Fortification Friday: In a “hollow formed by two eminences”? Reverse Defilement

Last week we discussed the defiling of an earthwork … as in planning, thence constructing, the works so as to intercept plunging fires from higher elevation.  This process of defilement was, while “not indispensable” in Mahan’s words, was highly recommended where terrain allowed an adversary to dominate the point of defense.

Where we left that lesson, Mahan advised to define a plane of defilement out to 1000 yards from the works – that being the extreme range of musketry and (at the time of writing) practical range of artillery.  We can easily place this into the “common sense” category… saying it just makes sense to pile dirt high enough that one is protected from the enemy’s guns.  Now that is a simple matter if the eminences are a dozen feet above the height of the works.  Real world scenarios are rarely that simple, as anyone who has visited … say… Harpers Ferry or Chattanooga or Pilot Knob might attest.  The nature of terrain at such key points often allowed for an enemy to fire on the flanks, rear, and interior of the works. Piling dirt higher is just not practical.  So what would the engineer do in response to such lofty heights?

When a work is placed in a hollow formed by two eminences, and is exposed to both a direct and reverse fire from them, it cannot be defiled by the means just explained, without giving it a relief generally too great for field works.  To avoid this the method of reverse defilement must be resorted to.

Consider the crudely copied illustration below:

PlateIIFig16A

The lunette is defined by A-B-C-D-E of Figure 16.  There are two eminences here – O and O’.  We see the flank A-B-C is open to direct fire from O, and can be hit in reverse from O’.  And the same can be said for flank C-D-E, transposing the respective high ground.

Looking at this problem from the profile of the works, cited as Figure 17:

PlateIIFig17A

This is where the engineer needed to do his “figuring.”  We see a vertical plane from O to O’ (orange lines, corresponding to the lines seen in Figure 16).  We have the parapets of two sides of the lunette depicted as A and B.  And follow the proper labeling of points here, mindful of upper and lower case letters.  Also for this first snip ignore the structure labeled C (as in capital C) as we evolve the solution:

If in this plane a vertical, a b, be drawn [brown line], corresponding to the capital of the work, and eight feet be set off on this vertical form the point a, and two verticals be drawn through the points O and O’, and five feet be set off on each of them [black lines]; and then the points c and c’ be joined with d [blue line], it is obvious that the interior crest of the parapet A, being placed on the line c d, will screen all the ground in the rear of it, as far as the capital, from direct fire from O.  The parapet B being regulated in a similar manner, will screen all the ground behind it as far as the same line.

Sounds good so far, but what about that reverse fire?

But the fire from O’ would take the parapet A in reverse, and that from O the parapet B; to prevent this, a covering mass, denominated a traverse, must be erected on the line of the capital, and a sufficient height be given to it to screen both A and B from a reverse fire. To effect this, let eighteen inches be set off the interior crests of A and B; the point e being joined with c’, and the point e’ with c; it is here also obvious, that if the top of the traverse be placed on the line c e’ [dashed green line], it will effectually screen both the parapets from all reverse fire; because every shot that strikes the top of it will pass at least eighteen inches above the two parapets, and since the banquettes are four feet three inches below the interior crests, the shot must pass five feet nine inches above the banquettes, which will be quite sufficient to clear the heads of the men when on the banquettes.  This illustration explains the spirit of the method of reverse defilement.

At this point in the engineering process, we’ve defined one particular needed to form that traverse, labeled C on the diagram.  However, that is still a far cry from actually determining the full nature of that traverse.  More measurements, observations, and planning was required to properly plan and place the traverse.  We’ll turn to that in the next installment.

(Citation from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, page 26-7.)

Fortification Friday: Relief of Intrenchments… talking elevation, not reinforcements!

Back from the holidays and getting back to the Friday installments with titles prompting double-takes from those with dirty minds…..

