Miss Eells apparently knows something about Civil War era spy codes - and what she knows is apparently interesting enough to share with, and impress, Anthony Monday [The Treasure of Alpheus Winterborn; 7].
Author Michael Antonucci notes the American Civil War (1861-65) presented new challenges for cryptographers because it was the first war in which the telegraph played an important role: it "greatly increased the number of messages that could be sent and the speed at which they could travel, but the wires were not secure. There was little that Civil War armies could do to stop the wiretappers, so they sought to minimize the danger by encoding their transmissions. Even encrypted messages, however, were not safe, thanks to...code-cracking...both sides in the Civil War used ciphers, and both sides tried to break its opponent's ciphers - with varying degrees of success."
Interestingly enough many of the spies during the American Civil War were women, such as Sarah Emma Edmonds and Pauline Cushman (Union) and Rose O’Neal and Belle Boyd (Confederacy).