I bet this came in handy.
Richard

NASHVILLE DISPATCH, March 18, 1863, p. 4, c. 2

Among the most important articles captured on the Queen of the West, by the rebels at Fort Taylor, was the revised signal book of the United States Navy. Such books are always kept, on a man-of-war, tied up in a canvas bag, with a leaden weight attached to it, so that, in case the ship is captured by the enemy, it can be thrown overboard and sunk. When the Queen was disabled and abandoned, the signal book was lying on a table in Colonel Ellet's room, and it is, doubtless, ere this, in the Navy Department at Richmond. By its aid, they can learn the meaning of every flag hoisted on a Federal ship-of-war or gunboat in the Eastern or Western navy. The correspondent of the Cincinnati Commercial heard Commodore Foote say, to a master's mate on board the gunboat St. Louis, just before the battle of Fort Donelson, "Take good care of the signal book, and throw it overboard, if any thing happens to the fleet. I had rather the rebels get a gunboat than to have that fall into their hands."