Last week we looked at the use of outlets in the field fortifications. Certainly it was impractical to insist the garrison stockpile sufficient food and supplies for the anticipated duration of the conflict at hand… not to mention cross their legs the whole time. Some sort of gate or entrance was required. However, that presented a weak point that an attacker might exploit. Thus Mahan urged his students to build additional structures, namely traverses, to cover those entrances. That reduced the risk to a degree.
But where the point where the outlet crossed the ditch (if it had to cross a ditch, that is) presented a greater problem. As we’ve seen in detail, one of the important properties of the ditch was serving as an obstacle to any attack. But the outlet would require a six to ten foot wide path across the ditch. Thus granting an attacker a potential highway across. Common sense response to that problem was to simply build some sort of retractable bridge. But pre-war, Mahan did not offer that as a direct solution (indeed, avoiding mention of the problem altogether). But in his post-war edition, Mahan offered:
Draw-Bridge. For the usually narrow ditches of field works, either a light rolling bridge may be used for a communication, from the outlet, across the ditch; or else an ordinary wooden draw-bridge. A very simple one, and of easy construction, was proposed by Colonel Bergère of the French engineers.
I believe this refers to Colonel Pierre Bergère (1785-1868), but I don’t know of a specific work to reference in regard to the bridges in question here. I find it interesting the more detailed discussion of bridges enters Mahan’s post-war edition with Bergère serving as a reference and introduction. Either as if the 1861-65 experience simply followed that of the French engineer, or could not be applied directly. I have my thoughts, but let’s save that for later.
Mahan offered Figure 45, bis to illustrate the proposed bridge:
Technical details followed:
The bridge is a light platform a,a’, of joists and boards, long enough to span the ditch D, and so arranged as to turn around an axle at A, the crest of the scarp. At the point B, on each side of the platform, an iron gudgeon is firmly attached to it and turns in the eye of a socket at the end of lever C B. This lever is formed of two pieces of scantling of some tough flexible wood, each about four inches square. The lever has an eye, at the middle point O, which receives a strong iron bolt that connects two ordinary gun carriage wheels. The two pieces which form the lever are firmly fastened together, as shown in the figure; a weight, consisting of shells filled with sand, or shot, being fastened at the end c, and serving as a counterpoise to the bridge. Two rails A, of heavy scantling are laid for the wheels to run upon in maneuvering the bridge; which is done simply by one or two men taking hold of the spokes of the wheels, and so, by turning them, causing them to run backwards or forwards, and thus raise or lower the bridge.
The arrangement is a clean piece of engineering. But nothing novel or particularly advanced, technically speaking. Noteworthy is the re-purposing of common equipment, such as carriage wheels and projectiles, on hand with the field army.
But I have to throw a flag out here. If those building the fortification had ample time to build fancy drawbridges, they were more so working as garrison troops and probably not actively campaigning. The premise behind field fortifications was those would be temporary structures established as part of a field army’s operations. Not structures to be garrisoned, which should shade more to the permanent fortifications with far more elaborate arrangements all around. But experience in the Civil War took the American officers away from some pre-war assumptions about how fortifications would be use. And more importantly how to classify them. We’ve already noted that Junius Wheeler began classifying semi-permanent and temporary garrison fortifications within the discussion of field fortifications.
Now this is not to say no self-respecting American engineer in 1861 would consider the drawbridge when planning the outlet to a fort. Rather that in 1861, the use of drawbridges were not emphasized in the classroom. But by 1870, that changed to provide a couple of paragraphs and a nice illustration based on a French officer’s recommendations. As we will see by looking at Wheeler’s text, by the next decade, those instructions evolved even further. Pages, mind you, dedicated to the discussion of small bridges!
(Citation from Mahan, An Elementary Course of Military Engineering: Part 1: Field Fortifications, Military Mining, and Siege Operations, New York: John Wiley & Son, 1870, page 57-8.)