Report from the Fort21 January 1802 · Major J. J. Ulrich Rivardi
The bombproofs are six in number and well arched.
No.1 - used as a guard house has two large bake ovens - its dimensions are 50 feet in length, 18 in width and 12 high in the center of the arch - it is well floored - is apt to leak in the spring.No. 2 - used as powder magazine, being very dry, and isolated, is 68 feet long, 17½ wide and 12 feet high in the center of the arch.No. 3 (empty) is 50 feet long, 18 wide, 12 high, has a good underfloor and a chimney.No. 4 (empty) is 43 feet long, 7 wide, 7½ high, no chimney.No. 5 (empty) is 34 feet long, 10 wide, 7 high, no chimney.No. 6 (empty) is 39 feet long, 7 wide, 7 high, no chimney.
The large Bombproofs, having 12 feet in height could in time of War have a double row of Bunks or a Gallery so as to accommodate easily one hundred men each.
Report from the Fort20 May 1807 · Jonathan Williams, Inspector of Fortifications
In the whole bastion at the northeast angle of the fort there are six large subterranean arches, and if they had been so constructed as to be tight, they would be a very valuable part of the fort. Unfortunately this is not the case and they of course are not habitable, nor capable of preserving stores that can be injured by moisture, one of them, however, appeared less damp than the others.National Archives
In nearly every report on the fort the casemates are cited as leaking and deteriorating. Fortunately, they never had to be used during an attack, though they did serve as prisons. A convict sentenced to hard labor in 1801, and a corporal who deserted from his troop in 1842 and was placed in "double irons" may well have been kept in the casemates. Between 1863 and 1865 during the Civil War, Confederate deserters as well as Union soldiers and citizens awaiting trial were housed here. Double-decker bunks were constructed here for their use. In 1864 the prisoner tally rose to 216 rebel prisoners, 55 Union soldiers awaiting trial or sentence, and 11 other offenders. The damp, cold quarters brought illness and complaints of inhuman treatment and encouraged letters from family, friends, and influential groups for the release of prisoners. A signed Oath of Allegiance to the United States provided freedom for some.
The iron gate on the casemate entrance thwarted most escapes, but alcoholic drinks were smuggled to the prisoners and disobedience often occurred. In 1863, on Christmas Day, an uprising among the prisoners was even attempted.
Fort Mifflin, PA
December 27, 1863
I have the honor to report that a very serious plot was discovered in the bomb proof on the night of the 25 inst. The Union prisoners, especially the prisoners whose sentences had been read to them on Parade, had it so planned that when the relief came into the bomb proof, knock the sentinel down on the outside, also the Sergeants of the guard whose duty it was to post the guard, take the keys from him and then the whole of the prisoners, both soldiers and citizens, make their escape. Happily the plot was discovered in good time to prevent it from being carried into execution. I have placed the Ringleader in close confinement.
I am Captain your obedient servant, J. Orr Finnie, Commander of the Post
One prisoner arrived with a murder charge, William H. Howe, a Union deserter who accidentally shot and killed an arresting officer, was tried and sentenced while at Fort Mifflin. He was found guilty, resulting in the only execution at the fort: death by hanging on August 26, 1864.