The mass of the ramparts of a masonry fort are the
casemates. Casemates are arched rooms that are most often used to house cannon. The arch has been long known as the strongest architectural form, and are used in masonry forts to bear the load of the ramparts along with barbette-mounted artillery above. To assist in spreading this weight across the foundation of the fort, a second arch, inverted, is often employed below the floor of the casemate. The picture below shows the arch above and the inverted arch below at Fort Pickens, guarding Pensacola Bay, Florida.
The arches of Fort Jefferson in the Dry Tortugas, Florida, seem to continue on infinitely.
These gunrooms opened to outside the fort with a penetration of the scarp, called an
embrasure. Embrasures could be simple openings in the wall that angled to a narrow area, called the
throat of the embrasure. The idea in embrasure design was to create the smallest opening in the wall that would provide the widest field of fire of the gun. To achieve this, an iron pin with a mushroom-shaped top called a
pintle was placed at the throat of the embrasure. This pin engaged the
tongue of the carriage, making this the point that the gun swiveled around. In the picture below, the embrasure, the opening for the pintle, and the slot below the embrasure where the tongue of the carriage would engage the pintle.
Here is a gun mounted in a casemate, showing the way the carriage was mounted.
The wheels at the front and rear of the carriage travel along iron rails in the floor of the casemate, called
traverse rails. These allow the horizontal aiming of the gun.
The rear of the casemate opens onto the parade of the fort. In two early Third System forts, the openings to the parade were long, narrow galleries. These casemates were called "tunnel casemates," and provided the gunners from protection in the rear. Unfortunately, this also caused the smoke from firing the gun to remain in the casemate rather than properly venting. This practice was discontinued, and all later forts had large openings to the parade.
A tunnel casemate at Fort Pike, near New Orleans, Louisiana.
The open-backed casemates at Fort Richmond, The Narrows, New York Harbor.
The cannon and carriage took up most of the area of the casemate, blocking the transverse arch. To run ammunition to the guns, the powder monkeys and brass monkeys would interfere with the gun crew. To solve this problem, a second set of arches, more narrow than the main arches, were created near the parade of the fort. These arches were called the
monkey run, as this is where the powder and brass would be run.
The monkey run at Fort Jefferson is at the left of this picture.