Thus far as we’ve examined, in meticulous detail, how to address relief in the planning of a field fortification, we’ve focused on the parapets. Getting those tailored would protect against direct fire against the defenders of a particular face – direct defilement as we call it. Mentioned earlier, the works also needed reverse defilement to protect the backs of those defenders. That was the job of the traverse.
Let us pause for the moment to explain the traverse. The traverse was an internal structure within a work which was designed to intercept enemy fires or reduce the impact of an explosion. Traverses filled a number of vital functions within the works and there were a number of variations, based on those functions. We see traverses along a line, designed to intercept flanking fire. Traverses might form a secondary facing along a line. We also see traverses as sort of a “backstop” behind a sallyport. And also traverses situated around magazines could prevent a disaster caused by a lucky shot (or stray ember). But here we are looking at traverses built within a salient and intended to block enemy fire from the rear of the face. Such are defilade traverses (though I would point out defilade traverses apply here to both reverse and enfilade defense).
Going back to our notional figures, thus far Mahan had instructed how to determine the height of the parapet.
To determine the height of the traverse is the next step. To do this, the height of the tread of the banquette is ascertained on the three poles, B,C,D, and a distance of nine inches is set off on each pole above the tread. Between the points thus determined a cord is stretched, or if the distance be too great for this, two pickets may be placed between B and C, and a cord, or straight edge, be fastened to them in the required direction.
Please note the mark set off on these well used poles. This mark is but nine inches above the banquette – that being the location where the defender stands to shoot over the parapet. The intent is to build a defense which will prevent the enemy from firing on that piece of ground from the rear.
An observer is then placed at the pole F, and another places himself behind the line B C, so as to bring the cord, and the points O’ and F, in the field of vision; he then shifts the position of the eye until the cord is brought to touch the point O’; he then directs the observer at F to mark the point on the pole where it is intersected by the plane of vision.
This is somewhat confusing at first read, but remember that at this time in the construction of the works, the parapets and other structures were not yet built. So the observers are walking over what is relatively level space. Still, this is difficult to depict on the diagram without the risk of confusion to the reader…
Work with me here – we have the line marked off between A-B-C. The second observer is somewhere there behind that line (and outside what would be come the works later on). Then the second observer directs the positioning of our original line of F-C until that cord, along with cord A-B-C and the line to O’ all sit in the same plane. That would determine the “mark” used as the baseline for the traverse. (I know what you are thinking… nine inches? Hold on to that for a second.)
A similar operation is performed with the point O, and the face C D, and above the highest point thus determined on F, a distance of five feet is set off for the top of the traverse at F; and five feet nine inches is set off above the tread of the banquette at C for the top of the traverse at that point.
So, pick the highest of the two baseline marks, add five feet to cover the backs of the standing musket-firing infantryman, and you have the desired height the traverse. Oh, and with that we have a new term to use:
The planes which determine the top of the traverse, are termed planes of reverse defilement.
That’s good because, as you probably have figured out, fortifications are built upon a firm foundations of technical terminology. The more buzzwords the engineer offered, the better the fort.
The height established, there were other details needed to finalize the traverse:
The traverse is finished on top like the roof of a house, with a slight pitch; its thickness at top should seldom exceed ten feet, and will be regulated by the means the enemy can bring to the attack; its sides are made with the natural slope of the earth; but, when the height of the traverse is considerable, the base of the side slopes would occupy a large portion of the interior space; to remedy this, in some measure, the portion of the sides which are below the planes of direct defilement, may be made steeper than the natural slope; the earth being retained by a facing of sods, &c.
I would offer that the “etc.” mentioned here included gabions.
OK, that’s the height of the traverse… what about the length needed?
When the salient of the work is arranged for defense, the traverse cannot be extended to the salient angle; it is usual to change its direction within some yards of the salient, and unite it with the face most exposed.
Keep in mind the traverse outlined above was designed to counter reverse fire on a face. We mentioned traverses also worked to stop enfilading fire. So more traverses were needed in the works:
Traverses are also used to cover the faces exposed to an enfilade fire; for this purpose they are placed perpendicular to the face to be covered. If several are required, they may be placed twenty or thirty yards apart; each traverse should be about twenty-four feet long, and thick enough to be cannon proof.
With all that figuring in place, the engineer could start pointing out were the shovels should go to work.
Keep in mind this process applied to the open works (with the open gorge). The same could be used for enclosed works, with a few other considerations. Next week, we’ll look at those along with some other “tips” offered by Mahan.
(Citation from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, page 28-30.)