ram:
and therefore in my opinion this is the worst construction of all. A
few face-stones may easily be dislodged, and then the sea entering
with this enormous force will speedily destroy the whole. This is the
form of the Cherbourg Digue.
From this you will gather that I have a full belief that Plymouth
Breakwater will last very long, and that the Digue of Cherbourg, at
least its upper wall, will not last long.
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