For the first three chapters of the Treatise on Field Fortifications, Mahan’s focus was on the construction and arrangement of the works without any detailed discussion of the external factors governing the layout of the works.  Yes, as if the works were build in a perfectly flat table top in conditions one would never see in real life.  But Mahan was writing a instructional text, so abstracting out reality for a bit was necessary to get the basics across.

Chapter four brought us to some of that “reality,” with Mahan discussing the impact of relief on the design of the works …. and that is “relief” in the sense of terrain elevations.   Sort of a new dimension to consider, beyond just horizontal and vertical, profiles and traces:

When a work is placed on level ground, it usually receives a uniform relief; but when the site is irregular, or there are commanding eminences within cannon range, a uniformity of relief cannot be preserved, because it might expose the interior of the work to the enemy’s view, from the commanding points.

Let us be fair, practical, and realistic here.  Excepting perhaps some seacoast defense, where field fortifications are not apt to be used, anything worth protecting with earthworks is probably within cannon range of some eminence or is sited on irregular terrain.  I’ve often marveled at the notion of “regular” terrain… as most terrain one will encounter is broken and, well, irregular.  So why call it irregular?  But I digress. The bottom line is that where the terrain did not provide a perfect, flat playground for the engineer to design his works, the earthwork had to consider those irregularities.

Mahan continued with the inputs needed to the fortification plan:

The plan will also be modified by the same causes. The principal faces should be placed as not only to guard all the points where an enemy might approach, but the enemy should not be able to take up their prolongations to obtain an enfilading, or a reverse fire upon them.  The position of the points to be guarded, and that of the commanding eminences, require to be carefully studied before adopting any definitive plans.

So… one should take into account these irregularities of terrain, in particular any place the enemy might gain an elevation advantage, before reaching a final plan for the earthworks.  Sounds simple.  But again we are at one of those points with military science where common sense sounds so simple but is darn difficult to apply.   Put yourself at, say, Harper’s Ferry with an adversary holding Maryland and Loudoun Heights.  Now try to apply the common sense espoused in the citation above.  Not like one can wave a hand and fix those problems.

How to remedy such sticky problems?  Some general guidance and “rules of thumb” to follow:

The only general rules that can be laid down, are to lay out the principal lines so as to obtain a direct and cross fire on the approaches of the enemy, and placing them, at the same time, as nearly parallel as practical, to the general direction of the crests of the commanding heights, in order that the enemy occupying the crest may have a direct fire along on these parts.

So this is some relief (the refreshing kind of relief, that is) to the poor engineer sitting in Harper’s Ferry.   But just a little.  By negating any advantage of flanking, enfilading, or reverse fires, the firepower arithmetic is simpler.  Though that still does not turn around the advantage of commanding heights, as Mahan spoke of next:

When the enemy occupies a position more elevated than the work, he is said to have a plunging fire on it; and when the relief of the work is so regulated as to intercept this fire, the work is said to be defiled.

I like this passage.  We see the problem defined and labeled as plunging fire.  Then we see the textbook remediation that can solve the problem… pile dirt higher!

Well not just pile dirt higher.  Rather think of ways to put dirt, rock, or other obstructions between the defender and any enemy on those elevations.  However, Mahan continued with a cautionary note that plans should not get carried away in this regard:

The defilement of field works is not indispensable to a good defense; nor is it generally practicable. It is, however, not only a conservative means, but it also inspires the assailed with confidence; for the soldier regards with distrust the strength of his position, when he finds himself exposed to the view of the enemy from an elevated point.

So call it an “optional” facet to the planning, but one that one should strongly consider picking up.

Mahan followed this up with a “practical example” of how to plan a defilement.  We’ll look at that in the next installment.  For now, consider again the play here of something sensible within military science.  It is very easy to stand at some spot and say “this is a bad spot as it is dominated by the high ground.”  But that assessment must consider that the defender of that “bad spot” was probably tied to that location by situational necessity.  It was, perhaps, a point of such value that the defense had to be made.  With that in mind, we really must be considering how vigorously and rigorously the defender worked to turn a “bad spot” into something at least a little less bad.

(Citation from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, page 24-5.)