Turning circles around the Confederates
Maj. Albert J. Myer, a U.S. Army Surgeon, created this cipher disk to protect Union Army communications. Letters on the upper two disks aligned with numbers on the two lower disks. In this image, for example, the letter "A" would be sent as "81." Myer's earlier work was reviewed by Lt. Col. Robert E. Lee. One of Myer's assistants was Lt. J.E.B. Stuart.
National Archives, Records of the Office of the Chief Signal Officer