Statistics for occurrence #1 of “Harper's Ferry” in chapter 12 of The Photographic History of The Civil War: in ten volumes, Thousands of Scenes Photographed 1861-65, with Text by many Special Authorities, Volume 5: Forts and Artillery.:
...f an engineer of renown, stated in a letter to Secretary of War Stanton that it was one of the most difficult operations of the kind ever performed.
Immediately after returning to Washington from Harper's Ferry , the engineer troops, with their bridge-equipage, were sent to Fort Monroe, in Virginia, and were moved thence, on April 4th, to a Camp near Yorktown, in preparation for the Peninsula campaign.
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