Fortification Friday: Don’t for get the door! Gotta get in and out of the fort

So you’ve built a strong fortification in accordance with Mahan’s instructions.  The fortification has wonderful fields of fire, properly sized parapets, protected batteries, and bombproofs for everything to include the commander’s liquor stash.  Great!

But sooner or later the fort’s garrison has to wonder outside the works.  Say for maintenance of the works, or patrolling the outskirts, or simply for resupply.  So there needs to be a passage to the exterior somewhere in the plan.  Mahan called these outlets:

Outlets are passages made through a parapet, or an enclosure of a gorge, for the service of the work.  They should, in all cases, be made in the least exposed part of the work.  Their width need not be more than six and a half feet, when used only for the service of the work; but when they serve as a common passage for wagons, &c., in the case of the intrenchment crossing a road, they should be at least ten feet wide.

Clear definition.  And some clear specifications with the idea width established at either 6 ½ feet, for “walk out” outlets, or ten feet, for “ride out” outlets.  That established as a rule of thumb, we turn to the structural components and advice for building these outlets:

When cut through the parapet, the sides receive a slope of three perpendicular to one base, and are riveted with sods, &c.

A gate, termed a barrier, serves as an enclosure to the outlet.  The framework of the barrier is made like an ordinary gate, consisting of two uprights, or stiles, a cross-piece, or rail, at the top and bottom, and a swinging bar, or a diagonal brace. Upright palisades, about seven feet long and four inches thick, are spiked to the frame about four inches apart; they are finished at top with spikes. A barrier, thus constructed, will not offer a shelter to the enemy should he attempt to cut it away.  The barrier is hung on hinges like an ordinary gate.  The posts of the framework should be very solidly braced to support the weight of the barrier.

Figure 41 illustrates such a barrier:

PlateVIFig41

Yes, they sell these as pre-fabricated products at Home Depot and Lowes.  Well… not exactly to military specifications.  But you get the point, the base form is a simple gate. For the annotations, Mahan offered:

  • A A – posts to which the gate is swung.
  • B B – the uprights of the gate.
  • c c’ – the upper and lower cross pieces. (c’ seems to have been left off).
  • D – the diagonal brace.
  • E – the bar of the gate.

Note also the mention of spikes on the gate. Specifically palisades.  These were not intended to impale an attacker, but rather to keep the attacker’s reach away from the structure of the gate.  So drop all those Medieval notions there.

Continuing, Mahan offered,  “A cheval-de-frise is sometimes used for a barrier, it presents but a trifling obstacle.”  So let us relegate that to the level of lazy engineering.

But just keeping the enemy at arms length was not enough.  One also had to protect the outlet from cannon fire. Toward that end:

The outlet should be covered by a mask, thrown up either on the interior, or on the exterior, to prevent the enemy from firing through it into the work.  A traverse is thrown across it, if placed on the interior.  Sufficient space should be left between the traverse and the parapet for the passage of a gun.  The length of the traverse is arranged to prevent the enemy from firing into the work, by an oblique fire through the outlet.  The traverse may be of earth or of wood; in either case it should be arranged for defense to enfilade the outlet. In some cases, and it would generally be safest, a barrier is erected between the parapet and the traverse, on each side of the outlet.

Figure 42 provides us Mahan’s suggested layout of the masking traverse:

PlateVIFig42

From the key for this figure:

  • O – the passage or outlet through the parapet.
  • P – (to the left of “Fig. 42.”) the passage between the parapet and the traverse
  • T – the traverse

Note Mahan gave us 6 ½ feet between the parapet and traverse for “P”.  We also see the prescribed cross fires built across the passage.

Mahan went on to suggest more elaborate defensive arrangements to protect important passages.  “In very frequented passages, a redan or a lunette, is thrown up on the exterior to cover the outlet, and thus ensure its safety in case of surprise.”  These arrangements followed the standard configurations for faces and flanks.  No doubt, such added more work for the defender’s labor force, as they would be clearing and leveling both front and rear.

The important take away with respect to passages is how a necessary weak spot in the defenses would be turned into a strong point by way of barriers, traverses, and other cover.

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, page 61-2.)

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