A subsequent paper1 from the same pen amplified a portion of
this theory, contending that, while modern engineering skill had
reduced the berm to the smallest possible dimensions (and it is
still required) by the introduction of a broad rampart, carefully
designed in every part and angle, the highest and heaviest part
of which is still far removed from the edge of the fosse, a
berm of considerable width was a structural necessity for the
Boman rampart, which resembled a tall erect wall. The
argument was set forth in a merely tentative manner, and
rather as a suggestion for further investigation, as the sections
available are still extremely limited in number. Briefly stated
it was as follows:-—In every case the surveying has proved that
the work was carefully designed and carefully executed and
stamped throughout with the idea of permanence. So much is
this the case, that subsidences in the sides of the fosse are very
rare after so many centuries. The site chosen almost invariably
slopes considerably to the north. The fosse is cut through a
sandy soil, frequently penetrating the upper stratum and pro-
jecting some distance into the underlying bed of boulder-clay.
These circumstances, combined with the natural drainage of the
country towards the valley north of the rampart, which would
tend to render the scarp unstable, would make the erection of a
high rampart near the edge of the fosse a matter of great risk.
In the two sections, which have been most fully detailed (Seabeg
No. 2 and Bonnyside No. 3), it is interesting to notice that the
slope of the counterscarp is the same in each—it is 27| degrees.
The scarps are different—in Seabeg it is 30| degrees and in
Bonnyside it is 26-J degrees. These angles represent the natural
slope of earth, or the angle of repose,2 and the stability of the work
has shown how wise has been their adoption. The desire would be
to make the slopes as steep as possible. The berm at Seabeg is
broader and lies at a much greater slope than that at Bonnymuir,
but it is worthy of remark that the protracted line of the scarp, at a
slope of 30|- degrees, touches a perpendicular line raised from the
1 " The Vallum, Berm, and Fosse : Their Correlation," read by Mr. Chalmers at
a meeting of the Society on 17th December, 1891. Mr. Chalmers does not wish
it published, as its full gist is contained here.
2 Prof. Rankine's Tables and Rules, p. 180.