Loop-hole

An aperture designed to allow small arms fire to be delivered from a covered position behind walls, gates, and doorways, or above the level of the superior slope of a parapet. The basic requirements for all loop-holes, whether in field or permanent fortifications, was that troops firing through be given sufficient space to point their weapons effectively toward ground defended by the loop-hole and that the aperture allowed to accomplish be as small as possible to maintain the greatest degree of cover and inhibit hostile fire from entering or ricocheting though the aperture. Just as important during a close assault, loop-holes also had to be arranged in such a fashion that they could not be used by attacking troops to fire with precision and deliberation into the interior of the work. The first requirement could usually be met by giving the aperture a splay, or spreading the opening out from the aperture to the front or back (or both to the front and back) of the wall where it was positioned. The splay could either be made smooth to form an aesthetically pleasing surface from the front of the wall to the limits of the aperture or it could be cut into a number of receding flat surfaces. Receding flat surfaces tended to promote ricochets away from the aperture while smoothed splays tended to allow ricochet shots and bullet fragments to fly into the aperture. The second condition was usually dealt with by raising the sole, or lower edge, of the aperture to a height that would make it very inconvenient and dangerous for attacking troops to try to reach.

In field fortifications loop-holes were usually erected along the crest of the interior slope using three or more sand-bags. Two sand-bags were laid either vertically or horizontally three to six inches apart and a third place on top to form the loop-hole. This arrangement allowed an individual to raise his head and shoulders above the cover of the parapet and fire through the gap between the sand-bags. Other arrangements could be hastily made when sand-bags were not available. Head-logs and skids, which consisted of heavy logs positioned along the parapet supported on long poles leaning from the top of the parapet to ground in rear of the protective work. The gap between the soil of the parapet and the log was used as a loop-hole while the skids forced head-logs struck by artillery fire to slide back over the work rather than tumbling down on the troops using the head-log as cover.

Loop-holes in stockades and blockhouses were shaped with the slay toward the interior so that the aperture was on the exterior side of the wall. D. H. Mahan's Treatise on Field Fortification prescribed blockhouse loop-holes that were 12 inches high and 8 or 12 inches wide on the interior side of the wall and 2 or 4 inches wide on the exterior side. These dimension could be considered rather standard dimensions that were, in practice, subject to alteration to meet the needs of particular blockhouses. Loop-holes cut through the walls of one of the blockhouses in the defenses of Washington, D.C., for instance, were 15 inches high with the aperture on the exterior wall 4 inches high and about two inches wide. These loop-holes were given a definite downward sloping splay (from the interior to the exterior) to assist troops firing through them depress their rifles at the proper angle to strike ground immediately surrounding the work.

Loop-hole designs in permanent fortifications differed quite markedly from those used in field fortifications for the very simple reason that masonry walls were usually much thicker than the wooden walls of blockhouses and stockades. It was an accepted condition that the muzzle of a musket had to protrude at least three inches though a loop-hole's aperture to adequately expel smoke produced by firing the piece and prevent it from being carried back into the protected enclosure. This was not a significant problem with wooden walls that were thinner than muskets were long, but when walls were thicker than muskets were long, some means had to be designed to both provide a good field of fire and insure that muskets' acrid residue was projected out of the loop-holed enclosure. Simple loop-holes splayed toward the interior of thick walls did not answer the purpose: 3 feet was taken as the mean length of muskets likely to be fired through loop-holes in permanent fortifications; if the loop-holed wall was 5 feet thick, a musket's muzzle would be two feet short of the aperture when fired and all of the resulting smoke and acrid fumes would remain inside the enclosure. Even the best natural system of heat induced ventilation could not carry fumes away quickly enough to prevent the smoke from interfering with the defenders' fire or, at worst, keep the defenders from being driven from the enclosure choking.

This made it necessary to move loop-hole apertures backward away from the exterior side of the wall and closer to the position where a defender would stand to fire through the loop-hole so that muskets' muzzles could be projected out of the aperture without greatly inconveniencing troops defending loop-holes. But it was still necessary to maintain an effective lateral field of fire so that a defender firing through a hole-hole could point his piece from side to side without interference. These two objects were best accomplished by giving loop-holes an hour-glass form, in plan, similar to that used for embrasures designed for cannon. Loop-holes for walls over 3 feet thick were splayed both toward the exterior of the wall and toward the interior; the aperture itself was reduced to the minimum dimensions possible to maintain the greatest amount of cover for troops firing through loop-holes.

January, 2003