Fortification Friday: Building a barbette in a bastion

Last week we discussed placement of artillery in barbettes to form batteries in a fortification defense.  Such an arrangement allowed the cannon to fire over the parapet, even with allowance for declination, at an attacker. Mahan’s description included details about spacing to allow for handling of the gun:

  • Mound of earth 2 feet 9 inches high.
  • Spacing along parapet – 16 ½ to 18 feet of length.
  • Depth of 24 feet (atop the tread of the banquette).
  • Ramp behind the mound at least 10 feet wide, sloped at 1:6 ratio.

Those figures were a rule of thumb to be adjusted to the situation.  And that rule of thumb best fit a situation were several guns were placed on a face, flank, or curtain wall.  In other words, a straight line of the defensive works.

But to illustrate the barbette, Mahan offered this illustration:


A barbette on a bastion’s salient angle.  Mahan observed:

As barbettes are usually placed in the salients, an arrangement is made for the guns to fire in the direction of the capital.  The construction in this case is somewhat different from the preceding. A pan-coupé of eleven feet is first made; from the foot of the interior slope at the pan-coupé, a distance of twenty-four feet is set off along the capital; at the extremity of this line a perpendicular is drawn to the capital; and five feet are set off on this perpendicular on each side of the capital; from these points, on the perpendicular, a line is drawn perpendicular to each face respectively; the hexagonal figure, thus laid out, is the surface of the barbette for one gun. The ramp in this case is made along the capitol [sic].

Let’s walk through this one step at a time, using Mahan’s illustration.  First we want to setup that pan-coupé within the salient angle:

As per Mahan’s guidance, this was eleven feet (indicated in red) perpendicular to the line of the capital (blue line included for reference).

From there, a distance of 24 feet – the depth prescribed for a barbette battery – was walked back towards the gorge:


Next, a width of five feet on either side of the capital was set aside within the depth:


With the depth and width established, this creates platform for the barbette, marked C on Mahan’s diagram:


From there, perpendiculars off each face of the bastion were defined:


Now the ramp was defined and laid out.  This would be ten feet wide, 1:6 slope, and along the line of the capital.  This is indicated as “B” on Mahan’s diagram:


With the ramp established, all sides of surface of the barbette are defined.  This being a hexagonal shape, indicated as “A” on the diagram:


One last bit of work to mention here. The parapet at the salient angle required adjustment to allow the cannon to depress.  Part of the parapet was cut down, indicated as “D” on the diagram:


This would cover much of the “sector without fire” at the capital. And it is called the superior slope of the pan-coupé.

Consider these features, the surface of the barbette, ramp, platform, and superior slope of the pan-coupé when seen on the horizontal:


This in place, the fortification had a position for one cannon on the bastion’s salient angle. And such could go a long way to reduce that sector without fire.

But one gun?  Just dangling out there over the parapet for the enemy to shoot away?  That won’t do!  So now we should look at arrangements made for several guns within the bastion.  That’s for next week….

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, page 53.)

Published by Craig Swain

"Historical marker hunter" and Civil War enthusiast.

3 thoughts on “Fortification Friday: Building a barbette in a bastion

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