Building upon the instructions for constructing the relief and defilement of open fortifications, Mahan’s lesson plan then turned to address the same factors within enclosed fortifications:
The cases of defilement here examined, are those of works open at the gorge; the same principles, and similar methods, would be applied to enclosed works. After the plan of the work has been regulated, the arrangement of the traverses next demands the attention; the only rule that can be laid down is, to place them in the most favorable position to intercept the reverse and enfilading fire of the enemy; and if there should be a choice with respect to several positions, to select the one which will give the lowest traverse.
With the open work considered in earlier instructions, there was but one set of angles to consider in the plan. But with an enclosed work, such as a bastion fort, there would be four or more sets. Each set, if considered alone, might demand its own elevated parapet or traverse. Mahan urged the engineer to simplify that where possible to reduce the space taken up by those expanded structures. Otherwise, the same rules, to define the planes of direct and reverse defilement applied.
That said, Mahan closed the discussion about the relief of works with three paragraphs which clarified, somewhat, the earlier instructions. The first of which sounded much like a “throw away”…
The difficulty of defilement, owing to the great relief that may be required for the parapets, the labor of erecting the traverses, and the room which they occupy within the work, which is frequently wanted for the defense, restricts its application mostly to enclosed works, which are to remain occupied during some time, and whose position, from some point to be defended, cannot be shifted.
I interpret this to mean the engineer should consider the situation, particularly the mission of the force employed, before embarking on a great plan for relief. If the expectation was to defend the point for a short period of time, then the labor of building larger parapets and traverses was not economical. Particularly where the terrain might require extensive defilement thrown up to provide the “textbook” defense. Of course, one may counter this by saying any work worth defending for a moment is worth defending for a month. Many a flawed work was started under the assumption, valid at the time, of temporary need.
If it is not even probable that a commanding eminence will be occupied by the enemy, nevertheless should the defense be not impaired, it will be better to place the work beyond the cannon range of the eminence.
This seems to redress the preceding paragraph, and get us back to the “common sense” rules. Unless the situation required such – as in the key point to defend required a work within range of the eminence – then place the works out of range from any fires from the high ground. Such would alleviate the need for those fat parapets and traverses. If only the real world worked that simply?
Finally, where such structures could not be avoided, Mahan urged the engineer not to become carried away:
The irregularity of the profile of the parapet, caused by defilement, will occasion a correspondent irregularity in that of the ditch. Where the parapet is highest, the ditch will require to be the widest and deepest; for, in order to avoid the removal of the earth to considerable distances, it is best that the earth for each portion of the parapet should be taken from the ditch in front of it. No other rule can be laid down in this case, than to keep the dimensions of the ditch within the prescribed limits; and, if this will not admit of its counterscarp being well defended, to raise a glacis in front of it, subject to the fire of the work.
Ah… remember the optional glacis? By artificially raising the ground in front of the ditch, the engineer could resolve defects caused by tall parapets. Of course, that also meant more work with more dirt being moved.
My closing comment on relief, is that defilement was much a process of risk management. We’ve discussed at length the process of defining the height of the parapet and particulars of the traverse. And we are also asked to balance the particulars of those structures against the effort needed. So if called upon to build a fortification in a position where high ground dominated, my first steps would be determining the likelihood an adversary might use that high ground (or perhaps just portions of it) against the works to be built. And from there, prioritize the work oriented on the most likely and risky options.
The reality is none of us will be building Civil War fortifications these days. However, the lessons which Mahan laid out in the text are still valuable tools for our use. These allow us to, in some ways, reverse engineer the works that survive so as to better interpret the history. In that way, the surviving earthworks become a primary source for our consultation and consideration. We just need to understand the dynamics behind the construction. And I’m spinning up the supporting illustrations for just such an exercise of reverse engineering – taking into mind these notions of relief and defilement, considering parapets and traverses. I’ll work that up for next week.
(Citation from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, page 30-31.)