NAVY DEPARTMENT, Washington, April 5, 1861. Capt. SAMUEL MERCER, Commanding U.S.S. Powhatan, New York: SIR: The United States steamers Powhatan, Pawnee, Pocahontas, and Harriet Lane will compose a naval force, under your command, to be sent to the vicinity of Charleston, S. C. for the purpose of aiding in carrying out the objects of an [...]