Crowdsourced to Decipher Civil War Military Telegrams

choi

Edward Rothstein, The Chance to Decode History; Anyone can help decipher Civil War telegrams, including messages from Lincoln himself. Wall Street Journal, June 21, 2016 (under the heading "Cryptography").
http://www.wsj.com/articles/here ... messages-1466544288
("As long as you had that book, you could work out the message. Six of some 10 code books now survive, with four in the Huntington and others in the possession of the George C. Marshall Foundation; one of the great cryptographers of the 20th century, William F. Friedman (1891-1969), had rescued them as they were about to be incinerated as trash")

Note:
(a) cryptography (n)
http://www.merriam-webster.com/dictionary/cryptography
(b) "the Huntington Library, Art Collections and Botanical Gardens [has a] project 'Decoding the Civil War,' collaborating with the Abraham Lincoln Presidential Library and Museum [located at Springfield, Illinois; opened in 2004; a division of the Illinois Historic Preservation Agency, an agency of Illinois state government], North Carolina State University’s Digital History and Pedagogy Project, and Zooniverse.org, a 'crowdsourcing' platform"
(c) "Lincoln may have had his Second Inaugural [delivered at DC on Mar 4, 1865] message in mind, 'with malice toward none, with charity for all.' He had visited Richmond's ruins and instructed Weitzel to 'let 'em up easy.' "
(i) ALL dictionaries says "let up" is intransitive. For example:

let up: "informal (of something undesirable) become less intense"
www.oxforddictionaries.com/definition/english/let-up?q=let+up
(ii) The phrase "Let 'em up easy" is attributed to Lincoln alone, and the source is this telegram, according to the Web.