“If the operation of the waters of the ocean be attended to in the formation of the shores, some useful hints may be gained. These shores will be found to be so many inclined planes, varying in declivity according to the tenacity of the matter of which they, are composed. Hence it is that the minute grains of sand and the light sea shell become a lasting barrier against the rapid river current and the tumultuous ocean, while the erect sea wall is levelled with the ground. For the truth of this it were needless to refer to the works of nature in different quarters of the world, or in distant parts of this country; it is only necessary to examine the shores on each side of Barnbougle Castle, where the beautiful beach, consisting of sand and shells, between the Cockle Burn and the sea, forms a complete defence to the low grounds behind it, while to the northward of the castle the massive wall is in danger of being completely thrown down. Without waiting to inquire into the causes which regulate these appearances, it will be more consonant to the business of this report to point out how their simple forms may be imitated and turned to advantage.”
In reporting on the defence of the lands of Trinity, on the Firth of Forth, Mr. Stevenson recommended the adoption of a cycloidal talus wall, which was executed under his direction in 1821:--
“In giving an opinion relative to the best mode of defending and preserving this property, the reporter observes that it fortunately happens that the beach is pretty closely covered with large boulder stones, which now form a kind of _chevaux de frise_ in breaking the force of the sea, and making it fall more gently towards high water mark. Were it not that these stones are proposed to be employed in the erection of a more effectual barrier against the waves, the reporter would not fail to disapprove of their removal for any other purpose.
“The reporter proposes that a _Talus wall_ or bulwark should be built of these boulder stones, roughly dressed and laid so as to form a cycloidal curve in the central part, as nearly as may be, as represented in the section with its tangents (Fig. 20). The properties of the cycloid as applicable to a sea wall in an exposed situation are very important. In particular, if compared with any other curve, in the same vertical line and down through the same points, it will be found of swiftest descent under similar circumstances, therefore the water in its rise must be proportionally retarded. The lower tangent to the curve alluded to also forms a wall towards low water, best adapted for admitting the sea to flow gently over it, while that connected with the upper extremity of the cycloidal part, tending towards the perpendicular, brings gravity into action against the rise of the waves. The practical execution of a wall upon this construction is simple, while the aggregate quantity of materials is less than for any of the curves of the conic sections of similar extent, and it seems upon the whole to be peculiarly applicable for the defence of the sea beach in question.