In the last installment on this series, we read that Mahan rated the bastion fort as satisfying “more fully the conditions of a good defense.” And his lesson plans to the cadets placed emphasis on the characteristics of this type of fortification. His lesson continued by describing the theory of the bastion fort’s layout with a bit of practical twist added:
The bastion of a fort may consist of a polygon of any number of sides; but for field forts, the square and the pentagon are generally preferred, owning to the labor and construction. To plan a work of this kind, a square [or] pentagon is laid out, and the sides bisected by perpendiculars; a distance of one eighth of the side is set off on the perpendiculars in the square, or one-seventh in the pentagon; from the angular points of the [polygon], lines are drawn through the points thus set off; these lines give the direction of the lines of defense; from the salient of the polygon distance, equal to two-sevenths the side, are set off on the direction of the line of defense, which give the faces; from the extremity of the faces, the flanks are drawn perpendicular to the lines of defense; the extremities of the flanks are connected by the curtains.
That, my readers, is in a nutshell how one builds a bastion fort. Let’s “rock drill” this.
We first start by drawing a polygon, preferably a square or pentagon, over the area on which the fort will sit. I’ll borrow one of Mahan’s diagrams to make this simple… and that being a square. Those lines are in red below:
As with last week, this is a partial diagram, with the dashed blue lines indicating the continuation. However this does serve a point – the practice for building any bastion, regardless if detached or enclosed, are the same. These are called the exterior sides. Notice this incorporates the “line of defense” as we defined in the discussion of fort traces.
Let me also point out, the square or pentagon or other polygon drawn at this step will contain all of the area that the engineer wished to defend. This is important. If some undesirable terrain feature, such as a steep slope, existed, the engineer had to adjust the polygon, and thus the exterior sides, to exclude. Yes, you’d not want a creek cutting through your square at this point in the design.
In a real situation, we’d measure the length of the exterior side, as that factors into the next steps taken. Mahan suggested a maximum of 250 yards for this length. But for now, allow me to construct this without a set measure. We’ll get into the math soon enough!
Next, draw lines perpendicular to those exterior sides:
Notice I’ve shifted the exterior sides to light blue and highlighted these new lines in red… I’ll stick to that convention through this lesson.
Next, walk down the perpendicular line, from the intersection of the perpendicular and the exterior line, the specified distance:
For a square, that distance is ⅛, and for a pentagon it is a seventh. From that point on the perpendicular, we draw a line back to the endpoints of the respective exterior line:
For clarity, I’ve only drawn two of those intersecting lines here.
Next mark off a distance equal to two-sevenths of the exterior side on the intersecting line:
This defines the face of the bastion – both in length and orientation.
From the interior terminus of the face, we draw a line that will intersect the opposite intersecting line at a right angle:
This defines the flank of the bastion. And it is important that the line of the flank cross that opposite line at a right angle. Otherwise it would not offer coverage of the opposite face.
Next we connect the two flanks:
This is the curtain wall. When the curtain is drawn for all respective sides, the fort becomes enclosed.
Now this is all well and good if you are going out to build a fort in the backyard… though perhaps too elaborate for a play-date with the kids. Of what value is this to us today? I’d submit this places us within the planning cycle for any 150-year-old fortification. In other words, this tells us how “they” would have approached this operation.
Let’s go one step further. We often read reports from primary sources rating defenses as “good” or “faulty.” Applying the method above to the works offer us the ability to quantify that. We can reverse engineer, to some degree, in order to derive a more exact identification of “superior” or the “faulty” elements of the works. Not only does that help us interpret the earthworks extant on the ground (or only known by way of maps), it also tells us something of the individual providing the observation. Yes, we can actually rate the guy making the rating – was he heeding the lessons of Mahan or not?
(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, page 14.)