Letter, 1863 August 16, Suess to Brooks on methods of sappingNational Park ServiceLetter to Brooks from Suess regarding the commencement of sapping operations. Suess writes that Capt. Walker has serious misgivings about the effectiveness of the fall sap, and that Capt. Walker proposes a different method of sapping, which was first used by the Turks in the 17th century.