William F. Fox, Lt. Col. U. S. V., Regimental Losses in the American Civil War, 1861-1865: A Treatise on the extent and nature of the mortuary losses in the Union regiments, with full and exhaustive statistics compiled from the official records on file in the state military bureaus and at Washington

The principal place of confinement for Union soldiers was at Andersonville, Ga. Out of 45,613 prisoners confined there, 12,912 died — or, 28 per cent. The greatest number present at any time was 33,114--on August 8, 1864. The greatest number of deaths in any one day was 127--on August 23, 1864. The daily average of deaths was 29 3/4.

The largest military prison in the North was located at Elmira, N. Y. As at Andersonville, it consisted of an open stockade or prison pen. In it were confined 11,916 prisoners, of whom 2,994 died, or 25 per cent. The greatest mortality occurred in March, 1865, in which month 495 died at Elmira. Of the total number that died, 2,988 were buried in a field which has since been ploughed over and planted with wheat; and now the grain of summer and the snow of winter show no sign of the hapless Confederates who are laid at rest beneath its surface.