Earlier we considered the pre-war instructions offered by Mahan in regard to outlets, particularly how those would be constructed. Training cadets before the war, Mahan covered the topic with a few paragraphs. Later, after the war, he added a section about bridges associated with the outlets. However, by the 1880s, Junius Wheeler’s instructions to cadets would sprawl over the better part of seven pages. Maybe this is simply a case where pre-war there was a premium on copy space for printed manuals. On the other hand, maybe Wheeler felt “his” cadets needed “special help” getting this particular aspect of fort-building in order. Or perhaps, which I tend to think, wartime experiences prompted the extended emphasis.
The first thing we see with Wheeler’s treatment of the subject is a focus shift. Before discussing the construction of these outlets, he steps back to discuss what these are used for. Notably, the section is titled “Communications, barriers, etc.”:
Communications. – The defenders of a closed work must have arrangements made by means of which they can enter or go out of the work when necessary. In the case of continued lines, arrangements should be provided by means of which the defenders can make sorties.
Word choice here. Mahan’s writing pressed that outlets were important for servicing the works. Left unstated, implied at best, were other uses for the outlets – communications, logistics, reconnaissance, or simply just getting out to stretch the garrison’s legs. On the other hand, by selecting this sub-heading, Wheeler set different functional requirements… although, much vaguer than Mahan’s.
Wheeler’s second paragraph discussed the basics. The outlets had to be made through the parapet, and that created a weak point in the work. To minimize the risk, he insisted the number of outlets be kept to a minimum and built where least exposed. Specifically, he gave these suggested placements:
In redoubts, the outlets are on the sides least exposed to attack; in half-enclosed works, they are placed near the middle of the gorge; in forts, they are usually placed near the re-entrants.
And that matches well to Mahan’s suggestions from decades before. But where we see variations is with the size of these outlets, which Wheeler relates based on functional requirements:
A passage for the use of infantry only should not, as a general thing, be less than six feet wide; for artillery, not less than ten feet wide; for sorties, the outlets in continued lines should be at least fifty yards wide.
See what happened here? Wheeler took Mahan’s two size categories and then added a third, based on an additional use-case. I think at this passage Wheeler is thinking back to works at Vicksburg or Petersburg or around Atlanta. And in those campaigns, field works had become part of an offensive operation – be that a very deliberate siege at some levels, but more dynamic than an “According to Vauban” siege. Again, not to say wide passages for sorties did not exist prior to 1861, but rather to say those took on different emphasis around about 1863-4… at least in the American context.
Demonstrating that the more things change, the more they tend to stay the same, Wheeler offered this diagram for an idea outlet through the parapet with masking traverse:
Yes, he ripped off Mahan. But give him a little credit for adding more notations and giving us that nice profile on the right. And he offered the lines (c and g, the “crossing” arrows through the middle) which were the enemy’s “extreme lines of fire”. Those lines, Wheeler instructed, governed the length of the traverse (T). Note how those lines are drawn off the corner of the superior slope of the parapet.
Toward that, Wheeler did add a lot more to the notion of masking the outlet:
The length of the traverse may be shortened by turning back the interior crest at right angles to its general direction, and extending it as far as the crest of the Banquette.
This “turn back” is indicated by “B” on the figure. One on each side of the outlet. Notice how that would send the lines of fire (again, lines c and g) into a tighter intersection, further back in the outlet, and thus reducing the area the enemy might fire upon. Such angles reduced, the traverse need not be so long. All in all a nice functional flourish to the outlet design.
Beyond that, Wheeler offered other options that would reduce the vulnerability:
Instead of having a road along its entire front, the traverse is sometimes joined to the parapet on one side of the opening, as shown by the dotted lines b d and e f, in Fig. 50.
And such would mean only one “extreme line of fire” need be considered in regard to traverse length. But that did mean traffic had a nasty chicane to deal with.
This is all good, but how about that fifty-yard wide outlet for sorties?
The method adopted to mask the interior of the work in this latter case, is to place the traverse opposite the outlet on the outside, and beyond the ditch….
The traverse in this case is usually broken, generally a redan in trace, with the profile of a parapet, but commanded by the parapet in rear.
Figure 51 illustrated this manner of masking:
Basically, if you have to provide a wide gap in the works, the only viable solution is to build a miniature work in front to cover it. Keep in mind, such sortie outlets would be placed on less exposed sections of the line. And of course, these would be covered by strong faces on either side. Such would reduce the chance the enemy might target the outlet for their own “sortie”. And the redan covering the front would further reduce the danger of direct artillery fire. Note, however, you don’t see a traverse in the interior of the works to cover the sortie outlet. The purpose of this outlet is to allow infantry in wide columns to emerge in quick order. Bad enough, though necessary, to have that redan in the way. Putting a traverse on the inside would impose yet another choke point for maneuver… and yet another delay in an operation where time was critical.
Before leaving Wheeler’s discussion of outlets, we’ll also examine the evolution of barriers and bridges. Furthermore, Wheeler introduced the notion of ramps within these outlets. All interesting facets to consider with respect to the art of military fortifications.
(Citations from Junius B. Wheeler, The Elements of Field Fortifications, New York: D. Van Nostrand, 1882, pages 148-51.)