Last week, we discussed the layout and arrangements for the blockhouse, when used as part of a keep in the interior of a fortification. An important requirement, if the keep was to function as intended, was the ability of the defenders to fire out of the blockhouse. Just as with building the banquette, embrasures, and other arrangements on the parapet, such arrangements within the blockhouse necessitated attention to details. And those details come in the form of loopholes and vents, as Mahan would write:
The loop-holes are three feet apart; their interior dimensions are twelve inches in height; and eight inches in width for sides twelve inches thick; and twelve inches square for sides two feet thick. The width on the exterior, for the same thicknesses, will be two-and-a-half and four inches. The height of the loop-hole on the exterior will depend on the points being defended; it should admit of the musket being fired under an elevation and depression. The height of the loop-hole above the exterior ground is six feet.
The visual you should have in mind is that of an aperture which is small on the exterior but larger for the interior. This would allow the defender to train the musket across a wide arc, as well as providing for elevation and declination. I don’t like mixing field fortifications with permanent fortifications, but in this case the application is along the same lines. So consider the loop-holes here at Fort Pulaski, to the right of an embrasure:
In this case, there was need for the muskets to cover two zones. So we see two loopholes incorporated as a pair. Note the placement of stone slab above and below to strengthen the loophole structurally. Something not needed within the wooden blockhouse – simple cut outs within the timbers usually sufficed.
But the major difference between the blockhouse and brick fortification’s loopholes is the height. Mahan specified only twelve inches for the blockhouse in a field fortification. Those at Fort Pulaski are two feet or so.
Another aspect to keep in mind is the depth of the wall. As the wall became thicker, the loophole’s lateral dimensions, particularly interior, increased. Geometry at play here, as the musket would need more clearance on the interior as depth increased.
Mahan did not directly discuss interior arrangements for the artillery’s embrasures. Partly, I think, as such an allocation would have pulled valuable cannons off the fort’s primary defensive line to that of the secondary or even tertiary defenses. But, we can deduce such arrangements would match those described for embrasures through the parapet. In short, a larger loophole… which is what we see to the left of the photo above.
All this is good thinking. But we also have to keep in mind the by-product of firing any weapon. In order to push the projectile out of the barrel, firing of the powder creates gasses. That foul air is not an issue out in the open or on the parapet. But in the enclosed space of the blockhouse, there is need to expel the gasses:
Vents for the escape of the smoke are made over each loop-hole, between the cap-sill and the top pieces.
Moving to another location in Fort Pulaski, we see a vent above one of the other embrasures:
See the weathering on the paint?
Mahan does not provide much information on constructing vents for the blockhouse. These could be vents between the ceiling and wall. Or vents incorporated in the wall itself. To maintain integrity of the structure, in terms of defense, those vents were best created using an interior angle. That would allow gasses to vent. But water… or things the enemy might want to push inside… would be restricted.
From there, Mahan gave brief descriptions of the camp bed (which we noted served as the banquette inside the blockhouse), racks, and other storage arrangements. But with that he left the interior arrangements. Instead he turned to an external details. We’ll look at those next week.
(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, page 64.)