Fortification Friday: Defilement of works by defining planes

Defilement, that is, by Mahan’s definition… as in constructing a work so as to protect against plunging fire.  You know, as we discussed last week.  Get your mind out of the gutter!

In his Treatise on Field Fortifications, Mahan offered a book example of how to build a defile in order to protect against otherwise dominating terrain.  The example used this basic diagram (which I’ve attempted to clean up a bit…):


Consider this a three-dimensional diagram, with “poles” and “stakes” extending upward from the points with capital letters (I’ve embellished in red).  At the top of the diagram are the dominating points of terrain, labeled “O”.

The defilement of a work is a practical operation performed on the ground in the following manner: –

Let A B C D E (Fig. 15) be the plan of a work, a lunette for example; and the points O, O, & c., the most elevated points of a commanding position in front of the work.  At the points A, B, &c., let straight poles be planted vertically, and on the poles along the gorge line let a point be marked, at three feet above the ground. Let two pickets be driven in the ground along the gorge line, and a cord a,b, or a straight edge of pine, be fastened to them, on the same level as the two points marked on the poles at A and E.

Referring to the diagram, we see those “poles” indicated – 5 feet tall at the points along the trace of the lunette.  Five feet allowed for a soldier to stand at the parapet, and have clearance to fire.   However, across the gorge (which, recall, was open for a lunette) Mahan called for a three foot measure.   Again, think three-dimensional here and put on those funny glasses so we can “get into” the works.  This is the “eye” on the drawing.  Oh… and do mind the difference here between “A” and “a”; “B” and “b”.

… Let an observer now place himself in the rear of a b, so as to bring the poles at B, C, and D, and the points O, O, &c., within the same field of vision.  Let observers be placed at B, C, and D.  The first observer now sights along a b, until he brings he eye in the position that a b will appear tangent to the most elevated of the points O. Having accurately determined this position, he next directs the other observers to slide their hands along the poles until they are brought into the same plane of vision with the point O, and the line a b, and to mark those points on the poles.  These points, together with the two first marked, will evidently be in the same plane, and this plane, produced, will be tangent to the highest point O.  It is denominated the Rampant Plane.

We see the points on the poles indicated with the short lines radiating from the “eye.”

Another way to define the rampant plane is thinking of it as a line of sight between two points.  These points being the three foot mark above the gorge line and the distant high ground.  So the line of sight starts on the interior of the works and extends through the interior of the works, out past to the high ground the enemy might occupy.

Having established some points to think about, Mahan began using those to frame up this defilement, by introducing yet another plane to consider:

Now if a point be marked on each pole, at five feet above the points thus determined, these points will be contained in a second idea plane, parallel to the first, and five feet above it.  This plane is denominated the Plane of Defilement, and the interior crests of the work are contained in this plane, being the lines joining the highest points marked on the poles.

Plane of defilement is sometimes shortened to plane of defilade (which sounds better for those with dirty minds).  The important aspect of this plane is consideration of the height of an extra five fight over the rampant plane to account for the height of the enemy soldier standing on that distant hill top.   Towards that consideration, Mahan elaborated:

As the gorge line is farthest from the heights, and the rampant plane ascends towards them, it will necessarily pass at more than three feet above every other point of the parade of the work; and the plane of defilement, in like manner, will pass at eight feet above the parade at the gorge, and at five feet above the highest point O.  A plane of defilement is therefore defined to be, that plane which, containing the interior crests of a work, passes at least eight feet above every point that the enemy can occupy within the range of cannon, which range may be taken, with safety, at one thousand yards.

Where that plane crosses the trace lines of the works, it is desirable for the parapet to rise to the same level… obviously.

This is common sense.  If one wishes to protect the gorge from enemy fires, then pile up the parapet a bit higher.  Well, recall we have some governing factors there.  One cannot pile the parapet up to great heights without compromising the close range properties.  Yes, adding a glacis will help to some degree.  But that is yet another large amount of earth to move.  Beyond that, there is a practical limitation as to how high parapets and glacis may be stacked.

Once again, something that appears simple common sense can easily become rather difficult to apply in real situations.  What if, for instance, the point to be defended is in a valley with dominating heights all around?  What is the engineer to do?  We’ll take up that lesson next Friday.

(Citation from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, page 25-6.)

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