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.)

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