Fortification Friday: Marking “Planes of Direct Defilement” on the Parapets

No doubt some readers are still battling with snow this Friday, with large berms of the white stuff piled high.  I’ve considered taking advantage of this to build our own little fortification with relief, Mahanian style.   But I must admit, despite encouragement… and bribery… I’ve yet to enlist my aide-de-camp to pose with a picket post in order to properly establish our fortification.  It is difficult enough just to get the aide to shovel the snow in the first place.  Seems it is much more fun to simply sled down the berm and assail neighborhood compatriots with snowballs.  Nobody wants to build up some intricate snow-works.  So, alas, readers will not have a depiction of relief in snow.

This week we will continue with Mahan’s lesson on building the relief of works. Thus far we’ve considered the need for relief in the works, defined defilement of the works,  and then begun planning the extents of that defilement.  Recall that by this point, the engineer has the basic idea of where he should place the traverse… as in this diagram:


Next the engineer needed to focus on the points e and e’ (again keep in mind the upper-case, lower-case distinctions here).  That brings us back to Figure 16 and the trace of the works:


Working on that plane, Mahan called for the engineer to address another set of planes… the planes of direct defilement.

Poles (Fig. 16) are planted at the points A B C, &c., and one at the point F, where the lines of the capital and gorge intersect.  On the pole F, a point is marked three feet above the ground and a point is likewise marked on the pole at C, which should be one foot six inches higher than that on F; that is, if the ground between the two poles be level, the point on C will be four feet six inches above the ground. Two stout pickets may next be planted between F and C, and a cord, or a straight edge, be fastened to them, so as to be in the same line as the points marked on the poles.

So let me dress up this figure to highlight what Mahan was referring to:


The blue lines are the posts at F and C.  The green are the two posts Mahan required on the line between F and C.  And the thin yellow line is the cord run between all four posts.  Due to the size and limitations of my graphical arts skills, I am not accurately demonstrating the height of the marks on those posts.  Keep in mind the desired marks for F (three feet) and C (four and a half, plus the height of the parapet).

These poles and cord in place, the engineer had a base line for further definition:

Observers are then placed at the poles A and B; and another places himself behind the cord [between F and C] so as to bring the posts O, A, and B, in the field of vision with it; then shifting the position of the eye until the cord is brought tangent to the highest point on O, he directs the observers at A and B, to mark on the respective poles the points where the plane of vision intersects them.  This operation will determine the rampant plane for one half of the work A B C F, and that for the other half will be determined by a similar process.  If the distance of five feet be set off on each pole above the points thus determined, these points will fix the position of the interior crests.

Looking at the diagram, let me attempt to illustrate this effort:


The poles at A and B are orange lines.  The line of observation, from the cord to O is in red.  Then we have the adjustments on B with the red arrows on that line.   With marks on the poles at A and B set, and a line (presumably another cord) between those posts, we set the height required for the parapet, specifically the interior crest in order to provide protection to the defenders.  The same could be repeated for D and E and set the height required on that side.  But there’s a catch here:

It is obvious that the interior crest of the part A B C is not the same plane as that of the part C D E.  These two planes are denominated planes of direct defilement.

Thus it may be that the interior crest on one side is higher than the other.  The engineer needed to “reconcile” or, simply ease the difference where the two planes come into contact on the parapet… that being at point C.  Thus there is an irregularity in the height of the parapet, which Mahan would address at a later point. But readers should keep this “conundrum” in mind as we go forward.

The poles, cords, and marks indicated here allowed the engineer to specify how much earth needed to be piled to make the parapet effective.  The verbiage used here is indicative of the desired effect – to intercept fires from the distant, high ground.  Thus we have “planes of direct defilement” here.  Next we need to look at the other side of this… that being the “planes of reverse defilement” and the details of the traverse.

Now, I ask, are you getting the feel for how complicated “pile it higher” really is?

(Citation from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, page 27-8.)